[ { "text": "The Black Holes of a General Two-Dimensional Dilaton Gravity Theory: A general dilaton gravity theory in 1+1 spacetime dimensions with a\ncosmological constant $\\lambda$ and a new dimensionless parameter $\\omega$,\ncontains as special cases the constant curvature theory of Teitelboim and\nJackiw, the theory equivalent to vacuum planar General Relativity, the first\norder string theory, and a two-dimensional purely geometrical theory. The\nequations of this general two-dimensional theory admit several different black\nholes with various types of singularities. The singularities can be spacelike,\ntimelike or null, and there are even cases without singularities. Evaluation of\nthe ADM mass, as a charge density integral, is possible in some situations, by\ncarefully subtrating the black hole solution from the corresponding linear\ndilaton at infinity.", "category": "gr-qc" }, { "text": "Improved methods for simulating nearly extremal binary black holes: Astrophysical black holes could be nearly extremal (that is, rotating nearly\nas fast as possible); therefore, nearly extremal black holes could be among the\nbinaries that current and future gravitational-wave observatories will detect.\nPredicting the gravitational waves emitted by merging black holes requires\nnumerical-relativity simulations, but these simulations are especially\nchallenging when one or both holes have mass $m$ and spin $S$ exceeding the\nBowen-York limit of $S/m^2=0.93$. We present improved methods that enable us to\nsimulate merging, nearly extremal black holes more robustly and more\nefficiently. We use these methods to simulate an unequal-mass, precessing\nbinary black hole coalescence, where the larger black hole has $S/m^2=0.99$. We\nalso use these methods to simulate a non-precessing binary black hole\ncoalescence, where both black holes have $S/m^2=0.994$, nearly reaching the\nNovikov-Thorne upper bound for holes spun up by thin accretion disks. We\ndemonstrate numerical convergence and estimate the numerical errors of the\nwaveforms; we compare numerical waveforms from our simulations with\npost-Newtonian and effective-one-body waveforms; we compare the evolution of\nthe black-hole masses and spins with analytic predictions; and we explore the\neffect of increasing spin magnitude on the orbital dynamics (the so-called\n\"orbital hangup\" effect).", "category": "gr-qc" }, { "text": "Directed search for continuous gravitational-wave signals from the\n Galactic Center in the Advanced LIGO second observing run: In this work we present the results of a search for continuous gravitational\nwaves from the Galactic Center using LIGO O2 data. The search uses the\nBand-Sampled-Data directed search pipeline, which performs a semi-coherent\nwide-parameter-space search, exploiting the robustness of the FrequencyHough\ntransform algorithm. The search targets signals emitted by isolated asymmetric\nspinning neutron stars, located within 25-150 parsecs from the Galactic Center.\nThe frequencies covered in this search range between 10 and 710 Hz with a\nspin-down range from $-1.8\\times10^{-9}$ to $3.7\\times10^{-11}$ Hz/s. No\ncontinuous wave signal has been detected and upper limits on the gravitational\nwave amplitude are presented. The most stringent upper limit at $95\\%$\nconfidence level, for the Livingston detector, is $\\sim 1.4 \\times 10^{-25}$ at\nfrequencies near 160 Hz. To date, this is the most sensitive directed search\nfor continuous gravitational-wave signals from the Galactic Center and the\nfirst search of this kind using the LIGO second observing run.", "category": "gr-qc" }, { "text": "Constraining $f(Q,T)$ gravity from energy conditions: We are living in a golden age for experimental cosmology. New experiments\nwith high accuracy precision are been used to constrain proposals of several\ntheories of gravity, as it has been never done before. However, important roles\nto constrain new theories of gravity in a theoretical perspective are the\nenergy conditions. Throughout this work, we carefully constrained some free\nparameters of two different families of $f(Q,T)$ gravity using different energy\nconditions. This theory of gravity combines the gravitation effects through the\nnon-metricity scalar function $Q$, and manifestations from the quantum era of\nthe Universe in the classical theory (due to the presence of the trace of the\nenergy-momentum tensor $T$). Our investigation unveils the viability of\n$f(Q,T)$ gravity to describe the accelerated expansion our Universe passes\nthrough. Besides, one of our models naturally provides a phantom regime for\ndark energy and satisfies the dominant energy condition. The results here\nderived strength the viability of $f(Q,T)$ as a promising complete theory of\ngravity, lighting a new path towards the description of the dark sector of the\nUniverse.", "category": "gr-qc" }, { "text": "Crossing the Phantom Divide Line in a DGP-Inspired $F(R,\u03c6)$-Gravity: We study possible crossing of the phantom divide line in a DGP-inspired\n$F(R,\\phi)$ braneworld scenario where scalar field and curvature quintessence\nare treated in a unified framework. With some specific form of $F(R,\\phi)$ and\nby adopting a suitable ansatz, we show that there are appropriate regions of\nthe parameters space which account for late-time acceleration and admit\ncrossing of the phantom divide line.", "category": "gr-qc" }, { "text": "Global Portraits of Nonminimal Inflation: Metric and Palatini: In this paper, we study the global phase space dynamics of single\nnonminimally coupled scalar field inflation models in the metric and Palatini\nformalisms. Working in the Jordan frame, we derive the scalar-tensor general\nfield equations and flat FLRW cosmological equations, and present the Palatini\nand metric equations in a common framework. We show that inflation is\ncharacterized by a \"master\" trajectory from a saddle-type de Sitter fixed point\nto a stable node fixed point, approximated by slow roll conditions (presented\nfor the first time in the Palatini formalism). We show that, despite different\nunderlying equations, the fixed point structure and properties of many models\nare congruent in metric and Palatini, which explains their qualitative\nsimilarities and their suitability for driving inflation. On the other hand,\nthe global phase portraits reveal how even models which predict the same values\nfor observable perturbations differ, both to the extent of the phase space\nphysically available to their trajectories, as well as their past asymptotic\nstates. We also note how the slow roll conditions tend to underestimate the end\nof inflationary accelerated expansion experienced by the true nonlinear\n\"master\" solution. The explicit examples we consider range from the metric and\nPalatini induced gravity quintic potential with a Coleman-Weinberg correction\nfactor to Starobinsky, metric and Palatini nonminimal Higgs, second order pole,\nand several nontrivial Palatini models.", "category": "gr-qc" }, { "text": "Spin fluids in Bianchi-I f(R)-cosmology with torsion: We study Weyssenhoff spin fluids in Bianchi type-I cosmological models,\nwithin the framework of torsional f(R)-gravity; the resulting field equations\nare derived and discussed in both Jordan and Einstein frames, clarifying the\nrole played by the spin and the non-linearity of the gravitational Lagrangian\nf(R) in generating the torsional dynamical contributions. The general\nconservation laws holding for f(R)-gravity with torsion are employed to provide\nthe conditions needed to ensure the preservation of the Hamiltonian constraint\nand the consequent correct formulation of the associated initial value problem.\nExamples are eventually given.", "category": "gr-qc" }, { "text": "A theorem on topologically massive gravity: We show that for three dimensional space-times admitting a hypersurface\northogonal Killing vector field Deser, Jackiw and Templeton's vacuum field\nequations of topologically massive gravity allow only the trivial flat\nspace-time solution. Thus spin is necessary to support topological mass.", "category": "gr-qc" }, { "text": "Higher dimensional thin-shell wormholes in\n Einstein-Yang-Mills-Gauss-Bonnet gravity: We present thin-shell wormhole solutions in Einstein-Yang-Mills-Gauss-Bonnet\n(EYMGB) theory in higher dimensions d\\geq5. Exact black hole solutions are\nemployed for this purpose where the radius of thin-shell lies outside the event\nhorizon. For some reasons the cases d=5 and d>5 are treated separately. The\nsurface energy-momentum of the thin-shell creates surface pressures to resist\nagainst collapse and rendering stable wormholes possible. We test the stability\nof the wormholes against spherical perturbations through a linear\nenergy-pressure relation and plot stability regions. Apart from this restricted\nstability we investigate the possibility of normal (i.e. non-exotic) matter\nwhich satisfies the energy conditions. For negative values of the Gauss-Bonnet\n(GB) parameter we obtain such physical wormholes.", "category": "gr-qc" }, { "text": "Parameter estimation from Gravitational waves generated by non-spinning\n binary black holes with laser interferometers: beyond the Fisher information: In this paper we apply to gravitational waves from non-spinning binary\nsystems a recently intro- duced frequentist methodology to calculate\nanalytically the error for a maximum likelihood estimate (MLE) of physical\nparameters. While existing literature focuses on using the Cramer Rao Lower\nbound (CRLB) and Monte Carlo simulations, we use a power expansion of the bias\nand covariance in inverse powers of the signal to noise ratio. The use of\nhigher order derivatives of the likelihood function in the expansions makes the\nprediction also sensitive to the secondary lobes of the MLE probability\ndistribution. We discuss conditions for validity of the CRLB and predict new\nfeatures in regions of the parameter space currently not explored. For example,\nwe see how the bias can become the most important contributor to the\nparameters' errors for high mass systems (200M and above).", "category": "gr-qc" }, { "text": "Equivalence-principle Analog of the Gravitational Redshift: What happens when two synchronized clocks on a rigid beam are both given the\nexact same acceleration profile? Will they remain synchronized? What if we use\na rigid-rod Rindler acceleration profile? The special relativity prediction\nsurprises many people. This experimental setup is the special-relativity analog\nof the gravitational redshift. Just like two clocks higher and lower in a\ngravitational field lose synchronization, one sees a loss of synchronization in\nthese clocks with `identical' acceleration profiles. To the best of our\nknowledge this equivalence principle analog has never been directly measured,\nand current experimental techniques are sensitive enough to measure it. We\ndiscuss the origin of the essential physics behind this synchronization loss,\nand some special conditions which simplify its experimental observation. We\ndiscuss the origin of the essential physics behind this synchronization loss,\nand some special conditions which simplify its experimental observation. If\nvalidated this effect will not only test the equivalence principle from a new\nvantage, but it may one day aid in understanding and enhancing future\nultra-precise navigation systems.", "category": "gr-qc" }, { "text": "Cosmological models with a Hybrid Scale Factor: In this brief review, we present some cosmological models with a Hybrid Scale\nFactor (HSF) in the framework of general relativity (GR). The hybrid scale\nfactor fosters an early deceleration as well as a late time acceleration and\nmimics the present Universe. The dynamical aspects of different cosmological\nmodels with HSF in the presence of different matter fields have been discussed.", "category": "gr-qc" }, { "text": "Accretion of perfect fluids onto a class of regular black holes: We consider the stationary spherical accretion process of perfect fluids onto\na class of spherically symmetric regular black holes corresponding to\nquantum-corrected Schwarzschild spacetimes. We show that the accretion rates\ncan differ from the Schwarzschild case, suggesting that the de Sitter core\ninside these regular black holes, which indeed precludes the central\nsingularity, can act for some cases as a sort of antigravitational source,\ndecreasing the fluid's radial infall velocity in the accretion process, and for\nothers as a gravitational enhancer, increasing the fluid flow into the black\nhole horizon. Our analysis and results can be extended and also applied to the\nproblem of black hole evaporation in cosmological scenarios with phantom\nfluids. In particular, we show that the mass of typical regular black holes can\nbe used in order to constrain turnaround events in cyclic cosmologies.", "category": "gr-qc" }, { "text": "Finite entanglement entropy from the zero-point-area of spacetime: The calculation of entanglement entropy S of quantum fields in spacetimes\nwith horizon shows that, quite generically, S (a) is proportional to the area A\nof the horizon and (b) is divergent. I argue that this divergence, which arises\neven in the case of Rindler horizon in flat spacetime, is yet another\nindication of a deep connection between horizon thermodynamics and\ngravitational dynamics. In an emergent perspective of gravity, which\naccommodates this connection, the fluctuations around the equipartition value\nin the area elements will lead to a minimal quantum of area, of the order of\nL_P^2, which will act as a regulator for this divergence. In a particular\nprescription for incorporating L_P^2 as zero-point-area of spacetime, this does\nhappen and the divergence in entanglement entropy is regularized, leading to S\nproportional to (A/L_P^2) in Einstein gravity. In more general models of\ngravity, the surface density of microscopic degrees of freedom is different\nwhich leads to a modified regularisation procedure and the possibility that the\nentanglement entropy - when appropriately regularised - matches the Wald\nentropy.", "category": "gr-qc" }, { "text": "A strongly hyperbolic and regular reduction of Einstein's equations for\n axisymmetric spacetimes: This paper is concerned exclusively with axisymmetric spacetimes. We want to\ndevelop reductions of Einstein's equations which are suitable for numerical\nevolutions. We first make a Kaluza-Klein type dimensional reduction followed by\nan ADM reduction on the Lorentzian 3-space, the (2+1)+1 formalism. We include\nalso the Z4 extension of Einstein's equations adapted to this formalism. Our\ngauge choice is based on a generalized harmonic gauge condition. We consider\nvacuum and perfect fluid sources.\n We use these ingredients to construct a strongly hyperbolic first-order\nevolution system and exhibit its characteristic structure. This enables us to\nconstruct constraint-preserving stable outer boundary conditions. We use\ncylindrical polar coordinates and so we provide a careful discussion of the\ncoordinate singularity on axis. By choosing our dependent variables\nappropriately we are able to produce an evolution system in which each and\nevery term is manifestly regular on axis.", "category": "gr-qc" }, { "text": "Asymptotic Behavior in Polarized {\\bf T}$^2$-symmetric Vacuum Spacetimes: We use Fuchsian Reduction to study the behavior near the singularity of a\nclass of solutions of Einstein's vacuum equations. These solutions admit two\ncommuting spacelike Killing fields like the Gowdy spacetimes, but their twist\ndoes not vanish. The spacetimes are also polarized in the sense that one of the\n`gravitational degrees of freedom' is turned off. Examining an analytic family\nof solutions with the maximum number of arbitrary functions, we find that they\nare all asymptotically velocity-term dominated as one approaches the\nsingularity.", "category": "gr-qc" }, { "text": "Impact of the wave-like nature of Proca stars on their\n gravitational-wave emission: We present a systematic study of the dynamics and gravitational-wave emission\nof head-on collisions of spinning vector boson stars, known as Proca stars. To\nthis aim we build a catalogue of about 800 numerical-relativity simulations of\nsuch systems. We find that the wave-like nature of bosonic stars has a large\nimpact on the gravitational-wave emission. In particular, we show that the\ninitial relative phase $\\Delta \\epsilon =\\epsilon_1-\\epsilon_2$ of the two\ncomplex fields forming the stars (or equivalently, the relative phase at\nmerger) strongly impacts both the emitted gravitational-wave energy and the\ncorresponding mode structure. This leads to a non-monotonic dependence of the\nemission on the frequency of the secondary star $\\omega_2$, for fixed frequency\n$\\omega_1$ of the primary. This phenomenology, which has not been found for the\ncase of black-hole mergers, reflects the distinct ability of the Proca field to\ninteract with itself in both constructive and destructive manners. We postulate\nthis may serve as a smoking gun to shed light on the possible existence of\nthese objects.", "category": "gr-qc" }, { "text": "Resonantly enhanced and diminished strong-field gravitational-wave\n fluxes: The inspiral of a stellar mass ($1 - 100\\,M_\\odot$) compact body into a\nmassive ($10^5 - 10^7\\,M_\\odot$) black hole has been a focus of much effort,\nboth for the promise of such systems as astrophysical sources of gravitational\nwaves, and because they are a clean limit of the general relativistic two-body\nproblem. Our understanding of this problem has advanced significantly in recent\nyears, with much progress in modeling the \"self force\" arising from the small\nbody's interaction with its own spacetime deformation. Recent work has shown\nthat this self interaction is especially interesting when the frequencies\nassociated with the orbit's $\\theta$ and $r$ motions are in an integer ratio:\n$\\Omega_\\theta/\\Omega_r = \\beta_\\theta/\\beta_r$, with $\\beta_\\theta$ and\n$\\beta_r$ both integers. In this paper, we show that key aspects of the self\ninteraction for such \"resonant\" orbits can be understood with a relatively\nsimple Teukolsky-equation-based calculation of gravitational-wave fluxes. We\nshow that fluxes from resonant orbits depend on the relative phase of radial\nand angular motions. The purpose of this paper is to illustrate in simple terms\nhow this phase dependence arises using tools that are good for strong-field\norbits, and to present a first study of how strongly the fluxes vary as a\nfunction of this phase and other orbital parameters. Future work will use the\nfull dissipative self force to examine resonant and near resonant strong-field\neffects in greater depth, which will be needed to characterize how a binary\nevolves through orbital resonances.", "category": "gr-qc" }, { "text": "The black hole that went away: A purported black hole solution in (2+1)-dimensions is shown to be nothing\nmore than flat space viewed from an accelerated frame.", "category": "gr-qc" }, { "text": "Incorporating information from LIGO data quality streams into the PyCBC\n search for gravitational waves: We present a new method which accounts for changes in the properties of\ngravitational-wave detector noise over time in the PyCBC search for\ngravitational waves from compact binary coalescences. We use information from\nLIGO data quality streams that monitor the status of each detector and its\nenvironment to model changes in the rate of noise in each detector. These data\nquality streams allow candidates identified in the data during periods of\ndetector malfunctions to be more efficiently rejected as noise. This method\nallows data from machine learning predictions of the detector state to be\nincluded as part of the PyCBC search, increasing the the total number of\ndetectable gravitational-wave signals by up to 5%. When both machine learning\nclassifications and manually-generated flags are used to search data from\nLIGO-Virgo's third observing run, the total number of detectable\ngravitational-wave signals is increased by up to 20% compared to not using any\ndata quality streams. We also show how this method is flexible enough to\ninclude information from large numbers of additional arbitrary data streams\nthat may be able to further increase the sensitivity of the search.", "category": "gr-qc" }, { "text": "An approximate global solution of Einstein's equations for a finite body: We obtain an approximate global stationary and axisymmetric solution of\nEinstein's equations which can be considered as a simple star model: a\nself-gravitating perfect fluid ball with constant mass density rotating in\nrigid motion. Using the post-Minkowskian formalism (weak-field approximation)\nand considering rotation as a perturbation (slow-rotation approximation), we\nfind approximate interior and exterior (asymptotically flat) solutions to this\nproblem in harmonic and quo-harmonic coordinates. In both cases, interior and\nexterior solutions are matched, in the sense of Lichnerowicz, on the surface of\nzero pressure to obtain a global solution. The resulting metric depends on\nthree arbitrary constants: mass density, rotational velocity and the star\nradius at the non-rotation limit. The mass, angular momentum, quadrupole moment\nand other constants of the exterior metric are determined by these three\nparameters. It is easy to show that this type of fluid cannot be a source of\nthe Kerr metric", "category": "gr-qc" }, { "text": "Thermoelastic Noise and Homogeneous Thermal Noise in Finite Sized\n Gravitational-Wave Test Masses: An analysis is given of thermoelastic noise (thermal noise due to\nthermoelastic dissipation) in finite sized test masses of laser interferometer\ngravitational-wave detectors. Finite-size effects increase the thermoelastic\nnoise by a modest amount; for example, for the sapphire test masses tentatively\nplanned for LIGO-II and plausible beam-spot radii, the increase is less than or\nof order 10 per cent. As a side issue, errors are pointed out in the currently\nused formulas for conventional, homogeneous thermal noise (noise associated\nwith dissipation which is homogeneous and described by an imaginary part of the\nYoung's modulus) in finite sized test masses. Correction of these errors\nincreases the homogeneous thermal noise by less than or of order 5 per cent for\nLIGO-II-type configurations.", "category": "gr-qc" }, { "text": "New Signals in Precision Gravity Tests and Beyond: We review the status of tests of spacetime symmetries with gravity. Recent\ntheoretical and experimental work has involved gravitational wave signals,\nprecision solar-system tests, and sensitive laboratory tests searching for\nviolations of spacetime symmetries. We present some new theoretical results\nrelevant for short-range gravity tests, with features of multiple length\nscales, and possible large non-Newtonian forces at short distances.", "category": "gr-qc" }, { "text": "Friedmann Thermodynamics and the Geometry of the Universe: In a recent article we have introduced Friedmann thermodynamics, where\ncertain geometric parameters in Friedmann models are treated like their\nthermodynamic counterparts (temperature, entropy, Gibbs potential etc.). This\nmodel has the advantage of allowing us to determine the geometry of the\nuniverse by thermodynamic stability arguments. In this article we review\nconnections between thermodynamics, geometry and cosmology.", "category": "gr-qc" }, { "text": "On the relation between ADM and Bondi energy-momenta III -- perturbed\n radiative spatial infinity: In a vacuum spacetime equipped with the Bondi's radiating metric which is\nasymptotically flat at spatial infinity including gravitational radiation ({\\bf\nCondition D}), we establish the relation between the ADM total energy-momentum\nand the Bondi energy-momentum for perturbed radiative spatial infinity. The\nperturbation is given by defining the \"real\" time the sum of the retarded time,\nthe Euclidean distance and certain function $f$.", "category": "gr-qc" }, { "text": "Jupiter, Saturn and the Pioneer anomaly: a planetary-based independent\n test: In this paper we use the ratio of the corrections to the standard\nNewtonian/Einsteinian secular precessions of the longitudes of perihelia of\nJupiter and Saturn, recently estimated by the Russian astronomer E.V. Pitjeva\nby fitting almost one century of data with the EPM ephemerides, to make an\nindependent, planetary-based test of the hypothesis that the Pioneer anomaly\n(PA), as it is presently known in the 5-10 AU region, is of gravitational\norigin. Accounting for the errors in the determined apsidal extra-rates and in\nthe values of the PA acceleration at the orbits of Jupiter and Saturn the\nanswer is negative. If and when the re-analysis of the entire Pioneer 10/11\nwill be completed more firm conclusions could be reached. Moreover, it would\nalso be important that other teams of astronomers estimate independently their\nown corrections to the perihelion precessions.", "category": "gr-qc" }, { "text": "Evolution of radial profiles in regular Lemaitre-Tolman-Bondi dust\n models: We undertake a comprehensive and rigorous analytic study of the evolution of\nradial profiles of covariant scalars in regular Lemaitre-Tolman-Bondi dust\nmodels. We consider specifically the phenomenon of \"profile inversions\" in\nwhich an initial clump profile of density, spatial curvature or the expansion\nscalar, might evolve into a void profile (and vice versa). Previous work in the\nliterature on models with density void profiles and/or allowing for density\nprofile inversions is given full generalization, with some erroneous results\ncorrected. We prove rigorously that if an evolution without shell crossings is\nassumed, then only the 'clump to void' inversion can occur in density profiles,\nand only in hyperbolic models or regions with negative spatial curvature. The\nprofiles of spatial curvature follow similar patterns as those of the density,\nwith 'clump to void' inversions only possible for hyperbolic models or regions.\nHowever, profiles of the expansion scalar are less restrictive, with profile\ninversions necessarily taking place in elliptic models. We also examine radial\nprofiles in special LTB configurations: closed elliptic models, models with a\nsimultaneous big bang singularity, as well as a locally collapsing elliptic\nregion surrounded by an expanding hyperbolic background. The general analytic\nstatements that we obtain allow for setting up the right initial conditions to\nconstruct fully regular LTB models with any specific qualitative requirements\nfor the profiles of all scalars and their time evolution. The results presented\ncan be very useful in guiding future numerical work on these models and in\nrevising previous analytic work on all their applications.", "category": "gr-qc" }, { "text": "Is conformal symmetry really anomalous?: The conformal anomaly (also known as the stress-energy trace anomaly) of an\ninteracting quantum theory, associated with violation of Weyl (conformal)\nsymmetry by quantum effects, can be amended if one endows the theory with a\ndilatation current coupled to a vector field that is the gauge connection of\nlocal Weyl symmetry transformations. The natural candidate for this Weyl\nconnection is the trace of the geometric torsion tensor, especially if one\nrecalls that pure (Cartan-Einstein) gravity with torsion is conformal. We first\npoint out that both canonical and path integral quantisation respect Weyl\nsymmetry. The only way quantum effects can violate conformal symmetry is by the\nprocess of regularization. However, if one calculates an effective action from\na conformally invariant classical theory by using a regularisation procedure\nthat is conform with Weyl symmetry, then the conformal Ward identities will be\nsatisfied. In this sense Weyl symmetry is not broken by quantum effects. This\nwork suggests that Weyl symmetry can be treated on equal footing with gauge\nsymmetries and gravity, for which an infinite set of Ward identities guarantees\nthat they remain unbroken by quantum effects.", "category": "gr-qc" }, { "text": "On the energy of the de Sitter-Schwarzschild black hole: Using Einstein's and Weinberg's energy complex, we evaluate the energy\ndistribution of the vaccum nonsingularity black hole solution. The energy\ndistribution is positive everywhere and be equal to zero at origin.", "category": "gr-qc" }, { "text": "An Interacting model of Dark Energy in Brans-Dicke theory: In this paper it is shown that in non-minimally coupled Brans-Dicke theory\ncontaining a self-interacting potential, a suitable conformal transformation\ncan automatically give rise to an interaction between the normal matter and the\nBrans-Dicke scalar field. Considering the scalar field in the Einstein frame as\nthe quintessence matter, it has been shown that such a non-minimal coupling\nbetween the matter and the scalar field can give rise to a late time\naccelerated expansion for the universe preceded by a decelerated expansion for\nvery high values of the Brans-Dicke parameter $\\omega$. We have also studied\nthe observational constraints on the model parameters considering the Hubble\nand Supernova data.", "category": "gr-qc" }, { "text": "The Quantum Echo of the Early Universe: We show that the fluctuations of quantum fields as seen by late comoving\nobservers are significantly influenced by the history of the early Universe,\nand therefore they transmit information about the nature of spacetime in\ntimescales when quantum gravitational effects were non-negligible. We discuss\nhow this may be observable even nowadays, and thus used to build falsifiability\ntests of quantum gravity theories.", "category": "gr-qc" }, { "text": "Cosmological Models in Lyra Geometry: Kinematics Tests: In this paper the observational consequence of the cosmological models and\nthe expression for the neoclassical tests, luminosity distance, angular\ndiameter distance and look back time are analyzed in the framework of Lyra\ngeometry. It is interesting to note that the space time of the universe is not\nonly free of Big Bang singularity but also exhibits acceleration during its\nevolution.", "category": "gr-qc" }, { "text": "Black hole induced spins from hyperbolic encounters in dense clusters: The black holes that have been detected via gravitational waves (GW) can have\neither astrophysical or primordial origin. Some GW events show significant spin\nfor one of the components and have been assumed to be astrophysical, since\nprimordial black holes are generated with very low spins. However, it is worth\nstudying if they can increase their spin throughout the evolution of the\nuniverse. Possible mechanisms that have already been explored are multiple\nblack hole mergers and gas accretion. We propose here a new mechanism that can\noccur in dense clusters of black holes: the spin-up of primordial black holes\nwhen they are involved in close hyperbolic encounters. We explore this effect\nnumerically with the Einstein Toolkit for different initial conditions,\nincluding variable mass ratios. For equal masses, there is a maximum spin that\ncan be induced on the black holes, $\\chi = a/m \\leq 0.2$. We find however that\nfor large mass ratios one can attain spins up to $\\chi \\simeq 0.8$, where the\nhighest spin is induced on the most massive black hole. For small induced spins\nwe provide simple analytical expressions that depend on the relative velocity\nand impact parameter.", "category": "gr-qc" }, { "text": "No Regularity Singularities Exist at Points of General Relativistic\n Shock Wave Interaction between Shocks from Different Characteristic Families: We give a constructive proof that coordinate transformations exist which\nraise the regularity of the gravitational metric tensor from $C^{0,1}$ to\n$C^{1,1}$ in a neighborhood of points of shock wave collision in General\nRelativity. The proof applies to collisions between shock waves coming from\ndifferent characteristic families, in spherically symmetric spacetimes. Our\nresult here implies that spacetime is locally inertial and corrects an error in\nour earlier RSPA-publication, which led us to the false conclusion that such\ncoordinate transformations, which smooth the metric to $C^{1,1}$, cannot exist.\nThus, our result implies that regularity singularities, (a type of mild\nsingularity introduced in our RSPA-paper), do not exist at points of\ninteracting shock waves from different families in spherically symmetric\nspacetimes. Our result generalizes Israel's celebrated 1966 paper to the case\nof such shock wave interactions but our proof strategy differs fundamentally\nfrom that used by Israel and is an extension of the strategy outlined in our\noriginal RSPA-publication. Whether regularity singularities exist in more\ncomplicated shock wave solutions of the Einstein Euler equations remains open.", "category": "gr-qc" }, { "text": "A Physical Interpretation of Gravitational Field Equations: It is possible to provide a thermodynamic interpretation for the field\nequations in any diffeomorphism invariant theory of gravity. This insight, in\nturn, leads us to the possibility of deriving the gravitational field equations\nfrom another variational principle without using the metric as a dynamical\nvariable. I review this approach and discuss its implications.", "category": "gr-qc" }, { "text": "Comments on absorption cross section for Chern-Simons black holes in\n five dimensions: In this paper we study the effects of black hole mass on the absorption cross\nsection for a massive scalar field propagating in a 5-dimensional topological\nChern-Simons black hole at the low-frequency limit. We consider the two\nbranches of black hole solutions $(\\alpha=\\pm 1)$ and we show that, if the mass\nof black hole increase the absorption cross section decreases at the\nzero-frequency limit for the branch $\\alpha=-1$ and for the other branch,\n$\\alpha=1$, the behavior is opposite, if the black hole mass increase the\nabsorption cross section increases. Also we find that beyond a certain\nfrequency value, the mass black hole does not affect the absorption cross\nsection.", "category": "gr-qc" }, { "text": "Supersymmetric double Darboux method in quantum cosmology: We briefly present the supersymmetric double Darboux method and next apply it\nto the continuum of the quantum Taub cosmological model as a toy model in order\nto generate a one-parameter family of bosonic Taub potentials and the\ncorresponding wavefunctions", "category": "gr-qc" }, { "text": "Dirac particle in gravitation field: Being considered is the motion of Dirac particle in gravitational field,\ndescribed by Kerr solution. It is proved, that evolution of the wave function\nis determined by Hermitian Hamiltonian, if the concomitant reference frame is\ninvolved.", "category": "gr-qc" }, { "text": "The influence of differential rotation on the detectability of\n gravitational waves from the r-mode instability: Recently, it was shown that differential rotation is an unavoidable feature\nof nonlinear r-modes. We investigate the influence of this differential\nrotation on the detectability of gravitational waves emitted by a newly born,\nhot, rapidly-rotating neutron star, as it spins down due to the r-mode\ninstability. We conclude that gravitational radiation may be detected by the\nadvanced laser interferometer detector LIGO if the amount of differential\nrotation at the time the r-mode instability becomes active is not very high.", "category": "gr-qc" }, { "text": "Geodesic Congruences and a Collapsing Stellar Distribution in f (T )\n Theories: Teleparallel Gravity (TG) describes gravitation as a torsional- rather than\ncurvature-based effect. As in curvature-based constructions of gravity, several\ndifferent formulations can be proposed, one of which is the Teleparallel\nequivalent of General Relativity (TEGR) which is dynamically equivalent to GR.\nIn this work, we explore the evolution of a spatially homogeneous collapsing\nstellar body in the context of two important modifications to TEGR, namely f\n(T) gravity which is the TG analogue of f (R) gravity, and a nonminimal\ncoupling with a scalar field which has become popular in TG for its effects in\ncosmology. We explore the role of geodesic deviation to study the congruence of\nnearby particles in lieu of the Raychaudhuri equation. We find f (T) models\nthat satisfy the null energy condition and describe interesting collapse\nprofiles. In the case of a nonminimally coupled scalar field, we also find\npotential collapse models with intriguing scalar field evolution profiles.", "category": "gr-qc" }, { "text": "Curvature invariants of static spherically symmetric geometries: We construct all independent local scalar monomials in the Riemann tensor at\narbitrary dimension, for the special regime of static, spherically symmetric\ngeometries. Compared to general spaces, their number is significantly reduced:\nthe extreme example is the collapse of all invariants ~ Weyl^k, to a single\nterm at each k. The latter is equivalent to the Lovelock invariant L_k.\nDepopulation is less extreme for invariants involving rising numbers of Ricci\ntensors, and also depends on the dimension. The corresponding local\ngravitational actions and their solution spaces are discussed.", "category": "gr-qc" }, { "text": "Chameleon scalar fields in relativistic gravitational backgrounds: We study the field profile of a scalar field $\\phi$ that couples to a matter\nfluid (dubbed a chameleon field) in the relativistic gravitational background\nof a spherically symmetric spacetime. Employing a linear expansion in terms of\nthe gravitational potential $\\Phi_c$ at the surface of a compact object with a\nconstant density, we derive the thin-shell field profile both inside and\noutside the object, as well as the resulting effective coupling with matter,\nanalytically. We also carry out numerical simulations for the class of inverse\npower-law potentials $V(\\phi)=M^{4+n} \\phi^{-n}$ by employing the information\nprovided by our analytical solutions to set the boundary conditions around the\ncentre of the object and show that thin-shell solutions in fact exist if the\ngravitational potential $\\Phi_c$ is smaller than 0.3, which marginally covers\nthe case of neutron stars. Thus the chameleon mechanism is present in the\nrelativistic gravitational backgrounds, capable of reducing the effective\ncoupling. Since thin-shell solutions are sensitive to the choice of boundary\nconditions, our analytic field profile is very helpful to provide appropriate\nboundary conditions for $\\Phi_c \\lesssim O(0.1)$.", "category": "gr-qc" }, { "text": "Black holes in nonlinear electrodynamics: quasi-normal spectra and\n parity splitting: We discuss the quasi-normal oscillations of black holes which are sourced by\na nonlinear electrodynamic field. While previous studies have focused on the\ncomputation of quasi-normal frequencies for the wave or higher spin equation on\na fixed background geometry described by such black holes, here we compute for\nthe first time the quasi-normal frequencies for the coupled\nelectromagnetic-gravitational linear perturbations.\n To this purpose, we consider a parametrized family of Lagrangians for the\nelectromagnetic field which contains the Maxwell Lagrangian as a special case.\nIn the Maxwell case, the unique spherically symmetric black hole solutions are\ndescribed by the Reissner-Nordstr\\\"om family and in this case it is well-known\nthat the quasi-normal spectra in the even- and odd-parity sectors are identical\nto each other. However, when moving away from the Maxwell case, we obtain\ndeformed Reissner-Nordstr\\\"om black holes, and we show that in this case there\nis a parity splitting in the quasi-normal mode spectra. A partial explanation\nfor this phenomena is provided by considering the eikonal (high-frequency)\nlimit.", "category": "gr-qc" }, { "text": "Dynamical Evolution of a Cylindrical Shell with Rotational Pressure: We prepare a general framework for analyzing the dynamics of a cylindrical\nshell in the spacetime with cylindrical symmetry. Based on the framework, we\ninvestigate a particular model of a cylindrical shell-collapse with rotational\npressure, accompanying the radiation of gravitational waves and massless\nparticles. The model has been introduced previously but has been awaiting for\nproper analysis. Here the analysis is put forward: It is proved that, as far as\nthe weak energy condition is satisfied outside the shell, the collapsing shell\nbounces back at some point irrespective of the initial conditions, and escapes\nfrom the singularity formation.\n The behavior after the bounce depends on the sign of the shell pressure in\nthe z-direction. When the pressure is non-negative, the shell continues to\nexpand without re-contraction. On the other hand, when the pressure is negative\n(i.e. it has a tension), the behavior after the bounce can be more complicated\ndepending on the details of the model. However, even in this case, the shell\nnever reaches the zero-radius configuration.", "category": "gr-qc" }, { "text": "The running vacuum in effective quantum gravity: We briefly review the previous works on the renormalization group in quantum\ngeneral relativity with the cosmological constant, based on the Vilkovisky and\nDeWitt version of effective action. On top of that, we discuss the prospects of\nthe applications of this version of renormalization group to the cosmological\nmodels with a running Newton constant and vacuum energy density.", "category": "gr-qc" }, { "text": "Gravitational waves in scalar-tensor theory to one-and-a-half\n post-Newtonian order: We compute the gravitational waves generated by compact binary systems in a\nclass of massless scalar-tensor (ST) theories to the 1.5 post-Newtonian (1.5PN)\norder beyond the standard quadrupole radiation in general relativity (GR).\nUsing and adapting to ST theories the multipolar-post-Minkowskian and\npost-Newtonian formalisms originally defined in GR, we obtain the tail and\nnon-linear memory terms associated with the dipole radiation in ST theory. The\nmultipole moments and GW flux of compact binaries are derived for general\norbits including the new 1.5PN contribution, and comparison is made with\nprevious results in the literature. In the case of quasi-circular orbits, we\npresent ready-to-use templates for the data analysis of detectors, and for the\nfirst time the scalar GW modes for comparisons with numerical relativity\nresults.", "category": "gr-qc" }, { "text": "On the existence of topological dyons and dyonic black holes in anti-de\n Sitter Einstein-Yang-Mills theories with compact semisimple gauge groups: Here we study the global existence of `hairy' dyonic black hole and dyon\nsolutions to four dimensional, anti-de Sitter Einstein-Yang-Mills theories for\na general simply-connected and semisimple gauge group $G$, for so-called\ntopologically symmetric systems, concentrating here on the regular case. We\ngeneralise here cases in the literature which considered purely magnetic\nspherically symmetric solutions for a general gauge group and topological\ndyonic solutions for $\\mathfrak{su}(N)$. We are able to establish the global\nexistence of non-trivial solutions to all such systems, both near existing\nembedded solutions and as $|\\Lambda|\\rightarrow\\infty$. In particular, we can\nidentify non-trivial solutions where the gauge field functions have no zeroes,\nwhich in the $\\mathfrak{su}(N)$ case proved important to stability. We believe\nthat these are the most general analytically proven solutions in 4D anti-de\nSitter Einstein-Yang-Mills systems to date.", "category": "gr-qc" }, { "text": "Entanglement partners and monogamy in de Sitter universes: We investigate entanglement of local spatial modes defined by a quantum field\nin a de Sitter universe. The introduced modes show dis-entanglement behavior\nwhen the separation between two regions where local modes are assigned becomes\nlarger than the cosmological horizon. To understand the emergence of\nseparability between these local modes, we apply the monogamy inequality\nproposed by S. Camalet. We embed the focusing bipartite mode defined by the\nquantum field in a pure four-mode Gaussian state, and identify its partner\nmodes. Then applying a Gaussian version of the monogamy relation, we show that\nthe external entanglement between the bipartite mode and its partner modes\nconstrains the entanglement of the bipartite mode. Thus the emergence of\nseparability of local modes in the de Sitter universe can be understood from\nthe perspective of entanglement monogamy.", "category": "gr-qc" }, { "text": "Dark Energy from Gauss-Bonnet and non-minimal couplings: We consider a scalar-tensor model of dark energy with Gauss-Bonnet and\nnon-minimal couplings. Exact cosmological solutions were found in absence of\npotential, that give equations of state of dark energy consistent with current\nobservational constraints, but with different asymptotic behaviors depending on\nthe couplings of the model. A detailed reconstruction procedure is given for\nthe scalar potential and the Gauss-Bonnet coupling for any given cosmological\nscenario. Particularly, we consider conditions for the existence of a variety\nof cosmological solutions with accelerated expansion, including quintessence,\nphantom, de Sitter, Little Rip. For the case of quintessence and phantom we\nhave found a scalar potential of the Albrecht-Skordis type, where the potential\nis an exponential with a polynomial factor.", "category": "gr-qc" }, { "text": "Observational Exclusion of a Consistent Quantum Cosmology Scenario: It is often argued that inflation erases all the information about what took\nplace before it started. Quantum gravity, relevant in the Planck era, seems\ntherefore mostly impossible to probe with cosmological observations. In\ngeneral, only very ad hoc scenarios or hyper fine-tuned initial conditions can\nlead to observationally testable theories. Here we consider a well-defined and\nwell motivated candidate quantum cosmology model that predicts inflation. Using\nthe most recent observational constraints on the cosmic microwave background B\nmodes, we show that the model is excluded for all its parameter space, without\nany tuning. Some important consequences are drawn for the deformed algebra\napproach to loop quantum cosmology. We emphasize that neither loop quantum\ncosmology in general nor loop quantum gravity are disfavored by this study but\ntheir falsifiability is established.", "category": "gr-qc" }, { "text": "Constraining Barrow entropy-based Cosmology with power-law inflation: We study the inflationary era of the Universe in a modified cosmological\nscenario based on the gravity-thermodynamics conjecture with Barrow entropy\ninstead of the usual Bekenstein-Hawking one. The former arises from the effort\nto account for quantum gravitational effects on the horizon surface of black\nholes and, in a broader sense, of the Universe. First, we extract modified\nFriedmann equations from the first law of thermodynamics applied to the\napparent horizon of a Friedmann- Robertson-Walker Universe in (n +\n1)-dimensions. Assuming a power-law behavior for the scalar inflaton field, we\nthen investigate how the inflationary dynamics is affected in Barrow\ncosmological setup. We find that the inflationary era may phenomenologically\nconsist of the slow-roll phase, while Barrow entropy is incompatible with\nkinetic inflation. By demanding observationally consistency of the scalar\nspectral index and tensor-to-scalar ratio with recent Planck data, we finally\nconstrain Barrow exponent to $\\Delta\\lesssim10^{-4}$.", "category": "gr-qc" }, { "text": "Spherically-symmetric gravitational fields in the metric-affine gauge\n theory of gravitation: Geometric structure of spherically-symmetric space-time in metric-affine\ngauge theory of gravity is studied. Restrictions on curvature tensor and\nBianchi identities are obtained. By using certain simple gravitational\nLagrangian the solution of gravitational equations for vacuum\nspherically-symmetric gravitational field is obtained.", "category": "gr-qc" }, { "text": "Bounce inflation with a conserved frame of rest: Some form of approximately exponential inflation is generally assumed to be\nthe origin of our present universe. The inflation is thought to be driven by a\nscalar field potential where the field first slowly slides along the potential\nand then comes to a steep slope where the field rapidly falls and then\noscillates around zero transforming into particles. The slowly sliding scalar\nfield inflation leads to an exponentially expanding de Sitter space. A scalar\nfield as well as the deSitter space are both Lorentz invariant. Thus no global\nframe of rest can be established in this scenario, while particle creation\nrequires a preferred frame of rest. Observations of the cosmic microwave\nbackground show, when the redshift is corrected for our local velocity, a very\neven temperature and redshift distribution requiring a global preferred frame\nof rest. We suggest here that a density dependent equilibrium relation between\nmatter/radiation and a scalar energy density could maintain a preferred frame\nof rest throughout the bounce and inflation and thereby solve the problem.", "category": "gr-qc" }, { "text": "Static spherically symmetric solutions in New General Relativity: We give a pedagogical introduction to static spherically symmetric solutions\nin models of New GR, both explaining the basics and showing how all such vacuum\nsolutions can be obtained in elementary functions. In doing so, we coherently\nintroduce the full landscape of these modified teleparallel spacetimes, and\nfind a few special cases. The equations of motion are turned into a very nice\nand compact form by using the Levi-Civita divergence of the torsion-conjugate;\nand generalised Bianchi identities are briefly discussed. Another important\npoint we make is that a convenient choice of the radial variable might be\ninstrumental for success of similar studies in other modified gravity models.", "category": "gr-qc" }, { "text": "4D spin-2 fields from 5D Chern-Simons theory: We consider a 5-dimensional Chern-Simons gauge theory for the isometry group\nof Anti-de-Sitter spacetime,\n$\\operatorname{AdS}_{4+1}\\simeq\\operatorname{SO}(4,2)$, and invoke different\ndimensional reduction schemes in order to relate it to 4-dimensional spin-2\ntheories. The AdS gauge algebra is isomorphic to a parametrized 4-dimensional\nconformal algebra, and the gauge fields corresponding to the generators of\nnon-Abelian translations and special conformal transformations reduce to two\nvierbein fields in $D=4$. Besides these two vierbeine, our reduction schemes\nleave only the Lorentz spin connection as an additional dynamical field in the\n4-dimensional theories. We identify the corresponding actions as particular\ngeneralizations of Einstein-Cartan theory, conformal gravity and ghost-free\nbimetric gravity in first-order form.", "category": "gr-qc" }, { "text": "Euclidean and Hamiltonian thermodynamics for regular black holes: We investigate the thermodynamic properties of the Hayward regular black hole\nusing both Euclidean path integral and Hamiltonian methods, in asymptotically\nanti-de Sitter, Minkowski, and de Sitter spacetimes. With the inclusion of\nmatter fields which act as a source for the regular black hole geometry, an\neffective temperature emerges that differs from the conventional definition\nrelated to the Killing surface gravity. We posit that this temperature is the\nappropriate choice for studying thermodynamic phenomena, by demonstrating\nconsistency between the Euclidean and Hamiltonian formulations in the\nappropriate limits. We examine the thermodynamic properties and phase structure\nof the Hayward black hole in the canonical ensemble and show that, counter to\nsome earlier indications, standard mean-field theory critical behavior is\nobserved when the cosmological constant is treated as a thermodynamic pressure.\nWe note the absence of a Hawking-Page transition, and conjecture that quantum\ngravity corrections which are suitably strong to regulate the Schwarzschild\nsingularity generically prevent the transition from occurring. We also show\nthat the Smarr relation remains linear in all cases, despite the absence of a\nlinearity proof for nonlinear electrodynamic theories with nonsymmetry\ninheriting fields.", "category": "gr-qc" }, { "text": "Thermodynamics of Spherically Symmetric Spacetimes in Loop Quantum\n Gravity: The choice of the area operator in loop quantum gravity is by no means\nunique. In addition to the area operator commonly used in loop quantum gravity\nthere is also an area operator introduced by Krasnov in 1998, which gives\nuniformly spaced area spectra for the horizons of spacetime. Using Krasnov's\narea operator we consider the thermodynamics of spherically symmetric\nspacetimes equipped with horizons in loop quantum gravity. Among other things,\nour approach implies, in a pretty simple manner, that every horizon of\nspacetime emits thermal radiation and possesses emtropy which, in the natural\nunits, is one-quarter of its area. When applied to the de Sitter spacetime loop\nquantum gravity provides an explanation both to the presence and the smallness\nof the cosmological constant.", "category": "gr-qc" }, { "text": "Quantized Intrinsic Redshift in Cosmological General Relativity: There are now several analyses reporting quantized differences in the\nredshifts between pairs of galaxies. In the simplest cases, these differential\nredshifts are found to be harmonics of fundamental periods of approximately 72\nkm/s and 37.5 km/s. In this paper a wave equation is derived based on\ncosmological general relativity, which is a space-velocity theory of the\nexpanding Universe. The wave equation is approximated to first order and\ncomparisons are made between the quantized solutions and the reported\nobservations.", "category": "gr-qc" }, { "text": "Features of gravity-Yang-Mills hierarchies in d-dimensions: Higher dimensional, direct analogues of the usual d=4 Einstein--Yang-Mills\n(EYM) systems are studied. These consist of the gravitational and Yang-Mills\nhierarchies in d=4p dimensional spacetimes, both consisting of 2p-form\ncurvature terms only. Regular and black hole solutions are constructed in\n$2p+2\\le d \\le 4p$, in which dimensions the total mass-energy is finite,\ngeneralising the familiar Bartnik-McKinnon solutions in EYM theory for p=1. In\nd=4p, this similarity is complete. In the special case of d=2p+1, just beyond\nthe finite energy range of d, exact solutions in closed form are found.\nFinally, d=2p+1 purely gravitational systems, whose solutions generalise the\nstatic d=3 BTZ solutions, are discussed.", "category": "gr-qc" }, { "text": "Is the Strong Anthropic Principle Too Weak?: We discuss the Carter's formula about the mankind evolution probability\nfollowing the derivation proposed by Barrow and Tipler. We stress the relation\nbetween the existence of billions of galaxies and the evolution of at least one\nintelligent life, whose living time is not trivial, all over the Universe. We\nshow that the existence probability and the lifetime of a civilization depend\nnot only on the evolutionary critical steps, but also on the number of places\nwhere the life can arise. In the light of these results, we propose a stronger\nversion of Anthropic Principle.", "category": "gr-qc" }, { "text": "A clarification on a common misconception about interferometric\n detectors of gravitational waves: The aims of this letter are two. First, to show the angular gauge-invariance\non the response of interferometers to gravitational waves (GWs). In this\nprocess, after resuming for completeness results on the Transverse-Traceless\n(TT) gauge, where, in general, the theoretical computations on GWs are\nperformed, we analyse the gauge of the local observer, which represents the\ngauge of a laboratory environment on Earth. The gauge-invariance between the\ntwo gauges is shown in its full angular and frequency dependences. In previous\nworks in the literature this gauge-invariance was shown only in the low\nfrequencies approximation or in the simplest geometry of the interferometer\nwith respect to the propagating GW (i.e. both of the arms of the interferometer\nare perpendicular to the propagating GW). Second, as far as the computation of\nthe response functions in the gauge of the local observer is concerned, a\ncommon misconception about interferometers is also clarified. Such a\nmisconception purports that, as the wavelength of laser light and the length of\nan interferometer's arm are both stretched by a GW, no effect should be\nvisible, invoking an analogy with cosmological redshift in an expanding\nuniverse.", "category": "gr-qc" }, { "text": "Mimetic Compact Stars: Modified gravity models have been constantly proposed with the purpose of\nevading some standard gravity shortcomings. Recently proposed by A.H.\nChamseddine and V. Mukhanov, the Mimetic Gravity arises as an optimistic\nalternative. Our purpose in this work is to derive Tolman-Oppenheimer-Volkoff\nequations and solutions for such a gravity theory. We solve them numerically\nfor quark star and neutron star cases. The results are carefully discussed.", "category": "gr-qc" }, { "text": "On the possible sources of gravitational wave bursts detectable today: We discuss the possibility that galactic gravitational wave sources might\ngive burst signals at a rate of several events per year, detectable by\nstate-of-the-art detectors. We are stimulated by the results of the data\ncollected by the EXPLORER and NAUTILUS bar detectors in the 2001 run, which\nsuggest an excess of coincidences between the two detectors, when the resonant\nbars are orthogonal to the galactic plane. Signals due to the coalescence of\ngalactic compact binaries fulfill the energy requirements but are problematic\nfor lack of known candidates with the necessary merging rate. We examine the\nlimits imposed by galactic dynamics on the mass loss of the Galaxy due to GW\nemission, and we use them to put constraints also on the GW radiation from\nexotic objects, like binaries made of primordial black holes. We discuss the\npossibility that the events are due to GW bursts coming repeatedly from a\nsingle or a few compact sources. We examine different possible realizations of\nthis idea, such as accreting neutron stars, strange quark stars, and the highly\nmagnetized neutron stars (``magnetars'') introduced to explain Soft Gamma\nRepeaters. Various possibilities are excluded or appear very unlikely, while\nothers at present cannot be excluded.", "category": "gr-qc" }, { "text": "General relativistic hydrodynamics in curvilinear coordinates: In this paper we report on what we believe is the first successful\nimplementation of relativistic hydrodynamics, coupled to dynamical spacetimes,\nin spherical polar coordinates without symmetry assumptions. We employ a\nhigh-resolution shock-capturing scheme, which requires that the equations be\ncast in flux-conservative form. One example of such a form is the :Valencia\"\nformulation, which has been adopted in numerous applications, in particular in\nCartesian coordinates. Here we generalize this formulation to allow for a\nreference-metric approach, which provides a natural framework for calculations\nin curvilinear coordinates. In spherical polar coordinates, for example, it\nallows for an analytical treatment of the singular r and sin(\\theta) terms that\nappear in the equations. We experiment with different versions of our\ngeneralized Valencia formulation in numerical implementations of relativistic\nhydrodynamics for both fixed and dynamical spacetimes. We consider a number of\ndifferent tests -- non-rotating and rotating relativistic stars, as well as\ngravitational collapse to a black hole -- to demonstrate that our formulation\nprovides a promising approach to performing fully relativistic astrophysics\nsimulations in spherical polar coordinates.", "category": "gr-qc" }, { "text": "Seeing through the cosmological bounce: Footprints of the contracting\n phase and luminosity distance in bouncing models: The evolution of the luminosity distance in a contracting universe is\nstudied. It is shown that for quite a lot of natural dynamical evolutions, its\nbehavior is far from trivial and its value can even decrease with an increasing\ntime interval between events. The consequences are investigated and it is\nunderlined that this could both put stringent consistency conditions on\nbouncing models and open a new observational window on \"pre Big Bang\" physics\nusing standard gravitational waves.", "category": "gr-qc" }, { "text": "Diamond-Shaped Regions as Microcosmoi: We give a geometrically intrinsic construction of a global time function for\nrelatively compact diamond-shaped regions in arbitrary spacetimes. In the case\nof Minkowski spacetime, the flow of diffeomorphisms associated to a suitably\nnormalized gradient of this time function becomes the conformal isotropy\nsubgroup of the diamond. In full generality, this time function is elegantly\nexpressed in terms of the Lorentzian distance function, and it has an\nasymptotic behavior at large absolute times similar to the one in Minkowski\nspacetime.", "category": "gr-qc" }, { "text": "Universe acceleration and nonlinear electrodynamics: A new model of nonlinear electrodynamics with a dimensional parameter $\\beta$\ncoupled to gravity is considered. We show that an accelerated expansion of the\nuniverse takes place if the nonlinear electromagnetic field is the source of\nthe gravitational field. A pure magnetic universe is investigated and the\nmagnetic field drives the universe to accelerate. In this model, after the big\nbang, the universe undergoes inflation, and the accelerated expansion and then\ndecelerates approaching Minkowski spacetime asymptotically. We demonstrate the\ncausality of the model and a classical stability at the deceleration phase.", "category": "gr-qc" }, { "text": "Observational constraints on varying fundamental constants in a minimal\n CPC model: A minimal model based on the Co-varying Physical Couplings (CPC) framework\nfor gravity is proposed. The CPC framework is based on the assumptions of a\nmetric-compatible four-dimensional Riemannian manifold where a covariantly\nconserved stress-energy tensor acts as source of the field equations which are\nformally the same as Einstein field equations, but where the couplings $\\{ G,\nc,\\Lambda \\}$ are allowed to vary simultaneously. The minimal CPC model takes\n$\\Lambda$ as a genuine constant while $c$ and $G$ vary in an entangled way that\nis consistent with Bianchi identity and the aforementioned assumptions. The\nmodel is constrained using the most recent galaxy cluster gas mass fraction\nobservational data. Our result indicates that the functions $c(z)$ and\n$G\\left(z\\right)=G_{0}\\left(c/c_{0}\\right)^{4}$ are compatible with constant\ncouplings for the three different parameterizations of $c=c(z)$ adopted here.", "category": "gr-qc" }, { "text": "Locally Anisotropic Black Holes in Einstein Gravity: By applying the method of moving frames modelling one and two dimensional\nlocal anisotropies we construct new solutions of Einstein equations on\npseudo-Riemannian spacetimes. The first class of solutions describes\nnon-trivial deformations of static spherically symmetric black holes to locally\nanisotropic ones which have elliptic (in three dimensions) and ellipsoidal,\ntoroidal and elliptic and another forms of cylinder symmetries (in four\ndimensions). The second class consists from black holes with oscillating\nelliptic horizons.", "category": "gr-qc" }, { "text": "Double Images from a Single Black Hole: In the simulations of the multi-black holes and merging black holes a larger\nprimary image and a secondary smaller image which looks like an eyebrow and the\ndeformation of the shadows have been observed. However, this kind of\neyebrow-like structure was considered as unique feature of multi black hole\nsystems. In this paper, we illustrate the new result that in the case of\noctupole distortions of a Schwarzschild black hole the local observer sees two\nshadows or two images for this single black hole, i.e., also an eyebrow-like\nstructure. Presence of two images in our case is remarkable, as we have only\none black hole, however, the observer sees two dark images of this single black\nhole.", "category": "gr-qc" }, { "text": "Imprints of dark matter on gravitational ringing of supermassive black\n holes: Gravitational waves emitted from the gravitational ringing of supermassive\nblack holes are important targets to test general relativity and probe the\nmatter environment surrounding such black holes. The main components of the\nringing waveform are black hole quasi-normal modes. In this paper, we study the\neffects of the dark matter halos with three different density profiles on the\ngravitational polar (even-parity) perturbations of a supermassive black hole.\nFor this purpose, we first consider modified Schwarzschild spacetime with three\ndifferent dark matter profiles and derive the equation of motion of the polar\nperturbations of the supermassive black hole. It is shown that by ignoring the\ndark matter perturbations, a Zerilli-like master equation with a modified\npotential for the polar perturbation can be obtained explicitly. Then we\ncalculate the complex frequencies of the quasi-normal modes of the supermassive\nblack hole in the dark matter halos. The corresponding gravitational wave\nspectra with the effects of the dark matter halos and their detectability have\nalso been discussed.", "category": "gr-qc" }, { "text": "Excision boundary conditions for the conformal metric: Shibata, Ury\\=u and Friedman recently suggested a new decomposition of\nEinstein's equations that is useful for constructing initial data. In contrast\nto previous decompositions, the conformal metric is no longer treated as a\nfreely-specifiable variable, but rather is determined as a solution to the\nfield equations. The new set of freely-specifiable variables includes only\ntime-derivatives of metric quantities, which makes this decomposition very\nattractive for the construction of quasiequilibrium solutions. To date, this\nnew formalism has only been used for binary neutron stars. Applications\ninvolving black holes require new boundary conditions for the conformal metric\non the domain boundaries. In this paper we demonstrate how these boundary\nconditions follow naturally from the conformal geometry of the boundary\nsurfaces and the inherent gauge freedom of the conformal metric.", "category": "gr-qc" }, { "text": "Study of Embedded Class-I Fluid Spheres in $f(R,T)$ Gravity with\n Karmarkar Condition: In this article, we explore some emerging properties of the stellar objects\nin the frame of the $f(R,T)$ gravity by employing the well-known Karmarkar\ncondition, where $R$ and $T$ represent Ricci scalar and trace of energy\nmomentum tensor respectively. It is worthy to highlight here that we assume the\nexponential type model of $f(R,T)$ theory of gravity $f(R,T)=R+\\alpha(e^{-\\beta\nR}-1)+\\gamma T$ along with the matter Lagrangian\n$\\mathcal{L}_{m}=-\\frac{1}{3}(p_{r}+2 p_{t})$ to classify the complete set of\nmodified field equations. We demonstrate the embedded class-I technique by\nusing the static spherically symmetric line element along with anisotropic\nfluid matter distribution. Further, to achieve our goal, we consider a specific\nexpression of metric potential $g_{rr}$, already presented in literature, and\nproceed by using the Karmarkar condition to obtain the second metric potential.\nIn particular, we use four different compact stars, namely $LMC~X-4,$\n$EXO~1785-248,$ $Cen~X-3$ and $4U~1820-30$ and compute the corresponding values\nof the unknown parameters appearing in metric potentials. Moreover, we conduct\nvarious physical evolutions such as graphical nature of energy density and\npressure progression, energy constraints, mass function, adiabatic index,\nstability and equilibrium conditions to ensure the viability and consistency of\nour proposed model. Our analysis indicates that the obtained anisotropic\noutcomes are physically acceptable with the finest degree of accuracy.", "category": "gr-qc" }, { "text": "Cosmological attractor inflation from the RG-improved Higgs sector of\n finite gauge theory: The possibility to construct an inflationary scenario for\nrenormalization-group improved potentials corresponding to the Higgs sector of\nfinite gauge models is investigated. Taking into account quantum corrections to\nthe renormalization-group potential which sums all leading logs of perturbation\ntheory is essential for a successful realization of the inflationary scenario,\nwith very reasonable parameter values. The inflationary models thus obtained\nare seen to be in good agreement with the most recent and accurate\nobservational data. More specifically, the values of the relevant inflationary\nparameters, $n_s$ and $r$, are close to the corresponding ones in the $R^2$ and\nHiggs-driven inflation scenarios. It is shown that the model here constructed\nand Higgs-driven inflation belong to the same class of cosmological attractors.", "category": "gr-qc" }, { "text": "Higher-dimensional black holes with multiple equal rotations: We study a limit of the Kerr-(A)dS spacetime in a general dimension where an\narbitrary number of its rotational parameters is set equal. The resulting\nmetric after the limit formally splits into two parts - the first part has the\nform of the Kerr-NUT-(A)dS metric analogous to the metric of the entire\nspacetime, but only for the directions not subject to the limit, and the second\npart can be interpreted as the K\\\"{a}hler metrics. However, this separation is\nnot integrable, thus it does not lead to a product of independent manifolds. We\nalso reconstruct the original number of explicit and hidden symmetries\nassociated with Killing vectors and Killing tensors. Therefore, the resulting\nspacetime represents a special subcase of the generalized Kerr-NUT-(A)dS metric\nthat retains the full Killing tower of symmetries. In $D=6$, we present\nevidence of an enhanced symmetry structure after the limit. Namely, we find\nadditional Killing vectors and show that one of the Killing tensors becomes\nreducible as it can be decomposed into Killing vectors.", "category": "gr-qc" }, { "text": "Conditions for negative specific heat in systems of attracting classical\n particles: We identify conditions for the presence of negative specific heat in\nnon-relativistic self-gravitating systems and similar systems of attracting\nparticles.\n The method used, is to analyse the Virial theorem and two soluble models of\nsystems of attracting particles, and to map the sign of the specific heat for\ndifferent combinations of the number of spatial dimensions of the system,\n$D$($\\geq 2$), and the exponent, $\\nu$($\\neq 0$), in the force potential,\n$\\phi=Cr^\\nu$. Negative specific heat in such systems is found to be present\nexactly for $\\nu=-1$, at least for $D \\geq 3$. For many combinations of $D$ and\n$\\nu$ representing long-range forces, the specific heat is positive or zero,\nfor both models and the Virial theorem. Hence negative specific heat is not\ncaused by long-range forces as such. We also find that negative specific heat\nappears when $\\nu$ is negative, and there is no singular point in a certain\ndensity distribution. A possible mechanism behind this is suggested.", "category": "gr-qc" }, { "text": "Cosmology of gravitational vacuum: Production of gravitational vacuum defects and their contribution to the\nenergy density of our Universe are discussed. These topological microstructures\n(defects) could be produced in the result of creation of the Universe from\n\"nothing\" when a gravitational vacuum condensate has appeared. They must be\nisotropically distributed over the isotropic expanding Universe. After Universe\ninflation these microdefects are smoothed, stretched and broken up. A part of\nthem could survive and now they are perceived as the structures of Lambda-term\nand an unclustered dark matter. It is shown that the parametrization\nnoninvariance of the Wheeler-De Witt equation can be used to describe\nphenomenologically vacuum topological defects of different dimensions\n(worm-holes, micromembranes, microstrings and monopoles). The mathematical\nillustration of these processes may be the spontaneous breaking of the local\nLorentz-invariance of the quasi-classical equations of gravity. Probably the\ngravitational vacuum condensate has fixed time in our Universe. Besides,\n3-dimensional topological defects renormalize Lambda-term.", "category": "gr-qc" }, { "text": "Linking Covariant and Canonical General Relativity via Local Observers: Hamiltonian gravity, relying on arbitrary choices of \"space,\" can obscure\nspacetime symmetries. We present an alternative, manifestly spacetime covariant\nformulation that nonetheless distinguishes between \"spatial\" and \"temporal\"\nvariables. The key is viewing dynamical fields from the perspective of a field\nof observers -- a unit timelike vector field that also transforms under local\nLorentz transformations. On one hand, all fields are spacetime fields,\ncovariant under spacetime symmeties. On the other, when the observer field is\nnormal to a spatial foliation, the fields automatically fall into Hamiltonian\nform, recovering the Ashtekar formulation. We argue this provides a bridge\nbetween Ashtekar variables and covariant phase space methods. We also outline a\nframework where the 'space of observers' is fundamental, and spacetime geometry\nitself may be observer-dependent.", "category": "gr-qc" }, { "text": "Asymptotically flat black holes with scalar hair: a review: We consider the status of black hole solutions with non-trivial scalar fields\nbut no gauge fields, in four dimensional asymptotically flat space-times,\nreviewing both classical results and recent developments. We start by providing\na simple illustration on the physical difference between black holes in\nelectro-vacuum and scalar-vacuum. Next, we review no-scalar-hair theorems. In\nparticular, we detail an influential theorem by Bekenstein and stress three key\nassumptions: 1) the type of scalar field equation; 2) the spacetime symmetry\ninheritance by the scalar field; 3) an energy condition. Then, we list regular\n(on and outside the horizon), asymptotically flat BH solutions with scalar\nhair, organizing them by the assumption which is violated in each case and\ndistinguishing primary from secondary hair. We provide a table summary of the\nstate of the art.", "category": "gr-qc" }, { "text": "The role of elliptic integrals in calculating the gravitational lensing\n of a charged Weyl black hole surrounded by plasma: In this paper, we mainly aim at highlighting the importance of\n(hyper-)elliptic integrals in the study of gravitational effects caused by\nstrongly gravitating systems. For this, we study the application of elliptic\nintegrals in calculating the light deflection as it passes a plasmic medium,\nsurrounding a charged Weyl black hole. To proceed with this, we consider two\nspecific algebraic ansatzes for the plasmic refractive index, and we\ncharacterize the photon sphere for each of the cases. This will be used further\nto calculate the angular diameter of the corresponding black hole shadow. We\nshow that the complexity of the refractive index expressions, can result in\nsubstantially different types of dependencies of the light behavior on the\nspacetime parameters.", "category": "gr-qc" }, { "text": "Anisotropic dark energy stars: A model of compact object coupled to inhomogeneous anisotropic dark energy is\nstudied. It is assumed a variable dark energy that suffers a phase transition\nat a critical density. The anisotropic Lambda-Tolman-Oppenheimer-Volkoff\nequations are integrated to know the structure of these objects. The anisotropy\nis concentrated on a thin shell where the phase transition takes place, while\nthe rest of the star remains isotropic. The family of solutions obtained\ndepends on the coupling parameter between the dark energy and the fermion\nmatter. The solutions share several features in common with the gravastar\nmodel. There is a critical coupling parameter that gives non-singular black\nhole solutions. The mass-radius relations are studied as well as the internal\nstructure of the compact objects. The hydrodynamic stability of the models is\nanalyzed using a standard test from the mass-radius relation. For each\npermissible value of the coupling parameter there is a maximum mass, so the\nexistence of black holes is unavoidable within this model.", "category": "gr-qc" }, { "text": "Effect of gravitational radiation reaction on circular orbits around a\n spinning black hole: The effect of gravitational radiation reaction on circular orbits around a\nspinning (Kerr) black hole is computed to leading order in $S$ (the magnitude\nof the spin angular momentum of the hole) and in the strength of gravity $M/r$\n(where $M$ is the mass of the black hole, $r$ is the orbital radius, and\n$G=c=1$). The radiation reaction makes the orbit shrink but leaves it circular,\nand drives the orbital plane very slowly toward antialignment with the spin of\nthe hole: $\\tan (\\iota /2) = \\tan (\\iota_0 /2) [1+(61/72)(S/M^2) (M/r)^{3/2}]$,\nwhere $\\iota$ is the angle between the normal to the orbital plane and the spin\ndirection, and $\\iota_0$ is the initial value of $\\iota$, when $r$ is very\nlarge.", "category": "gr-qc" }, { "text": "Compact objects in general relativity: From Buchdahl stars to quasiblack\n holes: A Buchdahl star is a highly compact star for which the boundary radius $R$\nobeys $R=\\frac98 r_+$, where $r_+$ is the gravitational radius of the star\nitself. A quasiblack hole is a maximum compact star, or more generically a\nmaximum compact object, for which the boundary radius $R$ obeys $R=r_+$.\nQuasiblack holes are objects on the verge of becoming black holes. Continued\ngravitational collapse ends in black holes and has to be handled with the\nOppenheimer-Snyder formalism. Quasistatic contraction ends in a quasiblack hole\nand should be treated with appropriate techniques. Quasiblack holes, not black\nholes, are the real descendants of Mitchell and Laplace dark stars. Quasiblack\nholes have many interesting properties. We develop the concept of a quasiblack\nhole, give several examples of such an object, define what it is, draw its\nCarter-Penrose diagram, study its pressure properties, obtain its mass formula,\nderive the entropy of a nonextremal quasiblack hole, and through an extremal\nquasiblack hole give a solution to the puzzling entropy of extremal black\nholes.", "category": "gr-qc" }, { "text": "On the discrete version of the Schwarzschild problem: We consider a Schwarzschild type solution in the discrete Regge calculus\nformulation of general relativity quantized within the path integral approach.\nEarlier, we found a mechanism of a loose fixation of the background scale of\nRegge lengths. This elementary length scale is defined by the Planck scale and\nsome free parameter of such a quantum extension of the theory. Besides, Regge\naction was reduced to an expansion over metric variations between the\ntetrahedra and, in the main approximation, is a finite-difference form of the\nHilbert-Einstein action. Using for the Schwarzschild problem a priori general\nnon-spherically symmetrical ansatz, we get finite-difference equations for its\ndiscrete version. This defines a solution which at large distances is close to\nthe continuum Schwarzschild geometry, and the metric and effective curvature at\nthe center are cut off at the elementary length scale. Slow rotation can also\nbe taken into account (Lense-Thirring-like metric). Thus we get a general\napproach to the classical background in the quantum framework in zero order: it\nis an optimal starting point for the perturbative expansion of the theory;\nfinite-difference equations are classical, the elementary length scale has\nquantum origin. Singularities, if any, are resolved.", "category": "gr-qc" }, { "text": "Exact solutions of an anisotropic universe in a modified teleparallel\n gravity model via the Noether and B.N.S. approaches: In this paper, we present the Noether symmetries of locally rotationally\nsymmetric Bianchi type I (LRS BI), an anisotropic model, in the context of the\nteleparallel gravity. We study a certain modified teleparallel theory based on\nthe action that, in particular, contains a coupling between the scalar field\nand field strength (magnetism part). We derive the symmetry generators and show\nthat, by means of cyclic variables approach, we can not obtain a suitable\nsolution for field equations. Hence by the use of B.N.S. approach, we solve the\nequations which carry Noether currents as well. By data analysis of the\nobtained results, we show compatible results with observational data at the\nlast half the age of universe which is accelerating.", "category": "gr-qc" }, { "text": "Quantum Gravity/String/M-theory/ as we approach the 3rd Millennium: I review some recent progress in String/M-theory", "category": "gr-qc" }, { "text": "Constructing $p,n$-forms from $p$-forms via the Hodge star operator and\n the exterior derivative: In this paper, we aim to explore the properties and applications on the\noperators consisting of the Hodge star operator together with the exterior\nderivative, whose action on an arbitrary $p$-form field in $n$-dimensional\nspacetimes makes its form degree remain invariant. Such operations are able to\ngenerate a variety of $p$-forms with the even-order derivatives of the\n$p$-form. To do this, we first investigate the properties of the operators,\nsuch as the Laplace-de Rham operator, the codifferential and their\ncombinations, as well as the applications of the operators in the construction\nof conserved currents. On basis of two general p-forms, then we construct a\ngeneral n-form with higher-order derivatives. Finally, we propose that such an\nn-form could be applied to define a generalized Lagrangian with respect to a\np-form field according to the fact that it incudes the ordinary Lagrangians for\nthe $p$-form and scalar fields as special cases.", "category": "gr-qc" }, { "text": "Gravity: a gauge theory perspective: The evolution of a generally covariant theory is under-determined. One\nhundred years ago such dynamics had never before been considered; its\nramifications were perplexing, its future important role for all the\nfundamental interactions under the name gauge principle could not be foreseen.\nWe recount some history regarding Einstein, Hilbert, Klein and Noether and the\nnovel features of gravitational energy that led to Noether's two theorems.\nUnder-determined evolution is best revealed in the Hamiltonian formulation. We\ndeveloped a covariant Hamiltonian formulation. The Hamiltonian boundary term\ngives covariant expressions for the quasi-local energy, momentum and angular\nmomentum. Gravity can be considered as a gauge theory of the local Poincar\\'e\ngroup. The dynamical potentials of the Poincar\\'e gauge theory of gravity are\nthe frame and the connection. The spacetime geometry has in general both\ncurvature and torsion. Torsion naturally couples to spin; it could have a\nsignificant magnitude and yet not be noticed, except on a cosmological scale\nwhere it could have significant effects.", "category": "gr-qc" }, { "text": "Covariant Spin Structure: Every Dirac spin structure on a world manifold is associated with a certain\ngravitational field, and is not preserved under general covariant\ntransformations. We construct a composite spinor bundle such that any Dirac\nspin structure is its subbundle, and this bundle admits general covariant\ntransformations.", "category": "gr-qc" }, { "text": "Weak cosmic censorship conjecture in BTZ black holes with scalar fields: The weak cosmic censorship conjecture in the near-extremal BTZ black hole has\nbeen tested by the test particles and fields. It was claimed that this black\nhole could be overspun. In this paper, we review the thermodynamics and weak\ncosmic censorship conjecture in BTZ black holes by the scattering of the scalar\nfield. The first law of thermodynamics in the non-extremal BTZ black hole is\nrecovered. For the extremal and near-extremal black holes, due to the\ndivergence of the variation of the entropy, we test the weak cosmic censorship\nconjecture by evaluating the minimum values of the function $f$. Both of the\nextremal and near-extremal black holes cannot be overspun.", "category": "gr-qc" }, { "text": "Stability and Instability of Extreme Reissner-Nordstr\u00f6m Black Hole\n Spacetimes for Linear Scalar Perturbations II: This paper contains the second part of a two-part series on the stability and\ninstability of extreme Reissner-Nordstrom spacetimes for linear scalar\nperturbations. We continue our study of solutions to the linear wave equation\non a suitable globally hyperbolic subset of such a spacetime, arising from\nregular initial data prescribed on a Cauchy hypersurface crossing the future\nevent horizon. We here obtain definitive energy and pointwise decay, non-decay\nand blow-up results. Our estimates hold up to and including the horizon. A\nhierarchy of conservations laws on degenerate horizons is also derived.", "category": "gr-qc" }, { "text": "Spectral Geometry and Causality: For a physical interpretation of a theory of quantum gravity, it is necessary\nto recover classical spacetime, at least approximately. However, quantum\ngravity may eventually provide classical spacetimes by giving spectral data\nsimilar to those appearing in noncommutative geometry, rather than by giving\ndirectly a spacetime manifold. It is shown that a globally hyperbolic\nLorentzian manifold can be given by spectral data. A new phenomenon in the\ncontext of spectral geometry is observed: causal relationships. The employment\nof the causal relationships of spectral data is shown to lead to a highly\nefficient description of Lorentzian manifolds, indicating the possible\nusefulness of this approach. Connections to free quantum field theory are\ndiscussed for both motivation and physical interpretation. It is conjectured\nthat the necessary spectral data can be generically obtained from an effective\nfield theory having the fundamental structures of generalized quantum\nmechanics: a decoherence functional and a choice of histories.", "category": "gr-qc" }, { "text": "Gravitation and vacuum entanglement entropy: The vacuum of quantum fields contains correlated fluctuations. When\nrestricted to one side of a surface these have a huge entropy of entanglement\nthat scales with the surface area. If UV physics renders this entropy finite,\nthen a thermodynamic argument implies the existence of gravity. That is, the\ncausal structure of spacetime must be dynamical and governed by the Einstein\nequation with Newton's constant inversely proportional to the entropy density.\nConversely, the existence of gravity makes the entanglement entropy finite.\nThis thermodynamic reasoning is powerful despite the lack of a detailed\ndescription of the dynamics at the cutoff scale, but it has its limitations. In\nparticular, we should not expect to understand corrections to Einstein gravity\nin this way.", "category": "gr-qc" }, { "text": "Scalar field propagation in higher dimensional black holes at a Lifshitz\n point: We study the complete time evolution of scalar fields propagating in\nspace-times of higher dimensional Lifshitz Black Holes with dynamical critical\nexponent $z=2$, obtained from a theory including the most general quadratic\ncurvature corrections to Einstein-Hilbert gravity in $D$ dimensions. We also\ncomputed the quasinormal spectrum after performing a numerical integration and\nsolving exactly the Klein-Gordon equation obeyed by the massive scalar field.\nWe found that quasinormal modes are purely imaginary for all dimensions.", "category": "gr-qc" }, { "text": "Behaviour of spin-half particles in curved space-time: We study the behaviour of spin-half particles in curved space-time. Since\nDirac equation gives the dynamics of spin-half particles, we mainly study the\nDirac equation in Schwarzschild, Kerr, Reissner-Nordstr\\\"om geometry. Due to\nthe consideration of existence of black hole in space-time (the curved\nspace-time), particles are influenced and equation will be modified. As a\nresult the solution will be changed from that due to flat space.", "category": "gr-qc" }, { "text": "The black hole challenge in Randall-Sundrum II model: Models postulating the existence of additional spacelike dimensions of\nmacroscopic or even infinite size, while viewing our observable universe as\nmerely a 3-brane living in a higher-dimensional bulk were a major breakthrough\nwhen proposed some 15 years ago. The most interesting among them both in terms\nof elegance of the setup and of the richness of the emerging phenomenology is\nthe Randall-Sundrum II model where one infinite extra spacelike dimension is\nconsidered with an AdS topology, characterized by the warping effect caused by\nthe presence of a negative cosmological constant in the bulk. A major drawback\nof this model is that despite numerous efforts no line element has ever been\nfound that could describe a stable, regular, realistic black hole. Finding a\nsmoothly behaved such solution supported by the presence of some more or less\nconventional fields either in the bulk and/or on the brane is the core of the\nblack hole challenge. After a comprehensive presentation of the details of the\nmodel and the analysis of the significance and the utility of getting a\nspecific analytic black hole solution, several (unsuccessful) analytic and\nnumerical approaches to the problem developed over the years are presented with\nsome discussion about their results. The chapter closes with the latest\nnumerical results that actually consists a major advancement in the effort to\naddress the challenge, the presentation of the most recent analytic work trying\n(and unfortunately failing) to build a solution assuming the existence of\nunconventional scalar fields and some ideas about the routes the forthcoming\nanalytic approaches should explore.", "category": "gr-qc" }, { "text": "A no-go result for covariance in models of loop quantum gravity: Based on the observation that the exterior space-times of Schwarzschild-type\nsolutions allow two symmetric slicings, a static spherically symmetric one and\na timelike homogeneous one, modifications of gravitational dynamics suggested\nby symmetry-reduced models of quantum cosmology can be used to derive\ncorresponding modified spherically symmetric equations. Generally covariant\ntheories are much more restricted in spherical symmetry compared with\nhomogeneous slicings, given by $1+1$-dimensional dilaton models if they are\nlocal. As shown here, modifications used in loop quantum cosmology do not have\na corresponding covariant spherically symmetric theory. Models of loop quantum\ncosmology therefore violate general covariance in the form of slicing\nindependence. Only a generalized form of covariance with a non-Riemannian\ngeometry could consistently describe space-time in models of loop quantum\ngravity.", "category": "gr-qc" }, { "text": "How often does the Unruh-DeWitt detector click beyond four dimensions?: We analyse the response of an arbitrarily-accelerated Unruh-DeWitt detector\ncoupled to a massless scalar field in Minkowski spacetimes of dimensions up to\nsix, working within first-order perturbation theory and assuming a smooth\nswitch-on and switch-off. We express the total transition probability as a\nmanifestly finite and regulator-free integral formula. In the sharp switching\nlimit, the transition probability diverges in dimensions greater than three but\nthe transition rate remains finite up to dimension five. In dimension six, the\ntransition rate remains finite in the sharp switching limit for trajectories of\nconstant scalar proper acceleration, including all stationary trajectories, but\nit diverges for generic trajectories. The divergence of the transition rate in\nsix dimensions suggests that global embedding spacetime (GEMS) methods for\ninvestigating detector response in curved spacetime may have limited validity\nfor generic trajectories when the embedding spacetime has dimension higher than\nfive.", "category": "gr-qc" }, { "text": "Gravitational Waves and Their Memory in General Relativity: General relativity explains gravitational radiation from binary black hole or\nneutron star mergers, from core-collapse supernovae and even from the inflation\nperiod in cosmology. These waves exhibit a unique effect called memory or\nChristodoulou effect, which in a detector like LIGO or LISA shows as a\npermanent displacement of test masses and in radio telescopes like NANOGrav as\na change in the frequency of pulsars' pulses. It was shown that electromagnetic\nfields and neutrino radiation enlarge the memory. Recently it has been\nunderstood that the two types of memory addressed in the literature as `linear'\nand `nonlinear' are in fact two different phenomena. The former is due to\nfields that do not and the latter is due to fields that do reach null infinity.", "category": "gr-qc" }, { "text": "Circularization vs. Eccentrification in Intermediate Mass Ratio\n Inspirals inside Dark Matter Spikes: Inspirals of an Intermediate Mass Black Hole (IMBH) and a solar mass type\nobject will be observable by space based gravitational wave detectors such as\nThe Laser Interferometer Space Antenna (LISA). A dark matter overdensity around\nan IMBH - a dark matter spike - can affect the orbital evolution of the system.\nWe consider here such Intermediate Mass Ratio Inspirals on eccentric orbits,\nexperiencing dynamical friction of the dark matter spike. We find that by\nincluding the phase space distribution of the dark matter, the dynamical\nfriction tends to circularize the orbit, in contrast to previous inquiries. We\nderive a general condition for circularization or eccentrification for any\ngiven dissipative force. In addition to the dephasing, we suggest using the\ncircularization rate as another probe of the dark matter spike. Observing these\neffects would be an indicator for the particle nature of dark matter.", "category": "gr-qc" }, { "text": "Detecting intermediate-mass black hole binaries with atom interferometer\n observatories: Using the resonant mode for the merger phase: Atom interferometry detectors like AION, ZAIGA, and AEDGE will be able to\ndetect gravitational waves (GWs) at dHz covering the band between large\nspace-based laser interferometers LISA/TianQin/Taiji and ground-based\nfacilities LIGO/Virgo/KAGRA. They will detect the late inspiral and merger of\nGW sources containing intermediate-mass black holes (IMBHs) in the mass range\n$10^2-10^5\\,{\\rm M_\\odot}$. We study how accurately the parameters of an IMBH\nbinary can be measured using the noise curve of AION. Furthermore, we propose a\ndetection scheme where the early inspiral of the binary is detected using the\nregular broadband mode while the merger is detected using the resonant mode. We\nfind that by using such a detection scheme the signal-to-noise ratio (SNR) of\nthe detection as well as the detection accuracy of the parameters can be\nenhanced compared to the full detection of the signal using the broadband mode.\nWe, further, assess the impact of the necessary detection gap while switching\nfrom broadband to resonant mode studying the case of a short ($30\\,{\\rm s}$)\nand a long ($600\\,{\\rm s}$) gap. We find that the improvement in SNR and\ndetection accuracy is bigger for the shorter gap but that even in the case of\nthe long gap such a scheme can be beneficial.", "category": "gr-qc" }, { "text": "Covariant coarse-graining of inhomogeneous dust flow in General\n Relativity: A new definition of coarse-grained quantities describing the dust flow in\nGeneral Relativity is proposed. It assigns the coarse--grained expansion, shear\nand vorticity to finite-size comoving domains of fluid in a covariant,\ncoordinate-independent manner. The coarse--grained quantities are all\nquasi-local functionals, depending only on the geometry of the boundary of the\nconsidered domain. They can be thought of as relativistic generalizations of\nsimple volume averages of local quantities in a flat space. The procedure is\nbased on the isometric embedding theorem for S^2 surfaces and thus requires the\nboundary of the domain in question to have spherical topology and positive\nscalar curvature. We prove that in the limit of infinitesimally small volume\nthe proposed quantities reproduce the local expansion, shear and vorticity. In\ncase of irrotational flow we derive the time evolution for the coarse-grained\nquantities and show that its structure is very similar to the evolution\nequation for their local counterparts. Additional terms appearing in it may\nserve as a measure of the backreacton of small-scale inhomogeneities of the\nflow on the large-scale motion of the fluid inside the domain and therefore the\nresult may be interesting in the context of the cosmological backreaction\nproblem. We also consider the application of the proposed coarse-graining\nprocedure to a number of known exact solutions of Einstein equations with dust\nand show that it yields reasonable results.", "category": "gr-qc" }, { "text": "Semi-realistic Bouncing Domain Wall Cosmology: In this paper we constructed a semi-realistic cosmological model in a dynamic\ndomain wall framework. Our universe is considered to be a (3+1) dimensional\ndynamic domain wall in a higher dimensional Einstein-Maxwell-Born-Infeld\nbackground. One of our interesting outcomes from the effective Hubble equation\nfor the domain wall dynamics is that it contains an additional component of\n\"dark matter\" which is induced from the charge of the bulk Born-Infeld gauge\nfield. In this background spacetime we have studied the cosmological dynamics\nof the domain wall. In addition to the Born-Infield gauge field if we consider\nadditional pure gauge field, a non-singular bounce happens at the early stage\nwith a smooth transition between contracting and expanding phase.", "category": "gr-qc" }, { "text": "Modelo An\u00e1logo Ac\u00fastico ao Buraco Negro de Schwarzschild: Um fluido em movimento pode agir sobre o som da mesma forma que os\nespa\\c{c}o-tempos curvos podem influenciar na trajet\\'oria da luz na\nrelatividade geral. Com isso, pode-se descrever a propaga\\c{c}\\~ao dessas ondas\nsonoras atrav\\'es de uma m\\'etrica efetiva, sob a qual elas seguir\\~ao\ngeod\\'esicas nulas. Esta disserta\\c{c}\\~ao faz uma revis\\~ao destes estudos\nconcentrando-se em uma analogia ac\\'ustica para um buraco negro de\nSchwarzschild, demonstrando suas vantagens e limita\\c{c}\\~oes quando aplicada\npara o estudo da teoria de Hawking.\n A fluid in moviment can act on the sound the same way that curved space-time\ncan influence on light trajectory in the general relativity. So, one can\ndescribe the propagation these sound waves through an effective metric, under\nwich they will follow null geodesics. This thesis makes a review of these\nstudies focusing in an analogy to Schwarzschild black hole using an acoustic\nsystem, showing its advantages and limitations when applied in the study of\nHawking's theory.", "category": "gr-qc" }, { "text": "Wavelet entropy filter and cross-correlation of gravitational wave data: We present a method for enhancing the cross-correlation of gravitational wave\nsignals eventually present in data streams containing otherwise uncorrelated\nnoise. Such method makes use of the wavelet decomposition to cast the\ncross-correlation time series in time-frequency space. Then an entropy\ncriterion is applied to identify the best time frequency resolution, i.e. the\nresolution allowing to concentrate the signal in the smallest number of wavelet\ncoefficients. By keeping only the coefficients above a certain threshold, it is\npossible to reconstruct a cross-correlation time series where the effect of\ncommon signal is stronger. We tested our method against signals injected over\ntwo data streams of uncorrelated white noise.", "category": "gr-qc" }, { "text": "Emergent modified gravity: The perfect fluid and gravitational collapse: Emergent modified gravity is a canonical theory based on general covariance\nwhere the spacetime is not fundamental, but rather an emergent object. This\nfeature allows for modifications of the classical theory and can be used to\nmodel new effects, such as those suggested by quantum gravity. We discuss how\nmatter fields can be coupled to emergent modified gravity, realize the coupling\nof the perfect fluid, identify the symmetries of the system, and explicitly\nobtain the Hamiltonian in spherical symmetry. We formulate the\nOppenheimer-Snyder collapse model in canonical terms, permitting us to extend\nthe model to emergent modified gravity and obtain an exact solution to the dust\ncollapsing from spatial infinity including some effects suggested by quantum\ngravity. In this solution the collapsing dust forms a black hole, then the star\nradius reaches a minimum with vanishing velocity and finite positive\nacceleration, and proceeds to emerge out now behaving as a white hole. While\nthe geometry on the minimum-radius surface is regular in the vacuum, it is\nsingular in the presence of dust. However, the fact that the geometry is\nemergent, and the fundamental fields that compose the phase-space are regular,\nallows us to continue the canonical solution in a meaningful way, obtaining the\nglobal structure for the interior of the star. The star-interior solution is\ncomplemented by the vacuum solution describing the star-exterior region by a\ncontinuous junction at the star radius. This gluing process can be viewed as\nthe imposition of boundary conditions, which is non-unique and does not follow\nfrom the equations of motion. This ambiguity gives rise to different possible\nphysical outcomes of the collapse. We discuss two such phenomena: the formation\nof a wormhole and the transition from a black hole to a white hole.", "category": "gr-qc" }, { "text": "The magnetic field of the relativistic stars in the 5D approach: It is well-known that the 5D equations without sources may be reduced to the\n4D ones with sources, provided an appropriate definition for the\nenergy-momentum tensor of matter in terms of the extra part of the geometry.The\nadvantage consists on the naturally appearance of gravitational and\nelectromagnetic fields from this decomposition. With this ansatz an algorithm\nis presented, which permits to express the physical parameters in terms of\ngauge potentials and scalar field. An explicit form for the exterior magnetic\nfield of neutron star in terms of the scalar field and the gauge potentials is\ndeduced for a static, spherically-symmetric metric.", "category": "gr-qc" }, { "text": "Linear potentials in galaxy halos by Asymmetric Wormholes: A spherically symmetric space-time solution for a diffusive two measures\ntheory is studied. An asymmetric wormhole geometry is obtained where the metric\ncoefficients have a linear term for galactic distances and the analysis of\nMannheim and collaborators, can then be used to describe the galactic rotation\ncurves. For cosmological distances, a de-Sitter space-time is realized. Centre\nof gravity coordinates for the wormhole is introduced which are the most\nsuitable for the collective motion of a wormhole. The wormholes connect\nuniverses with different vacuum energy densities which may represent different\nuniverses in a \"landscape scenario\". The metric coefficients depend on the\nasymmetric wormhole parameters. The coefficient of the linear potential is\nproportional to both the mass of the wormhole and the cosmological constant of\nthe observed universe. Similar results are also expected in other theories like\n$k$-essence theories, that may support wormholes.", "category": "gr-qc" }, { "text": "Warp Drive With Zero Expansion: It is commonly believed that Alcubierre's warp drive works by contracting\nspace in front of the warp bubble and expanding space behind it. We show that\nthis expansion/contraction is but a marginal consequence of the choice made by\nAlcubierre, and explicitly construct a similar spacetime where no\ncontraction/expansion occurs. Global and optical properties of warp drive\nspacetimes are also discussed.", "category": "gr-qc" }, { "text": "A simple method of constructing binary black hole initial data: By applying a parabolic-hyperbolic formulation of the constraints and\nsuperposing Kerr-Schild black holes, a simple method is introduced to\ninitialize time evolution of binary systems. As the input parameters are\nessentially the same as those used in the post-Newtonian (PN) setup the\nproposed method interrelates various physical expressions applied in PN and in\nfully relativistic formulations. The global ADM charges are also determined by\nthe input parameters, and no use of boundary conditions in the strong field\nregime is made.", "category": "gr-qc" }, { "text": "Mathematical Equivalence vs. Physical Equivalence between Extended\n Theories of Gravitations: We shall show that although Palatini f(R)-theories are equivalent to\nBrans-Dicke theories, still the first pass the Mercury precession of perihelia\ntest, while the second do not. We argue that the two models are not physically\nequivalent due to a different assumptions about free fall. We shall also go\nthrough perihelia test without fixing a conformal gauge (clocks or rulers) in\norder to highlight what can be measured in a conformal invariant way and what\ncannot. We shall argue that the conformal gauge is broken by choosing a\ndefinition of clock, rulers or, equivalently, of masses.", "category": "gr-qc" }, { "text": "On thermodynamics of charged black hole with scalar hair: It is shown that energy, entropy and the first law of the Martinez-Troncoso\nblack hole with electric charge and scalar hair (2006) can be consistently\ndescribed in a general Hamiltonian approach to black hole thermodynamics.", "category": "gr-qc" }, { "text": "Spin Gauge Theory of Gravity in Clifford Space: A theory in which 16-dimensional curved Clifford space (C-space) provides a\nrealization of Kaluza-Klein theory is investigated. No extra dimensions of\nspacetime are needed: \"extra dimensions\" are in C-space. We explore the spin\ngauge theory in C-space and show that the generalized spin connection contains\nthe usual 4-dimensional gravity and Yang-Mills fields of the U(1)xSU(2)xSU(3)\ngauge group. The representation space for the latter group is provided by\n16-component generalized spinors composed of four usual 4-component spinors,\ndefined geometrically as the members of four independent minimal left ideals of\nClifford algebra.", "category": "gr-qc" }, { "text": "Temperature and entropy of Schwarzschild-de Sitter space-time: In the light of recent interest in quantum gravity in de Sitter space, we\ninvestigate semi-classical aspects of 4-dimensional Schwarzschild-de Sitter\nspace-time using the method of complex paths. The standard semi-classical\ntechniques (such as Bogoliubov coefficients and Euclidean field theory) have\nbeen useful to study quantum effects in space-times with single horizons;\nhowever, none of these approaches seem to work for Schwarzschild-de Sitter or,\nin general, for space-times with multiple horizons. We extend the method of\ncomplex paths to space-times with multiple horizons and obtain the spectrum of\nparticles produced in these space-times. We show that the temperature of\nradiation in these space-times is proportional to the effective surface gravity\n-- inverse harmonic sum of surface gravity of each horizon. For the\nSchwarzschild-de Sitter, we apply the method of complex paths to three\ndifferent coordinate systems -- spherically symmetric, Painleve and Lemaitre.\nWe show that the equilibrium temperature in Schwarzschild-de Sitter is the\nharmonic mean of cosmological and event horizon temperatures. We obtain\nBogoliubov coefficients for space-times with multiple horizons by analyzing the\nmode functions of the quantum fields near the horizons. We propose a new\ndefinition of entropy for space-times with multiple horizons analogous to the\nentropic definition for space-times with a single horizon. We define entropy\nfor these space-times to be inversely proportional to the square of the\neffective surface gravity. We show that this definition of entropy for\nSchwarzschild-de Sitter satisfies the D-bound conjecture.", "category": "gr-qc" }, { "text": "Spinors and Twistors in Loop Gravity and Spin Foams: Spinorial tools have recently come back to fashion in loop gravity and spin\nfoams. They provide an elegant tool relating the standard holonomy-flux algebra\nto the twisted geometry picture of the classical phase space on a fixed graph,\nand to twistors. In these lectures we provide a brief and technical\nintroduction to the formalism and some of its applications.", "category": "gr-qc" }, { "text": "Gravitational radiation for extreme mass ratio inspirals to the 14th\n post-Newtonian order: We derive gravitational waveforms needed to compute the 14th post-Newtonian\n(14PN) order energy flux for a particle in circular orbit around a\nSchwarzschild black hole, i.e. $v^{28}$ beyond the leading Newtonian\napproximation where $v$ is the orbital velocity of a test particle. We\ninvestigate the convergence of the energy flux in the PN expansion and suggest\na fitting formula which can be used to extract unknown higher order PN\ncoefficients from accurate numerical data for more general orbits around a Kerr\nblack hole. The phase difference between the 14PN waveforms and numerical\nwaveforms after two years inspiral is shown to be about $10^{-7}$ for\n$\\mu/M=10^{-4}$ and $10^{-3}$ for $\\mu/M=10^{-5}$ where $\\mu$ is the mass of a\ncompact object and $M$ the mass of the central supermassive black hole. In\nfirst order black hole perturbation theory, for extreme mass ratio inspirals\nwhich are one of the main targets of Laser Interferometer Space Antenna, the\n14PN expressions will lead to the data analysis accuracies comparable to the\nones resulting from high precision numerical waveforms.", "category": "gr-qc" }, { "text": "Dirac Equation in Gauge and Affine-Metric Gravitation Theories: We show that the covariant derivative of Dirac fermion fields in the presence\nof a general linear connection on a world manifold is universal for Einstein's,\ngauge and affine-metric gravitation theories.", "category": "gr-qc" }, { "text": "A morphology-independent search for gravitational wave echoes in data\n from the first and second observing runs of Advanced LIGO and Advanced Virgo: Gravitational wave echoes have been proposed as a smoking-gun signature of\nexotic compact objects with near-horizon structure. Recently there have been\nobservational claims that echoes are indeed present in stretches of data from\nAdvanced LIGO and Advanced Virgo immediately following gravitational wave\nsignals from presumed binary black hole mergers, as well as a binary neutron\nstar merger. In this paper we deploy a morphology-independent search algorithm\nfor echoes introduced in Tsang et al., Phys. Rev. D 98, 024023 (2018), which\n(a) is able to accurately reconstruct a possible echoes signal with minimal\nassumptions about their morphology, and (b) computes Bayesian evidences for the\nhypotheses that the data contain a signal, an instrumental glitch, or just\nstationary, Gaussian noise. Here we apply this analysis method to all the\nsignificant events in the first Gravitational Wave Transient Catalog (GWTC-1),\nwhich comprises the signals from binary black hole and binary neutron star\ncoalescences found during the first and second observing runs of Advanced LIGO\nand Advanced Virgo. In all cases, the ratios of evidences for signal versus\nnoise and signal versus glitch do not rise above their respective \"background\ndistributions\" obtained from detector noise, the smallest $p$-value being 3%\n(for event GW170823). Hence we find no statistically significant evidence for\nechoes in GWTC-1.", "category": "gr-qc" }, { "text": "Invariant conserved currents in gravity theories with local Lorentz and\n diffeomorphism symmetry: We discuss conservation laws for gravity theories invariant under general\ncoordinate and local Lorentz transformations. We demonstrate the possibility to\nformulate these conservation laws in many covariant and noncovariant(ly\nlooking) ways. An interesting mathematical fact underlies such a diversity:\nthere is a certain ambiguity in a definition of the (Lorentz-) covariant\ngeneralization of the usual Lie derivative. Using this freedom, we develop a\ngeneral approach to construction of invariant conserved currents generated by\nan arbitrary vector field on the spacetime. This is done in any dimension, for\nany Lagrangian of the gravitational field and of a (minimally or nonminimally)\ncoupled matter field. A development of the \"regularization via relocalization\"\nscheme is used to obtain finite conserved quantities for asymptotically nonflat\nsolutions. We illustrate how our formalism works by some explicit examples.", "category": "gr-qc" }, { "text": "Cosmological models with Big rip and Pseudo rip Scenarios in extended\n theory of gravity: In this paper, we have presented the big rip and pseudo rip cosmological\nmodels in an extended theory of gravity. The matter field is considered to be\nthat of perfect fluid. The geometrical parameters are adjusted in such a manner\nthat it matches the prescriptions given by cosmological observations, to be\nspecific to the range of Hubble tension $(H_{0})$. The models favor phantom\nbehavior. The violation of strong energy conditions are shown in both the\nmodels, as it has become essential in an extended gravity. The representative\nvalues of the coupling parameter are significant on the evolution of the\nuniverse.", "category": "gr-qc" }, { "text": "Light, delayed: The Shapiro Effect and the Newtonian Limit: The Shapiro effect, also known as the gravitational time delay, is close kin\nto the gravitational deflection of light that was the central topic of our\nSummer School. It is also an interesting test bed for exploring a topic that\nprovides the foundations for most of the calculations we have done in this\nschool, yet is highly complex when treated more rigorously: the question of the\nNewtonian limit, and of the post-Newtonian corrections that must be applied to\ninclude the leading-order effects of general relativity. This contribution\ndiscusses simplified derivations for the gravitational redshift and the Shapiro\neffect, as well as astrophysical situations in which the Shapiro effect can be\nmeasured.", "category": "gr-qc" }, { "text": "Ghost Free Theory in Unitary Gauge: A New Candidate: We propose an algebraic analysis using a 3+1 decomposition to identify\nconditions for a clever cancellation of the higher derivatives, which plagued\nthe theory with Ostrogradsky ghosts, by exploiting some existing degeneracy in\nthe Lagrangian. We obtain these conditions as linear equations (in terms of\ncoefficients of the higher derivative terms) and demand that they vanish, such\nthat the existence of nontrivial solutions implies that the theory is\ndegenerate. We find that, for the theory under consideration, no such solutions\nexist for a general inhomogeneous scalar field, but that the theory is\ndegenerate in the unitary gauge. We, then, find modified FLRW equations and\nnarrow down conditions for which there could exist a de Sitter inflationary\nepoch. We further find constraints on the coefficients of the remaining\nhigher-derivative interaction terms, based on power-counting renormalizability\nand tree-level unitarity up to the Planck scale.", "category": "gr-qc" }, { "text": "Two Fluid Shear-Free Composites: Shear-free composite fluids are constructed from two Letelier rotated\nunaligned perfect fluids. The component fluid parameters necessary to construct\na shear-free composite are investigated. A metric in the Stephani-Barnes\nsolution family and a simple stationary metric are discussed.", "category": "gr-qc" }, { "text": "Causal simplicity and (maximal) null pseudoconvexity: We consider pseudoconvexity properties in Lorentzian and Riemannian manifolds\nand their relationship in static spacetimes. We provide an example of a\ncausally continuous and maximal null pseudoconvex spacetime that fails to be\ncausally simple. Its Riemannian factor provides an analogous example of a\nmanifold that is minimally pseudoconvex, but fails to be convex.", "category": "gr-qc" }, { "text": "Charged black holes on Kaluza-Klein bubbles: We construct exact solutions of the Einstein-Maxwell-Dilaton field equations\nin five dimensions, which describe general configurations of charged and static\nblack holes sitting on a Kaluza-Klein bubble. More specifically we discuss the\nconfigurations describing two black holes sitting on a Kaluza-Klein bubble and\nalso the general charged static black Saturn balanced by a Kaluza-Klein bubble.\nA straightforward extension of the solution generating technique leads to a new\nsolution describing the charged static black Saturn on the Taub-bolt instanton.\nWe compute the conserved charges and investigate some of the thermodynamic\nproperties of these systems.", "category": "gr-qc" }, { "text": "Ricci Collineations for Non-Degenerate, Diagonal and Spherically\n Symmetric Ricci Tensors: The expression of the vector field generator of a Ricci Collineation for\ndiagonal, spherically symmetric and non-degenerate Ricci tensors is obtained.\nThe resulting expressions show that the time and radial first derivatives of\nthe components of the Ricci tensor can be used to classify the collineation,\nleading to 64 families.\n Some examples illustrate how to obtain the collineation vector.", "category": "gr-qc" }, { "text": "Geodesic motion in equal angular momenta Myers-Perry-AdS spacetimes: We study the geodesic motion of massive and massless test particles in the\nbackground of equally spinning Myers-Perry-anti-de Sitter (AdS) black holes in\nfive dimensions. By adopting a coordinate system that makes manifest the\ncohomogeneity-1 property of these spacetimes, the equations of motion simplify\nconsiderably. This allows us to easily separate the radial motion from the\nangular part and to obtain solutions for angular trajectories in a compact\nclosed form. For the radial motion we focus our attention on spherical orbits.\nIn particular, we determine the timelike innermost stable circular orbits\n(ISCOs) for these asymptotically AdS spacetimes, as well as the location of\nnull circular orbits. We find that the ISCO dives below the ergosurface for\nblack holes rotating close to extremality and merges with the event horizon\nexactly at extremality, in analogy with the four-dimensional Kerr case. For\nsufficiently massive black holes in AdS there exists a spin parameter range in\nwhich the background spacetime is stable against superradiance and the ISCO\nlies inside the ergoregion. Our results for massless geodesics show that there\nare no stable circular null orbits outside the horizon, but there exist such\norbits inside the horizon, as well as around over-extremal spacetimes, i.e.,\nnaked singularities. We also discuss how these orbits deform from the static to\nthe rotating case.", "category": "gr-qc" }, { "text": "Unified Lagrangian for Tachyon, Quintessence, and Phantom Scalar Fields: This paper presents a novel unified Lagrangian density that combines the\nbehaviors of tachyon, quintessence, and phantom scalar fields within the realms\nof theoretical physics and cosmology. The unified Lagrangian is formulated,\nwhere \\(f(\\alpha)\\) is a function that accommodates different scenarios and\ncaptures the distinct characteristics of these scalar fields. The potential\nfunction \\(V(\\phi)\\) is incorporated to account for the specific properties of\nthe scalar fields. The study employs numerical simulations in Python to\nthoroughly analyze the dynamics of the scalar fields for various \\(\\alpha\\)\nvalues, investigating the cosmological behavior.", "category": "gr-qc" }, { "text": "Discreteness of Space from GUP in a Weak Gravitational Field: Quantum gravity effects modify the Heisenberg's uncertainty principle to a\ngeneralized uncertainty principle (GUP). Earlier work showed that the\nGUP-induced corrections to the Schr\\\"odinger equation, when applied to a\nnon-relativistic particle in a one-dimensional box, led to the quantization of\nlength. Similarly, corrections to the Klein-Gordon and the Dirac equations,\ngave rise to length, area and volume quantizations. These results suggest a\nfundamental granular structure of space. In this work, it is investigated how\nspacetime curvature and gravity might influence this discreteness of space. In\nparticular, by adding a weak gravitational background field to the above three\nquantum equations, it is shown that quantization of lengths, areas and volumes\ncontinue to hold. However, it should be noted that the nature of this new\nquantization is quite complex and under proper limits, it reduces to cases\nwithout gravity. These results suggest that quantum gravity effects are\nuniversal.", "category": "gr-qc" }, { "text": "Quantum interaction between two gravitationally polarizable objects in\n presence of boundaries: We investigate, in the framework of the linearized quantum gravity and the\nleading-order perturbation theory, the quantum correction to the classical\nNewtonian interaction between a pair of gravitationally polarizable objects in\nthe presence of both Neumann and Dirichlet boundaries. We obtain general\nresults for the interaction potential and find that the presence of a boundary\nalways strengthens in the leading-order the interaction as compared with the\ncase in absence of boundaries. But different boundaries yield a different\ndegree of strengthening. In the limit when one partner of the pair is placed\nvery close to the Neumann boundary, the interaction potential is larger when\nthe pair is parallel with the boundary than when it is perpendicular to, which\nis just opposite to the case when the boundary is Dirichlet where the latter is\nlarger than the former. In addition, we find that the pair-boundary separation\ndependence of the higher-order correction term is determined by the orientation\nof the pair with respect to boundary, with the parallel case giving a quadratic\nbehavior and the perpendicular case a linear one.", "category": "gr-qc" }, { "text": "Compact sources and cosmological horizons in lower dimensional\n bootstrapped Newtonian gravity: We study the bootstrapped Newtonian potential generated by a localised source\nin one and two spatial dimensions, and show that both cases naturally lead to\nfinite spatial extensions of the outer vacuum. We speculate that this implies\nthe necessary existence of a cosmological (particle) horizon associated with\ncompact sources. In view of the possible dimensional reduction occurring in\nultra-high energy processes - like scatterings at Planckian energies, the\ngravitational collapse of compact objects or the end-point of black hole\nevaporation - one can consider such lower-dimensional \"bubbles\" immersed in our\nUniverse as describing (typically Planckian size) baby universes relevant to\nthose dynamics.", "category": "gr-qc" }, { "text": "Thermodynamics in $f(\\mathcal{G},T)$ Gravity: This paper explores the non-equilibrium behavior of thermodynamics at the\napparent horizon of isotropic and homogeneous universe model in\n$f(\\mathcal{G},T)$ gravity ($\\mathcal{G}$ and $T$ represent the Gauss-Bonnet\ninvariant and trace of the energy-momentum tensor, respectively). We construct\nthe corresponding field equations and analyze the first as well as generalized\nsecond law of thermodynamics in this scenario. It is found that an auxiliary\nterm corresponding to entropy production appears due to the non-equilibrium\npicture of thermodynamics in first law. The universal condition for the\nvalidity of generalized second law of thermodynamics is also obtained. Finally,\nwe check the validity of generalized second law of thermodynamics for the\nreconstructed $f(\\mathcal{G},T)$ models (de Sitter and power-law solutions). We\nconclude that this law holds for suitable choices of free parameters.", "category": "gr-qc" }, { "text": "Observable currents and a covariant Poisson algebra of physical\n observables: Observable currents are locally defined gauge invariant conserved currents;\nphysical observables may be calculated integrating them on appropriate\nhypersurfaces. Due to the conservation law the hypersurfaces become irrelevant\nup to homology, and the main objects of interest become the observable currents\nthem selves. Gauge inequivalent solutions can be distinguished by means of\nobservable currents. With the aim of modeling spacetime local physics, we work\non spacetime domains $U\\subset M$ which may have boundaries and corners.\nHamiltonian observable currents are those satisfying ${\\sf\nd_v}F=-\\iota_V\\Omega_L+{\\sf d_h}\\sigma^F$ and a certain boundary condition. The\nfamily of Hamiltonian observable currents is endowed with a bracket that gives\nit a structure which generalizes a Lie algebra in which the Jacobi relation is\nmodified by the presence of a boundary term. If the domain of interest has no\nboundaries the resulting algebra of observables is a Lie algebra. In the\nresulting framework algebras of observable currents are associated to bounded\ndomains, and the local algebras obey interesting gluing properties. These\nresults are due to considering currents that defined only locally in field\nspace and to a revision of the concept of gauge invariance in bounded spacetime\ndomains. A perturbation of the field on a bounded spacetime domain is regarded\nas gauge if: (i) the \"first order holographic imprint\" that it leaves in any\nhypersurface locally splitting a spacetime domain into two subdomains is\nnegligible according to the linearized gluing field equation, and (ii) the\nperturbation vanishes at the boundary of the domain. A current is gauge\ninvariant if the variation in them induced by any gauge perturbation vanishes\nup to boundary terms.", "category": "gr-qc" }, { "text": "Gravitomagnetic effects: This paper contains a review of the theory and practice of gravitomagnetism,\nwith particular attention to the different and numerous proposals which have\nbeen put forward to experimentally or observationally verify its effects. The\nbasics of the gravitoelectromagnetic form of the Einstein equations is\nexpounded. Then the Lense-Thirring and clock effects are described, reviewing\nthe essentials of the theory. Space based and Earth based experiments are\nlisted. Other effects, such as the coupling of gravitomagnetism with spin, are\ndescribed and orders of magnitude are considered to give an idea of the\nfeasibility of actual experiments.", "category": "gr-qc" }, { "text": "Brown-York Energy and Radial Geodesics: We compare the Brown-York (BY) and the standard Misner-Sharp (MS) quasilocal\nenergies for round spheres in spherically symmetric space-times from the point\nof view of radial geodesics. In particular, we show that the relation between\nthe BY and MS energies is precisely analogous to that between the\n(relativistic) energy E of a geodesic and the effective (Newtonian) energy\nE_{eff} appearing in the geodesic equation, thus shedding some light on the\nrelation between the two. Moreover, for Schwarzschild-like metrics we establish\na general relationship between the BY energy and the geodesic effective\npotential which explains and generalises the recently observed connection\nbetween negative BY energy and the repulsive behaviour of geodesics in the\nReissner-Nordstrom metric. We also comment on the extension of this connection\nbetween geodesics and the quasilocal BY energy to regions inside a horizon.", "category": "gr-qc" }, { "text": "Validity of energy conditions of matter in traversable wormholes under\n the $f(Q)$ modified gravity theory: In the framework of the theory of general relativity, in order to obtain\nstable traversable wormholes, matter needs to violate the null energy\ncondition. It is well known that the violation of the energy condition (EC) of\nmatter leads to various physical problems. To address this issue, researchers\nhave turned their attention to exploring modified theories of gravity, aiming\nto avoid the violation of ECs by introducing geometric terms. In this paper,\nwithin the framework of the $f(Q)$ modified gravitational theory, we\ninvestigate the effectiveness of ECs for matter in traversable wormholes. We\nexamine the compliance of four types of energy conditions (weak energy\ncondition, null energy condition, dominant energy condition, and strong energy\ncondition) in the model by selecting a power-law model for $f(Q)$ and\nconsidering different shape functions $b(r)$. Our study reveals that for\ntraversable wormholes realized through the $f(Q)$ modified gravity theory using\nthe power-law model $f(Q)=a(-Q)^n$, all four types of ECs for matter can be\nsatisfied. There is no need to introduce exotic matter (violating the null\nenergy condition) or special matter (violating other energy conditions)\nartificially in the physics of wormholes.", "category": "gr-qc" }, { "text": "Dark Matter and Charged Exotic Dust: The density profiles of dark matter halos are often modeled by an approximate\nsolution to the isothermal Lane-Emden equation with suitable boundary\nconditions at the origin. It is shown here that such a model corresponds to an\nexact solution of the Einstein-Maxwell equations for exotic charged dust. It is\nalso shown that, because of its necessarily very small charge to mass ratio,\nthe fact that the particles are charged does not necessarily rule out such\nmaterial as a candidate for dark matter.", "category": "gr-qc" }, { "text": "Localization of gravitational field energy and a procedure proposed for\n its experimental verification: Introduction of Vaidya metrics into the Expansive Nondecelerative Universe\nmodel allows to localize the energy of gravitational field. On the assumption\nthat there is an interaction of long-range gravitational and electromagnetic\nfields, the localization might be verified experimentally. In this contribution\nsome details on such an experiment are given.", "category": "gr-qc" }, { "text": "Quantum Observables Algebras and Abstract Differential Geometry: The\n Topos-Theoretic Dynamics of Diagrams of Commutative Algebraic Localizations: We construct a sheaf-theoretic representation of quantum observables algebras\nover a base category equipped with a Grothendieck topology, consisting of\nepimorphic families of commutative observables algebras, playing the role of\nlocal arithmetics in measurement situations. This construction makes possible\nthe adaptation of the methodology of Abstract Differential Geometry (ADG), a la\nMallios, in a topos-theoretic environment, and hence, the extension of the\n\"mechanism of differentials\" in the quantum regime. The process of gluing\ninformation, within diagrams of commutative algebraic localizations, generates\ndynamics, involving the transition from the classical to the quantum regime,\nformulated cohomologically in terms of a functorial quantum connection, and\nsubsequently, detected via the associated curvature of that connection.", "category": "gr-qc" }, { "text": "Group field theory for quantum gravity minimally coupled to a scalar\n field: We construct a group field theory model for quantum gravity minimally coupled\nto relativistic scalar fields, defining as well a corresponding discrete\ngravity path integral (and, implicitly, a coupled spin foam model) in its\nFeynman expansion. We also analyze a number of variations of the same model,\nthe corresponding discrete gravity path integrals, its generalization to the\ncoupling of multiple scalar fields and discuss its possible applications to the\nextraction of effective cosmological dynamics from the full quantum gravity\nformalism, in the context of group field theory condensate cosmology.", "category": "gr-qc" }, { "text": "Model-independent search for the quasinormal modes of gravitational wave\n echoes: Postmerger gravitational wave echoes provide a unique opportunity to probe\nthe near-horizon structure of astrophysical black holes, which may be modified\ndue to nonperturbative quantum gravity phenomena. However, since the waveform\nis subject to large theoretical uncertainties, it is necessary to develop\nsearch methods that are less reliant on specific models for detecting echoes\nfrom observational data. A promising strategy is to identify the characteristic\nquasinormal modes (QNMs) associated with echoes, {\\it in frequency space},\nwhich complements existing searches of quasiperiodic pulses in time. In this\nstudy, we build upon our previous work targeting these modes by incorporating\nrelative phase information to optimize the Bayesian search algorithm. Using a\nnew phase-marginalized likelihood, the performance can be significantly\nimproved for well-resolved QNMs. This enables an efficient search for QNMs of\nvarious shapes, utilizing a simple search template that is independent of\nspecific models. To demonstrate the robustness of the search algorithm, we\nconstruct four complementary benchmarks for the echo waveform that span a\ndiverse range of different theoretical possibilities for the near-horizon\nstructure. We then validate our Bayesian search algorithms by injecting the\nbenchmark models into different realizations of Gaussian noise. Using two types\nof phase-marginalized likelihoods, we find that the search algorithm can\nefficiently detect the corresponding QNMs. Therefore, our search strategy\nprovides a concrete Bayesian and model-independent approach to \"quantum black\nhole seismology.\"", "category": "gr-qc" }, { "text": "Naked Singularities for the Einstein Vacuum Equations: The Interior\n Solution: In a previous work [I. Rodnianski and Y. Shlapentokh-Rothman, Naked\nSingularities for the Einstein Vacuum Equations: The Exterior Solution,\narXiv:1912.08478] we constructed solutions to the Einstein vacuum equations in\n3+1 dimensions which corresponded to the exterior region of a naked\nsingularity. In this work we construct solutions which correspond to the\ninterior region and show that the two solutions may be glued together to\nproduce a naked singularity. Fundamental to our construction is the novel type\nof self-similarity for the Einstein vacuum equations that we introduced in our\nprevious work and also the study of a new class of quasilinear PDE's of mixed\ndegenerate elliptic-hyperbolic type.", "category": "gr-qc" }, { "text": "Measuring precession in asymmetric compact binaries: Gravitational-wave observations of merging compact binaries hold the key to\nprecision measurements of the objects' masses and spins. General-relativistic\nprecession, caused by spins misaligned with the orbital angular momentum, is\nconsidered a crucial tracer for determining the binary's formation history and\nenvironment, and it also improves mass estimates -- its measurement is\ntherefore of particular interest with wide-ranging implications. Precession\nleaves a characteristic signature in the emitted gravitational-wave signal that\nis even more pronounced in binaries with highly unequal masses. The recent\nobservations of GW190412 and GW190814 have confirmed the existence of such\nasymmetric compact binaries. Here, we perform a systematic study to assess the\nconfidence in measuring precession in gravitational-wave observations of high\nmass ratio binaries and, our ability to measure the mass of the lighter\ncompanion in neutron star -- black hole type systems. Using Bayesian model\nselection, we show that precession can be decisively identified for low-mass\nbinaries with mass ratios as low as $1:3$ and mildly precessing spins with\nmagnitudes $\\lesssim 0.4$, even in the presence of systematic waveform errors.", "category": "gr-qc" }, { "text": "Static black hole solutions with axial symmetry: We construct a new class of asymptotically flat black hole solutions in\nEinstein-Yang-Mills and Einstein-Yang-Mills-dilaton theory. These black hole\nsolutions are static, and they have a regular event horizon. However, they\npossess only axial symmetry. Like their regular counterparts, the black hole\nsolutions are characterized by two integers, the winding number $n$ and the\nnode number $k$ of the gauge field functions.", "category": "gr-qc" }, { "text": "Energetics of the Quantum Graphity Universe: Quantum graphity is a background independent model for emergent geometry, in\nwhich space is represented as a complete graph. The high-energy pre-geometric\nstarting point of the model is usually considered to be the complete graph,\nhowever we also consider the empty graph as a candidate pre-geometric state.\nThe energetics as the graph evolves from either of these high-energy states to\na low-energy geometric state is investigated as a function of the number of\nedges in the graph. Analytic results for the slope of this energy curve in the\nhigh-energy domain are derived, and the energy curve is plotted exactly for\nsmall number of vertices $N$. To study the whole energy curve for larger (but\nstill finite) $N$, an epitaxial approximation is used. It is hoped that this\nwork may open the way for future work to compare predictions from quantum\ngraphity with observations of the early universe, making the model falsifiable.", "category": "gr-qc" }, { "text": "Cosmic strings in axionic-dilatonic gravity: We first consider local cosmic strings in dilaton-axion gravity and show that\nthey are singular solutions. Then we take a supermassive Higgs limit and\npresent expressions for the fields at far distances from the core by applying a\nPecci-Quinn and a duality transformation to the dilatonic Melvin's magnetic\nuniverse.", "category": "gr-qc" }, { "text": "The role of shell crossing on the existence and stability of trapped\n matter shells in spherical inhomogeneous \u039b-CDM models: We analyse the dynamics of trapped matter shells in spherically symmetric\ninhomogeneous \\Lambda-CDM models. The investigation uses a Generalised\nLema\\^itre-Tolman-Bondi description with initial conditions subject to the\nconstraints of having spatially asymptotic cosmological expansion, initial\nHubble-type flow and a regular initial density distribution. We discuss the\neffects of shell crossing and use a qualitative description of the local\ntrapped matter shells to explore global properties of the models. Once shell\ncrossing occurs, we find a splitting of the global shells separating expansion\nfrom collapse into, at most, two global shells: an inner and an outer limit\ntrapped matter shell. In the case of expanding models, the outer limit trapped\nmatter shell necessarily exists. We also study the role of shear in this\nprocess, compare our analysis with the Newtonian framework and give concrete\nexamples using density profile models of structure formation in cosmology.", "category": "gr-qc" }, { "text": "Modeling Ringdown: Beyond the Fundamental Quasi-Normal Modes: While black hole perturbation theory predicts a rich quasi-normal mode\nstructure, technical challenges have limited the numerical study of excitations\nto the fundamental, lowest order modes caused by the coalescence of black\nholes. Here, we present a robust method to identify quasi-normal mode\nexcitations beyond the fundamentals within currently available numerical\nrelativity waveforms. In applying this method to waveforms of initially\nnon-spinning black hole binaries, of mass ratios 1 to 15, we find not only the\nfundamental quasi-normal mode amplitudes, but also overtones, and evidence for\n2nd order quasi-normal modes. We find that the mass-ratio dependence of\nquasi-normal mode excitation is very well modeled by a Post-Newtonian like sum\nin symmetric mass ratio. Concurrently, we find that the mass ratio dependence\nof some quasi-normal modes is qualitatively different from their Post-Newtonian\ninspired counterparts, suggesting that the imprints of nonlinear merger are\nmore evident in some modes than in others. We present new fitting formulas for\nthe related quasi-normal mode excitations, as well as for remnant black hole\nspin and mass. We also discuss the relevance of our results in terms of\ngravitational wave detection and characterization.", "category": "gr-qc" }, { "text": "Time-dependent mass of cosmological perturbations in the hybrid and\n dressed metric approaches to loop quantum cosmology: Loop quantum cosmology has recently been applied in order to extend the\nanalysis of primordial perturbations to the Planck era and discuss the possible\neffects of quantum geometry on the cosmic microwave background. Two approaches\nto loop quantum cosmology with admissible ultraviolet behavior leading to\npredictions that are compatible with observations are the so-called hybrid and\ndressed metric approaches. In spite of their similarities and relations, we\nshow in this work that the effective equations that they provide for the\nevolution of the tensor and scalar perturbations are somewhat different. When\nbackreaction is neglected, the discrepancy appears only in the time- dependent\nmass term of the corresponding field equations. We explain the origin of this\ndifference, arising from the distinct quantization procedures. Besides, given\nthe privileged role that the big bounce plays in loop quantum cosmology, e.g.\nas a natural instant of time to set initial conditions for the perturbations,\nwe also analyze the positivity of the time-dependent mass when this bounce\noccurs. We prove that the mass of the tensor perturbations is positive in the\nhybrid approach when the kinetic contribution to the energy density of the\ninflaton dominates over its potential, as well as for a considerably large\nsector of backgrounds around that situation, while this mass is always\nnonpositive in the dressed metric approach. Similar results are demonstrated\nfor the scalar perturbations in a sector of background solutions that includes\nthe kinetically dominated ones; namely, the mass then is positive for the\nhybrid approach, whereas it typically becomes negative in the dressed metric\ncase. More precisely, this last statement is strictly valid when the potential\nis quadratic for values of the inflaton mass that are phenomenologically\nfavored.", "category": "gr-qc" }, { "text": "Covariant Renormalizable Gravity Theories on (Non) Commutative Tangent\n Bundles: The field equations in modified gravity theories possess an important\ndecoupling property with respect to certain classes of nonholonomic frames.\nThis allows us to construct generic off--diagonal solutions depending on all\nspacetime coordinates via generating and integration functions containing\n(un--)broken symmetry parameters. Some corresponding analogous models have a\nnice ultraviolet behavior and seem to be (super) renormalizable in a sense of\ncovariant modifications of Ho\\v{r}ava--Lifshits (HL) and ghost free gravity.\nThe apparent noncommutativity and breaking of Lorentz invariance by quantum\neffects can be encoded into geometric objects and basic equations on\nnoncommutative tangent Lorentz. The constructions can be extended to include\nconjectured covariant reonormalizable models with effective Einstein fields\nwith (non)commutative variables.", "category": "gr-qc" }, { "text": "Vacuum polarization of massive fields in the spacetime of the\n higher-dimensional black holes: We construct and study the vacuum polarization, $\\langle\n\\phi^{2}\\rangle_{D},$ of the quantized massive scalar field with a general\ncurvature coupling parameter in higher-dimensional static and\nspherically-symmetric black hole spacetimes, with a special emphasis put on the\nelectrically charged Tangherlini solutions and the extremal and ultraextremal\nconfigurations. For $4 \\leq D \\leq 7$ the explicit analytic expressions for the\nvacuum polarization are given. For the conformally coupled fields the relation\nbetween the trace of the stress-energy tensor and the vacuum polarization is\nexamined, which requires knowledge of the higher-order terms in the\nSchwinger-DeWitt expansion.", "category": "gr-qc" }, { "text": "A new general purpose event horizon finder for 3D numerical spacetimes: I present a new general purpose event horizon finder for full 3D numerical\nspacetimes. It works by evolving a complete null surface backwards in time. The\nnull surface is described as the zero level set of a scalar function, that in\nprinciple is defined everywhere. This description of the surface allows the\nsurface, trivially, to change topology, making this event horizon finder able\nto handle numerical spacetimes, where two (or more) black holes merge into a\nsingle final black hole.", "category": "gr-qc" }, { "text": "Darboux class of cosmological fluids with time-dependent adiabatic\n indices: A one-parameter family of time dependent adiabatic indices is introduced for\nany given type of cosmological fluid of constant adiabatic index by a\nmathematical method belonging to the class of Darboux transformations. The\nprocedure works for zero cosmological constant at the price of introducing a\nnew constant parameter related to the time dependence of the adiabatic index.\nThese fluids can be the real cosmological fluids that are encountered at\ncosmological scales and they could be used as a simple and efficient\nexplanation for the recent experimental findings regarding the present day\naccelerating universe. In addition, new types of cosmological scale factors,\ncorresponding to these fluids, are presented", "category": "gr-qc" }, { "text": "Killing vectors and anisotropy: We consider an action that can generate fluids with three unequal stresses\nfor metrics with a spacelike Killing vector. The parameters in the action are\ndirectly related to the stress anisotropies. The field equations following from\nthe action are applied to an anisotropic cosmological expansion and an\nextension of the Gott-Hiscock cosmic string.", "category": "gr-qc" }, { "text": "Torsion Dilaton and Novel Minimal Coupling Principle: We propose a novel self consistent minimal coupling principle in presence of\ntorsion dilaton field. This principle yields a new local dilatation symmetry\nand predicts the interactions of torsion dilaton with the real matter and with\nmetric. The soft violation of this symmetry yields a physical dilaton and a\nsimple relation between Cartan scalar curvature and cosmological constant in\nthis new model of gravity with propagating torsion. Its relation with\nscalar-tensor theories of gravity and a possible use of torsion dilaton in the\ninflation scenario is discussed.\n \\noindent{PACS number(s): 04.50.+h, 04.40.Nr, 04.62.+v}", "category": "gr-qc" }, { "text": "Skyrme Fluid in Anisotropic Universe: Cosmological solutions are obtained in an anisotropic Kantowski-Sachs (KS)\nand Bianchi Type-I universes considering a cosmological constant with Skyrme\nfluid. Interestingly, the solutions obtained here in both the KS and Bianchi-I\nanisotropic universes are found isotropize at late time due to the presence of\nthe Skyrme fluid even in the absence of $\\Lambda$ term or any inflationary\nmechanism involving the inflaton field. A comparative study of both the\nanisotropic cosmological models are carried out here and found that Bianchi-I\nuniverse admits oscillatory solutions for a given matter configuration. We also\nnote that the emergent universe model can be obtained with Skyrme fluid. The\nanisotropy, deceleration and jerk parameters have been studied along with the\nlinear perturbative stability to explore efficacy of the model. Both the\ncosmological models are stable in the absence of cosmological constant besides\ntheir compatibility with observational data. Thus, we claim Skyrme fluid a\npossible source for isotropization of an anisotropic universe via accelerated\nexpansion, which is capable of reproducing some observed features of the\nuniverse.", "category": "gr-qc" }, { "text": "Gravitational Goldstone fields from affine gauge theory: In order to facilitate the application of standard renormalization\ntechniques, gravitation should be decribed, if possible, in pure connection\nformalism, as a Yang-Mills theory of a certain spacetime group, say the\nPoincare or the affine group. This embodies the translational as well as the\nlinear connection. However, the coframe is not the standard Yang-Mills type\ngauge field of the translations, since it lacks the inhomogeneous gradient term\nin the gauge transformations. By explicitly restoring the \"hidden\" piece\nresponsible for this behavior within the framework of nonlinear realizations,\nthe usual geometrical interpretation of the dynamical theory becomes possible,\nand in addition one can avoid the metric or coframe degeneracy which would\notherwise interfere with the integrations within the path integral. We claim\nthat nonlinear realizations provide a general mathematical scheme clarifying\nthe foundations of gauge theories of spacetime symmetries. When applied to\nconstruct the Yang-Mills theory of the affine group, tetrads become identified\nwith nonlinear translational connections; the anholonomic metric does not\nconstitute any more an independent gravitational potential, since its degrees\nof freedom reveal to correspond to eliminable Goldstone bosons. This may be an\nimportant advantage for quantization.", "category": "gr-qc" }, { "text": "The origins of length contraction: I. The FitzGerald-Lorentz deformation\n hypothesis: One of the widespread confusions concerning the history of the 1887\nMichelson-Morley experiment has to do with the initial explanation of this\ncelebrated null result due independently to FitzGerald and Lorentz. In neither\ncase was a strict, longitudinal length contraction hypothesis invoked, as is\ncommonly supposed. Lorentz postulated, particularly in 1895, any one of a\ncertain family of possible deformation effects for rigid bodies in motion,\nincluding purely transverse alteration, and expansion as well as contraction;\nFitzGerald may well have had the same family in mind. A careful analysis of the\nMichelson-Morley experiment (which reveals a number of serious inadequacies in\nmany text-book treatments) indeed shows that strict contraction is not\nrequired.", "category": "gr-qc" }, { "text": "Gibbs' paradox and black-hole entropy: In statistical mechanics Gibbs' paradox is avoided if the particles of a gas\nare assumed to be indistinguishable. The resulting entropy then agrees with the\nempirically tested thermodynamic entropy up to a term proportional to the\nlogarithm of the particle number. We discuss here how analogous situations\narise in the statistical foundation of black-hole entropy. Depending on the\nunderlying approach to quantum gravity, the fundamental objects to be counted\nhave to be assumed indistinguishable or not in order to arrive at the\nBekenstein--Hawking entropy. We also show that the logarithmic corrections to\nthis entropy, including their signs, can be understood along the lines of\nstandard statistical mechanics. We illustrate the general concepts within the\narea quantization model of Bekenstein and Mukhanov.", "category": "gr-qc" }, { "text": "Cohomogeneity-1 solutions in Einstein-Maxwell-dilaton gravity: The field equations for Einstein-Maxwell-dilaton gravity in $D$ dimensions\nare reduced to an effective one-dimensional system under the influence of\nexponential potentials. Various cases where exact solutions can be found are\nexplored. With this procedure, we present interesting solutions such as a\none-parameter generalisation of the dilaton-Melvin spacetime and a\nthree-parameter solution that interpolates between the Reissner-Nordstr\\\"{o}m\nand Bertotti-Robinson solution. This procedure also allows simple, alternative\nderivations of known solutions such as the Lifshitz spacetime and the planar\nAnti-de Sitter naked singularity. In the latter case, the metric is cast in a\nsimpler form which reveals the presence of an additional curvature singularity.", "category": "gr-qc" }, { "text": "Inflationary magnetogenesis in the perturbative regime: While during inflation a phase of increasing gauge coupling allows for a\nscale-invariant hyperelectric spectrum, when the coupling decreases a flat\nhypermagnetic spectrum can be generated for typical wavelengths larger than the\neffective horizon. After the gauge coupling flattens out the late-time\nhypermagnetic power spectra outside the horizon in the radiation epoch are\ndetermined by the hyperelectric fields at the end of inflation whereas the\nopposite is true in the case of decreasing coupling. Instead of imposing an\nabrupt freeze after inflation, we consider a smooth evolution of the mode\nfunctions by positing that the gauge couplings and their conformal time\nderivatives are always continuous together with the background extrinsic\ncurvature. The amplified gauge power spectra are classified according to their\ntransformation properties under the duality symmetry. After clarifying the role\nof the comoving and of the physical spectra in the formulation of the relevant\nmagnetogenesis constraints, the parameter space of the scenario is scrutinized.\nIt turns out that a slightly blue hyperelectric spectrum during inflation may\nlead to a quasi-flat hypermagnetic spectrum prior to matter radiation equality\nand before the relevant wavelengths reenter the effective horizon. In this\nframework the gauge coupling is always perturbative but the induced large-scale\nmagnetic fields can be of the order of a few hundredths of a nG and over\ntypical length scales between a fraction of the Mpc and $100$ Mpc prior to the\ngravitational collapse of the protogalaxy.", "category": "gr-qc" }, { "text": "Interaction between Tachyon and Hessence (or Hantom) dark energies: In this paper, we have considered that the universe is filled with tachyon,\nhessence (or hantom) dark energies. Subsequently we have investigated the\ninteractions between tachyon and hessence (hantom) dark energies and calculated\nthe potentials considering the power law form of the scale factor. It has been\nrevealed that the tachyonic potential always decreases and hessence (or hantom)\npotential increases with corresponding fields. Furthermore, we have considered\na correspondence between the hessence (or hantom) dark energy density and new\nvariable modified Chaplygin gas energy density. From this, we have found the\nexpressions of the arbitrary positive constants B0 and C of new variable\nmodified Chaplygin gas.", "category": "gr-qc" }, { "text": "Experimentally obtaining metrics in general relativity: It seems to be not well known that the metrics of general relativity (GR) can\nbe obtained without integrating Einstein equations. To that, we need only\ndefine a unit for GR-interval $\\Delta s$, and observe 10 geodesics (out of\nwhich at least one must be nonnull). Even without using any unit, we can have\n$\\kappa g_{\\mu\\nu}(x^\\rho)$, where $\\kappa=$const. Our notes attempt to\nsimplify the articles of E. Kretschmann (1917) and of H.A. Lorentz (1923) about\nthis last subject. The text of this article in English will soon be available,\nin LaTeX. Please ask the author.\n -----\n /Sajne estas malmulte konata ke la metrikoj de /generala relativeco (/GR)\npovas esti havataj sen integri Einstein-ajn ekvaciojn. Por tio, ni bezonas\ndifini nur unuon por /GR-tempo $\\Delta s$, kaj observi 10 geodezajn (el kiuj,\nalmena/u unu devas esti nenulan). E/c sen uzi iun unuon, ni povas havi $\\kappa\ng_{\\mu\\nu}(x^\\rho)$, kie $\\kappa$=konst. Niaj notoj tentas simpligi la\nartikolojn de E. Kretschmann (1917) kaj de H.A. Lorentz (1923) pri tiu lasta\nafero.", "category": "gr-qc" }, { "text": "Sharpening the dark matter signature in gravitational waveforms II:\n Numerical simulations with the NbodyIMRI code: Future gravitational wave observatories can probe dark matter by detecting\nthe dephasing in the waveform of binary black hole mergers induced by dark\nmatter overdensities. Such a detection hinges on the accurate modelling of the\ndynamical friction, induced by dark matter on the secondary compact object in\nintermediate and extreme mass ratio inspirals. In this paper, we introduce\nNbodyIMRI, a new publicly available code designed for simulating binary systems\nwithin cold dark matter `spikes'. Leveraging higher particle counts and finer\ntimesteps, we validate the applicability of the standard dynamical friction\nformalism and provide an accurate determination of the maximum impact parameter\nof particles which can effectively scatter with a compact object, across\nvarious mass ratios. We also show that in addition to feedback due to dynamical\nfriction, the dark matter also evolves through a `stirring' effect driven by\nthe time-dependent potential of the binary. We introduce a simple\nsemi-analytical scheme to account for this effect and demonstrate that\nincluding stirring tends to slow the rate of dark matter depletion and\ntherefore enhances the impact of dark matter on the dynamics of the binary.", "category": "gr-qc" }, { "text": "Dynamical photon sphere and time evolving shadow around black holes with\n temporal accretion: A photon sphere is known as the geometrical structure shaping a black hole\nshadow. The mechanism is well understood for static or stationary black hole\nspacetimes such as the Schwarzschild and the Kerr spacetimes. In this paper, we\ninvestigate and explicitly specify a photon sphere that shapes a black hole\nshadow in a dynamical spacetime while taking the global structure of the\nspacetime into account. We consider dynamical and eternal black hole cases of\nthe Vaidya spacetime, which represents a spherically symmetric black hole with\naccreting null dust. First, we numerically show that there are the dynamical\nphoton sphere and photon orbits corresponding to the shadow edge in a moderate\naccretion case. Second, the photon spheres are derived analytically in special\ncases. Finally, we discuss the relation between our photon sphere and the\nseveral notions defined as a photon sphere generalization.", "category": "gr-qc" }, { "text": "Bound gravitational waves in a dielectric medium and a constant magnetic\n field: A description is made of the process of excitation of bound\nlongitudinal-transverse gravitational waves during the propagation of\nelectromagnetic waves in a dielectric medium. It is shown that the speed of\nsuch gravitational waves is less than the speed of light in a vacuum and\ncoincides with the speed of an electromagnetic wave in matter. A description of\nthe propagation of a bound gravitational waves in a dielectric in the presence\nof a constant magnetic field is suggested as well. It is claimed that these\ngravitational waves in a dielectric medium are forced ones and they cannot\nexist in a free state.", "category": "gr-qc" }, { "text": "Hamiltonian formulation of gravity as a spontaneously-broken gauge\n theory of the Lorentz group: A number of approaches to gravitation have much in common with the gauge\ntheories of the standard model of particle physics. In this paper, we develop\nthe Hamiltonian formulation of a class of gravitational theories that may be\nregarded as spontaneously-broken gauge theories of the complexified Lorentz\ngroup $SO(1,3)_C$ with the gravitational field described entirely by a gauge\nfield valued in the Lie algebra of $SO(1,3)_C$ and a `Higgs field' valued in\nthe group's fundamental representation. The theories have one free parameter\n$\\beta$ which appears in a similar role to the inverse of the Barbero-Immirzi\nparameter of Einstein-Cartan theory. However, contrary to that parameter, it is\nshown that the number of degrees of freedom crucially depends on the value of\n$\\beta$. For non-zero values of $\\beta$, it is shown that three complex degrees\nof freedom propagate on general backgrounds, and for the specific values\n$\\beta=\\pm i$ an extension to General Relativity is recovered in a\nsymmetry-broken regime. For the value $\\beta=0$, the theory propagates no local\ndegrees of freedom. A non-zero value of $\\beta$ corresponds to the self-dual\nand anti-self-dual gauge fields appearing asymmetrically in the action,\ntherefore in these models, the existence of gravitational degrees of freedom is\ntied to chiral asymmetry in the gravitational sector.", "category": "gr-qc" }, { "text": "Quantization of Midisuperspace Models: We give a comprehensive review of the quantization of midisuperspace models.\nThough the main focus of the paper is on quantum aspects, we also provide an\nintroduction to several classical points related to the definition of these\nmodels. We cover some important issues, in particular, the use of the principle\nof symmetric criticality as a very useful tool to obtain the required\nHamiltonian formulations. Two main types of reductions are discussed: those\ninvolving metrics with two Killing vector fields and spherically symmetric\nmodels. We also review the more general models obtained by coupling matter\nfields to these systems. Throughout the paper we give separate discussions for\nstandard quantizations using geometrodynamical variables and those relying on\nloop quantum gravity inspired methods.", "category": "gr-qc" }, { "text": "From spinning to non-spinning cosmic string spacetimes: We analyse the properties of a fluid generating a spinning cosmic string\nspacetime with flat limiting cases corresponding to a constant angular momentum\nin the infinite past and static configuration in the infinite future. The\nspontaneous loss of angular momentum of a spinning cosmic string due to\nparticle emission is discussed. The rate of particle production between the\nspinning and non-spinning cosmic string spacetimes is calculated.", "category": "gr-qc" }, { "text": "Moving mirrors and black hole evaporation in non-commutative space-times: We study the evaporation of black holes in non-commutative space-times. We do\nthis by calculating the correction to the detector's response function for a\nmoving mirror in terms of the noncommutativity parameter $\\Theta$ and then\nextracting the number density as modified by this parameter. We find that\nallowing space and time to be non-commutative increases the decay rate of a\nblack hole.", "category": "gr-qc" }, { "text": "Exact black hole solutions in higher-order scalar-tensor theories: In this chapter, we discuss explicit black hole solutions in higher-order\nscalar-tensor theories. After a brief recap of no-hair theorems, we start our\ndiscussion by so-called stealth solutions present in theories with parity and\nshift symmetry. Stealth solutions are such that their metric are Ricci flat\nGeneral Relativity solutions, but they are accompanied by a non-trivial scalar\nfield, in both spherically-symmetric and rotating cases. The stealth metrics\nthen enable to construct an analytic stationary solution of scalar-tensor\ntheory which is called disformed Kerr metric. This solution constitutes a\nmeasurable departure from the usual Kerr geometry of GR. We discuss within\nparity and shift symmetric theories several non-stealth solutions. We then\nconsider scalar-tensor theories stemming from a Kaluza-Klein reduction of a\nhigher-dimensional Lovelock theory. These theories encompass all Horndeski\nfunctionals and hence go beyond parity and shift symmetry. Reduction and\nsingular limits allow one to obtain non-stealth black holes with differing\ninteresting properties which are not Ricci flat metrics. We analyse the\nsolutions obtained and classify them with respect to the geometry of the\ninternal space according to their Kaluza-Klein origin.", "category": "gr-qc" }, { "text": "Extending the generalized Chaplygin gas model by using\n geometrothermodynamics: We use the formalism of geometrothermodynamics (GTD) to derive fundamental\nthermodynamic equations that are used to construct general relativistic\ncosmological models. In particular, we show that the simplest possible\nfundamental equation, which corresponds in GTD to a system with no internal\nthermodynamic interaction, describes the different fluids of the standard model\nof cosmology. In addition, a particular fundamental equation with internal\nthermodynamic interaction is shown to generate a new cosmological model that\ncorrectly describes the dark sector of the Universe and contains as a special\ncase the generalized Chaplygin gas model.", "category": "gr-qc" }, { "text": "Spinning ring wormholes: a classical model for elementary particles?: Static horizonless solutions to the Einstein--Maxwell field equations, with\nonly a circular cosmic string singularity, are extended to exact rotating\nasymptotically flat solutions. The possible interpretation of these field\nconfigurations as spinning elementary particles or as macroscopic rotating\ncosmic rings is discussed.", "category": "gr-qc" }, { "text": "A Quantum-Driven Time (QDT) Quantization of the Taub Cosmology: We present here an application of a new quantization scheme. We quantize the\nTaub cosmology by quantizing only the anisotropy parameter $\\beta$ and imposing\nthe super-Hamiltonian constraint as an expectation-value equation to recover\nthe relationship between the scale factor $\\Omega$ and the time $t$. This\napproach appears to avoid the problem of time. (Paper to appear in the Seventh\nMarcel Grossmann Conference Proceedings.)", "category": "gr-qc" }, { "text": "Hyperbolicity of the BSSN system of Einstein evolution equations: We discuss an equivalence between the Baumgarte-Shapiro-Shibata-Nakamura\n(BSSN) formulation of the Einstein evolution equations, a subfamiliy of the\nKidder--Scheel--Teukolsky formulation, and other strongly or symmetric\nhyperbolic first order systems with fixed shift and densitized lapse. This\nallows us to show under which conditions the BSSN system is, in a sense to be\ndiscussed, hyperbolic. This desirable property may account in part for the\nempirically observed better behavior of the BSSN formulation in numerical\nevolutions involving black holes.", "category": "gr-qc" }, { "text": "Event horizons and apparent horizons in spherically symmetric geometries: Spherical configurations that are very massive must be surrounded by apparent\nhorizons. These in turn, when placed outside a collapsing body, must propagate\noutward with a velocity equal to the velocity of radially outgoing photons.\nThat proves, within the framework of (1+3) formalism and without resorting to\nthe Birkhoff theorem, that apparent horizons coincide with event horizons.", "category": "gr-qc" }, { "text": "Spin-driven inflation: Following recent studies of Ford, we suggest -- in the framework of general\nrelativity -- an inflationary cosmological model with the self-interacting\nspinning matter. A generalization of the standard fluid model is discussed and\nestimates of the physical parameters of the evolution are given.", "category": "gr-qc" }, { "text": "Weyl geometry and gauge-invariant gravitation: We provide a gauge-invariant theory of gravitation in the context of Weyl\nIntegrable Space-Times. After making a brief review of the theory's postulates,\nwe carefully define the observers' proper-time and point out its relation with\nspace-time description. As a consequence of this relation and the theory's\ngauge symmetry we recover all predictions of General Relativity. This feature\nis made even clearer by a new exact solution we provide which reveals the\nimportance of a well defined proper-time. The thermodynamical description of\nthe source fields is given and we observe that each of the geometric fields\nhave a certain physical significance, despite the gauge-invariance. This is\nshown by two examples, where one of them consists of a new cosmological\nconstant solution. Our conclusions highlight the intimate relation among test\nparticles trajectories, proper-time and space-time description which can also\nbe applied in any other situation, whether or not it recovers General\nRelativity results and also in the absence of a gauge symmetry.", "category": "gr-qc" }, { "text": "An ASSF and the Teleparallelisim: This paper has been removed by arXiv administrators because it plagiarizes\ngr-qc/0203084, gr-qc/0607138, gr-qc/0011087, gr-qc/0102070, gr-qc/0607138,\ngr-qc/0109017, gr-qc/0212018, and gr-qc/9409039.", "category": "gr-qc" }, { "text": "Research of Gravitation in Flat Minkowski Space: In this paper it is introduced and studied an alternative theory of\ngravitation in flat Minkowski space. Using an antisymmetric tensor, which is\nanalogous to the tensor of electromagnetic field, a non-linear connection is\nintroduced. It is very convenient for studying the perihelion/periastron shift,\ndeflection of the light rays near the Sun and the frame dragging together with\ngeodetic precession, i.e. effects where angles are involved. Although the\ncorresponding results are obtained in rather different way, they are the same\nas in the General Relativity. The results about the barycenter of two bodies\nare also the same as in the General Relativity. Comparing the derived equations\nof motion for the $n$-body problem with the Einstein-Infeld-Hoffmann equations,\nit is found that they differ from the EIH equations by Lorentz invariant terms\nof order $c^{-2}$.", "category": "gr-qc" }, { "text": "The Hawking-Page-like Phase Transition from FRW Spacetime to McVittie\n Black Hole: In this paper, we investigate the thermodynamics especially the\nHawking-Page-like phase transition of the McVittie space-time. We formulate the\nfirst law of thermodynamics for the McVittie black hole, and find that the work\ndensity $W$ of the perfect fluid plays the role of the thermodynamic pressure,\ni.e. $P$:=$-W$. We also construct the thermodynamic equation of state for the\nMcVittie black hole. Most importantly, by analysing the Gibbs free energy, we\nfind that the Hawking-Page-like phase transition from FRW spacetime to McVittie\nblack hole is possible in the case $P>0$.", "category": "gr-qc" }, { "text": "Cubic Derivative Interactions and Asymptotic Dynamics of the Galileon\n Vacuum: In this paper we apply the tools of the dynamical systems theory in order to\nuncover the whole asymptotic structure of the vacuum interactions of a galileon\nmodel with a cubic derivative interaction term. It is shown that, contrary to\nwhat occurs in the presence of background matter, the galileon interactions of\nvacuum appreciably modify the late-time cosmic dynamics. In particular, a local\nlate-time attractor representing phantom behavior arises which is inevitably\nassociated with a big rip singularity. It seems that the gravitational\ninteractions of the background matter with the galileon screen the effects of\nthe gravitational self-interactions of the galileon, thus erasing any potential\nmodification of the late-time dynamics by the galileon vacuum processes. Unlike\nother galileon models inspired in the DGP scenario, self-accelerating solutions\ndo not arise in this model.", "category": "gr-qc" }, { "text": "LISA detections of massive black hole inspirals: parameter extraction\n errors due to inaccurate template waveforms: The planned Laser Interferometer Space Antenna (LISA) is expected to detect\nthe inspiral and merger of massive black hole binaries (MBHBs) at z <~ 5 with\nsignal-to-noise ratios (SNRs) of hundreds to thousands. Because of these high\nSNRs, and because these SNRs accrete over periods of weeks to months, it should\nbe possible to extract the physical parameters of these systems with high\naccuracy; for instance, for a ~ 10^6 Msun MBHBs at z = 1 it should be possible\nto determine the two masses to ~ 0.1% and the sky location to ~ 1 degree.\nHowever, those are just the errors due to noise: there will be additional\n\"theoretical\" errors due to inaccuracies in our best model waveforms, which are\nstill only approximate. The goal of this paper is to estimate the typical\nmagnitude of these theoretical errors. We develop mathematical tools for this\npurpose, and apply them to a somewhat simplified version of the MBHB problem,\nin which we consider just the inspiral part of the waveform and neglect\nspin-induced precession, eccentricity, and PN amplitude corrections. For this\nsimplified version, we estimate that theoretical uncertainties in sky position\nwill typically be ~ 1 degree, i.e., comparable to the statistical uncertainty.\nFor the mass and spin parameters, our results suggest that while theoretical\nerrors will be rather small absolutely, they could still dominate over\nstatistical errors (by roughly an order of magnitude) for the strongest\nsources. The tools developed here should be useful for estimating the magnitude\nof theoretical errors in many other problems in gravitational-wave astronomy.", "category": "gr-qc" }, { "text": "Collapse driven by a scalar field without final singularity: We explore a collapsing cosmology driven by a scalar field which is minimally\ncoupled to gravity in a spatially at and spherically symmetric, isotropic and\nhomogeneous space-time, with a variable timescale that avoids the final\nsingularity. The equation of state that describes the collapse is $\\omega=1$.\nWe calculate the back-reaction of the space-time during the collapse and the\nenergy density fluctuations related to this back-reaction has a spectral index\n$n_s = 0$, favouring short-wavelengths modes to be detected. The interesting is\nthat the amplitude of these fluctuations increase with time when the collapse\nis sufficiently strong.", "category": "gr-qc" }, { "text": "Radial oscillations and stability of multiple-fluid compact stars: I derive a system of pulsation equations for compact stars made up of an\narbitrary number of perfect fluids that can be used to study radial\noscillations and stability with respect to small perturbations. I assume\nspherical symmetry and that the only inter-fluid interactions are\ngravitational. My derivation is in line with Chandrasekhar's original\nderivation for the pulsation equation of a single-fluid compact star and keeps\nthe contributions from the individual fluids manifest. I illustrate solutions\nto the system of pulsations equations with one-, two-, and three-fluid\nexamples.", "category": "gr-qc" }, { "text": "Bose-Einstein Condensate and Liquid Helium He$^4$: Implications of GUP\n and Modified Gravity Correspondence: Utilizing the recently established connection between Palatini-like gravity\nand linear Generalized Uncertainty Principle (GUP) models, we have formulated\nan approach that facilitates the examination of Bose gases. Our primary focus\nis on the ideal Bose-Einstein condensate and liquid helium, chosen as\nillustrative examples to underscore the feasibility of tabletop experiments in\nassessing gravity models. The non-interacting Bose-Einstein condensate imposes\nconstraints on linear GUP and Palatini $f(R)$ gravity (Eddington-inspired\nBorn-Infeld gravity) within the ranges of $-10^{12}\\lesssim\\sigma\\lesssim\n3\\times 10^{24}{\\text{ s}}/{\\text{kg m}}$ and\n$-10^{-1}\\lesssim\\bar\\beta\\lesssim 10^{11} \\text{ m}^2$\n($-4\\times10^{-1}\\lesssim\\epsilon\\lesssim 4\\times 10^{11} \\text{ m}^2$),\nrespectively. In contrast, the properties of liquid helium suggest more\nrealistic bounds, specifically $-10^{23}\\lesssim\\sigma\\lesssim 10^{23}{\\text{\ns}}/{\\text{kg m}}$ and $-10^{9}\\lesssim\\bar\\beta\\lesssim 10^{9} \\text{ m}^2$.\nAdditionally, we argue that the newly developed method employing Earth seismic\nwaves provides improved constraints for quantum and modified gravity by\napproximately one order of magnitude.", "category": "gr-qc" }, { "text": "Development of a Double Pendulum for Gravitational Wave Detectors: Seismic noise will be the dominant source of noise at low frequencies for\nground based gravitational wave detectors, such as LIGO now under construction.\nFuture interferometers installed at LIGO plan to use at least a double pendulum\nsuspension for the test masses to help filter the seismic noise. We are\nconstructing an apparatus to use as a test bed for double pendulum design. Some\nof the tests we plan to conduct include: dynamic ranges of actuators, and how\nto split control between the intermediate mass and lower test mass;\nmeasurements of seismic transfer functions; measurements of actuator and\nmechanical cross couplings; and measurements of the noise from sensors and\nactuators. All these properties will be studied as a function of mechanical\ndesign of the double pendulum.", "category": "gr-qc" }, { "text": "On the Occurrence of Finite-time Singularities in Swampland-related\n Quintessence Dark Energy Models: In this work we focus on the phase space singularities of interactive\nquintessence model in the presence of matter fluid. This model is related to\nswampland studies, that the outcomes affect all these Swampland related models\nwith the same dynamical system. We shall form the dynamical system\ncorresponding to the cosmological system, which is eventually autonomous, and\nby using the dominant balances technique we shall investigate the occurrence or\nnot of finite-time singularities. Our results indicate that the dynamical\nsystem of the model may develop finite-time singularities, but these are not\ngeneral singularities, like in the case that the matter fluids were absent, in\nwhich case singularities occurred for general initial conditions. Hence, the\npresence of matter fluids affects the dynamical system of the cosmological\nsystem, making the singularities to depend on the initial conditions, instead\nof occurring for general initial conditions.", "category": "gr-qc" }, { "text": "The third law of thermodynamics, non-extensivity, and energy definition\n in black hole physics: Working in the framework of generalized statistics, the problem of\nestablishing the third law of thermodynamics in the black hole physics is\nstudied by focusing on Schwarzschild black hole which easily and clearly\nexposes the violation of this law in the common approach based on Bekenstein\nentropy. Additionally, it is addressed that some inconsistencies between the\npredictions of quantum field theory and thermodynamics about the black hole\ntemperature may be reconciled by using the thermodynamics laws in order to\nbroaden energy definition. It claims that thermodynamics should be employed as\na powerful tool in looking for more comprehensive energy definitions in\nhigh-energy physics, still mysterious.", "category": "gr-qc" }, { "text": "Gravitational Radiation from hyperbolic encounters in the presence of\n dark matter: In this study, we look into binaries undergoing gravitational radiation\nduring a hyperbolic passage. Such hyperbolic events can be a credible source of\ngravitational waves in future detectors. We systematically calculate fluxes of\ngravitational radiation from such events in the presence of dark matter, also\nconsidering the effects of dynamical friction. We also investigate the binary\ndynamics through the changes in the orbital parameters by treating the\npotential due to dark matter spike and the dynamical friction effects as a\nperturbation term. An insight into the effects of such a medium on the binaries\nfrom the corresponding osculating elements opens up avenues to study binary\ndynamics for such events.", "category": "gr-qc" }, { "text": "Conformal symmetries and integrals of the motion in pp waves with\n external electromagnetic fields: The integrals of the motion associated with conformal Killing vectors of a\ncurved space-time with an additional electromagnetic background are studied for\nmassive particles. They involve a new term which might be non-local. The\ndifficulty disappears for pp-waves, for which explicit, local conserved charges\nare found. Alternatively, the mass can be taken into account by \"distorting\"\nthe conformal Killing vectors. The relation of these non-point symmetries to\nthe charges is analysed both in the Lagrangian and Hamiltonian approaches, as\nwell as in the framework of Eisenhart-Duval lift.", "category": "gr-qc" }, { "text": "Cosmological solutions and finite time singularities in Finslerian\n geometry: We consider a very general scenario of our universe where its geometry is\ncharacterized by the Finslerian structure on the underlying spacetime manifold,\na generalization of the Riemannian geometry. Now considering a general\nenergy-momentum tensor for matter sector, we derive the gravitational field\nequations in such spacetime. Further, to depict the cosmological dynamics in\nsuch spacetime proposing an interesting equation of state identified by a sole\nparameter $\\gamma$ which for isotropic limit is simply the barotropic equation\nof state $p= (\\gamma- 1) \\rho$ ($\\gamma \\in \\mathbb{R}$ being the barotropic\nindex), we solve the background dynamics. The dynamics offers several\npossibilities depending on this sole parameter as follows $-$ (i) only an\nexponential expansion, or (ii) a finite time past singualrity (big bang) with\nlate accelerating phase, or (iii) a nonsingular universe exhibiting an\naccelerating scenario at late time which finally predicts a big rip type\nsingularity. We also discuss several energy conditions and the possibility of\ncosmic bounce. Finally, we establish the first law of thermodynamics in such\nspacetime.", "category": "gr-qc" }, { "text": "Gravity-Gradient Subtraction in 3rd Generation Underground\n Gravitational-Wave Detectors in Homogeneous Media: In this paper, we develop a new approach to gravity-gradient noise\nsubtraction for underground gravitational-wave detectors in homogeneous rock.\nThe method is based on spatial harmonic expansions of seismic fields. It is\nshown that gravity-gradient noise produced by seismic fields from distant\nsources, stationary or non-stationary, can be calculated from seismic data\nmeasured locally at the test mass. Furthermore, the formula is applied to\nseismic fields from stationary local sources. It is found that gravity\ngradients from these fields can be subtracted using local seismic measurements.\nThe results are confirmed numerically with a finite-element simulation. A new\nseismic-array design is proposed that provides the additional information about\nthe seismic field required to ensure applicability of the approach to realistic\nscenarios even with inhomogeneous rock and non-stationary local sources.", "category": "gr-qc" }, { "text": "Holonomy invariance, orbital resonances, and kilohertz QPOs: Quantized orbital structures are typical for many aspects of classical\ngravity (Newton's as well as Einstein's). The astronomical phenomenon of\norbital resonances is a well-known example. Recently, Rothman, Ellis and\nMurugan (2001) discussed quantized orbital structures in the novel context of a\nholonomy invariance of parallel transport in Schwarzschild geometry. We present\nhere yet another example of quantization of orbits, reflecting both orbital\nresonances and holonomy invariance. This strong-gravity effect may already have\nbeen directly observed as the puzzling kilohertz quasi-periodic oscillations\n(QPOs) in the X-ray emission from a few accreting galactic black holes and\nseveral neutron stars.", "category": "gr-qc" }, { "text": "Particle-Like Solutions of the Einstein-Dirac Equations: The coupled Einstein-Dirac equations for a static, spherically symmetric\nsystem of two fermions in a singlet spinor state are derived. Using numerical\nmethods, we construct an infinite number of soliton-like solutions of these\nequations. The stability of the solutions is analyzed. For weak coupling (i.e.,\nsmall rest mass of the fermions), all the solutions are linearly stable (with\nrespect to spherically symmetric perturbations), whereas for stronger coupling,\nboth stable and unstable solutions exist. For the physical interpretation, we\ndiscuss how the energy of the fermions and the (ADM) mass behave as functions\nof the rest mass of the fermions. Although gravitation is not renormalizable,\nour solutions of the Einstein-Dirac equations are regular and well-behaved even\nfor strong coupling.", "category": "gr-qc" }, { "text": "Gravitation and the noise needed in objective reduction models: I briefly recall intersections of my research interests with those of John\nBell. I then argue that the noise needed in theories of objective state vector\nreduction most likely comes from a fluctuating complex part in the classical\nspacetime metric, that is, state vector reduction is driven by {\\it complex\nnumber valued} \"spacetime foam\".", "category": "gr-qc" }, { "text": "Hamiltonian formulation of teleparallel gravity: The Hamiltonian formulation of the teleparallel equivalent of general\nrelativity (TEGR) is developed from an ordinary second-order Lagrangian, which\nis written as a quadratic form of the coefficients of anholonomy of the\northonormal frames (vielbeins). We analyze the structure of eigenvalues of the\nmulti-index matrix entering the (linear) relation between canonical velocities\nand momenta to obtain the set of primary constraints. The canonical Hamiltonian\nis then built with the Moore-Penrose pseudo-inverse of that matrix. The set of\nconstraints, including the subsequent secondary constraints, completes a first\nclass algebra. This means that all of them generate gauge transformations. The\ngauge freedoms are basically the diffeomorphisms, and the (local) Lorentz\ntransformations of the vielbein. In particular, the ADM algebra of general\nrelativity is recovered as a sub-algebra.", "category": "gr-qc" }, { "text": "Kaluza-Klein Reduction of a Quadratic Curvature Model: Palatini variational principle is implemented on a five dimensional quadratic\ncurvature gravity model, rendering two sets of equations which can be\ninterpreted as the field equations and the stress-energy tensor. Unification of\ngravity with electromagnetism and the scalar dilaton field is achieved through\nthe Kaluza-Klein dimensional reduction mechanism. The reduced curvature\ninvariant, field equations and the stress-energy tensor in four dimensional\nspacetime are obtained. The structure of the interactions among the constituent\nfields is exhibited in detail. It is shown that the Lorentz force naturally\nemerges from the reduced field equations and the equations of the standard\nKaluza-Klein theory is demonstrated to be intrinsically contained in this\nmodel.", "category": "gr-qc" }, { "text": "How universe evolves with cosmological and gravitational constants: With a basic varying space-time cutoff $\\tilde\\ell$, we study a regularized\nand quantized Einstein-Cartan gravitational field theory and its domains of\nultraviolet-unstable fixed point $g_{\\rm ir}\\gtrsim 0$ and ultraviolet-stable\nfixed point $g_{\\rm uv}\\approx 4/3$ of the gravitational gauge coupling\n$g=(4/3)G/G_{\\rm Newton}$. Because the fundamental operators of quantum\ngravitational field theory are dimension-2 area operators, the cosmological\nconstant is inversely proportional to the squared correlation length\n$\\Lambda\\propto \\xi^{-2}$. The correlation length $\\xi$ characterizes an\ninfrared size of a causally correlate patch of the universe. The cosmological\nconstant $\\Lambda$ and the gravitational constant $G$ are related by a\ngeneralized Bianchi identity. As the basic space-time cutoff $\\tilde\\ell$\ndecreases and approaches to the Planck length $\\ell_{\\rm pl}$, the universe\nundergoes inflation in the domain of the ultraviolet-unstable fixed point\n$g_{\\rm ir}$, then evolves to the low-redshift universe in the domain of\nultraviolet-stable fixed point $g_{\\rm uv}$. We give the quantitative\ndescription of the low-redshift universe in the scaling-invariant domain of the\nultraviolet-stable fixed point $g_{\\rm uv}$, and its deviation from the\n$\\Lambda$CDM can be examined by low-redshift $(z\\lesssim 1)$ cosmological\nobservations, such as supernova Type Ia.", "category": "gr-qc" }, { "text": "GEO600 Online Detector Characterization System: A world-wide network of interferometric gravitational wave detectors is\ncurrently operational. The detectors in the network are still in their\ncommissioning phase and are expected to achieve their design sensitivity over\nthe next year or so. Each detector is a complex instrument involving many\nsubsystems and each subsystem is a source of noise at the output of the\ndetector. Therefore, in addition to recording the main gravitational wave data\nchannel at the output of the interferometer, the state of each detector\nsubsystem is monitored and recorded. This subsidiary data is both large in\nvolume as well as complex in nature. We require an online monitoring and\nanalysis tool which can process all the data channels for various noise\nartefacts and summarize the results of the analysis in a manner that can be\naccessed and interpreted conveniently.\n In this paper we describe the GEO600 Online Detector Characterization System\n(GODCS), which is the tool that is being used to monitor the output of the\nGEO600 gravitational wave detector situated near Hannover in Germany. We\ndescribe the various algorithms that we use and how the results of several\nalgorithms can be combined to make meaningful statements about the state of the\ndetector. This paper will be useful to researchers in the area of gravitational\nwave astronomy as a record of the various analyses and checks carried out to\nensure the quality and reliability of the data before searching the data for\nthe presence of gravitational waves.", "category": "gr-qc" }, { "text": "Radiation from a uniformly accelerating harmonic oscillator: We consider a radiation from a uniformly accelerating harmonic oscillator\nwhose minimal coupling to the scalar field changes suddenly. The exact time\nevolutions of the quantum operators are given in terms of a classical solution\nof a forced harmonic oscillator. After the jumping of the coupling constant\nthere occurs a fast absorption of energy into the oscillator, and then a slow\nemission follows. Here the absorbed energy is independent of the acceleration\nand proportional to the log of a high momentum cutoff of the field. The emitted\nenergy depends on the acceleration and also proportional to the log of the\ncutoff. Especially, if the coupling is comparable to the natural frequency of\nthe detector ($e^2/(4m) \\sim \\omega_0$) enormous energies are radiated away\nfrom the oscillator.", "category": "gr-qc" }, { "text": "Lifshitz cosmology: quantum vacuum and Hubble tension: Dark energy is one of the greatest scientific mysteries of today. The idea\nthat dark energy originates from quantum vacuum fluctuations has circulated\nsince the late '60s, but theoretical estimations of vacuum energy have\ndisagreed with the measured value by many orders of magnitude, until recently.\nLifshitz theory applied to cosmology has produced the correct order of\nmagnitude for dark energy. Furthermore, the theory is based on well-established\nand experimentally well-tested grounds in atomic, molecular and optical\nphysics. In this paper, we confront Lifshitz cosmology with astronomical data.\nWe find that the dark-energy dynamics predicted by the theory is able to\nresolve the Hubble tension, the discrepancy between the observed and predicted\nHubble constant within the standard cosmological model. The theory is\nconsistent with supernovae data, Baryon Acoustic Oscillations and the Cosmic\nMicrowave Background. Our findings indicate that Lifshitz cosmology is a\nserious candidate for explaining dark energy.", "category": "gr-qc" }, { "text": "A comment on \"How the cosmological constant is hidden by Planck scale\n curvature fluctuations'': A recent preprint by Wang and Unruh [arXiv:1911.06110] contains a number of\ncriticisms of my paper, \"Hiding the cosmological constant\" [Phys. Rev. Lett.\n123 (2019) 131302, arXiv:1809.08277]. While Wang and Unruh suggest an\ninteresting alternative scenario and raise an important conceptual question,\nmost of their criticisms are incorrect, in part because of misunderstandings\nabout averaging and about the nature of the \"foamy\" spacetimes considered in my\npaper.", "category": "gr-qc" }, { "text": "Perturbations in the relaxation mechanism for a large cosmological\n constant: Recently, a mechanism for relaxing a large cosmological constant (CC) has\nbeen proposed [arxiv:0902.2215], which permits solutions with low Hubble rates\nat late times without fine-tuning. The setup is implemented in the LXCDM\nframework, and we found a reasonable cosmological background evolution similar\nto the LCDM model with a fine-tuned CC. In this work we analyse analytically\nthe perturbations in this relaxation model, and we show that their evolution is\nalso similar to the LCDM model, especially in the matter era. Some tracking\nproperties of the vacuum energy are discussed, too.", "category": "gr-qc" }, { "text": "Anatomy of the binary black hole recoil: A multipolar analysis: We present a multipolar analysis of the gravitational recoil computed in\nrecent numerical simulations of binary black hole (BH) coalescence, for both\nunequal masses and non-zero, non-precessing spins. We show that multipole\nmoments up to and including l=4 are sufficient to accurately reproduce the\nfinal recoil velocity (within ~2%) and that only a few dominant modes\ncontribute significantly to it (within ~5%). We describe how the relative\namplitudes, and more importantly, the relative phases, of these few modes\ncontrol the way in which the recoil builds up throughout the inspiral, merger,\nand ringdown phases. We also find that the numerical results can be reproduced\nby an ``effective Newtonian'' formula for the multipole moments obtained by\nreplacing the radial separation in the Newtonian formulae with an effective\nradius computed from the numerical data. Beyond the merger, the numerical\nresults are reproduced by a superposition of three Kerr quasi-normal modes\n(QNMs). Analytic formulae, obtained by expressing the multipole moments in\nterms of the fundamental QNMs of a Kerr BH, are able to explain the onset and\namount of ``anti-kick'' for each of the simulations. Lastly, we apply this\nmultipolar analysis to help explain the remarkable difference between the\namplitudes of planar and non-planar kicks for equal-mass spinning black holes.", "category": "gr-qc" }, { "text": "A note on the wave equation on black hole spacetimes with small\n non-decaying first order terms: We present an elementary physical space argument to establish local\nintegrated decay estimates for the perturbed wave equation $\\Box_g \\phi =\n\\epsilon \\beta^a \\partial_a \\phi$ on the exterior of the Schwarzschild geometry\n$(\\mathcal{M},g)$. Here $\\beta$ is a regular vectorfield on $\\mathcal{M}$\ndecaying suitably in space but not necessarily in time. The proof is formulated\nto cover also perturbations of the Regge--Wheeler equation.", "category": "gr-qc" }, { "text": "Cosmological production of fermions in a flat Friedman universe with\n linearly growing scale factor: exactly solvable model: We consider an exactly solvable model for production of fermions in the\nFriedman flat universe with a scale factor linearly growing with time. Exact\nsolution expressed through the special functions admit an analytical\ncalculation of the number density of created particles. We also discuss in\ngeneral the role of the phenomenon of the cosmological particle production in\nthe history of universe.", "category": "gr-qc" }, { "text": "Black Hole-Neutron Star Binaries in General Relativity: Quasiequilibrium\n Formulation: We present a new numerical method for the construction of quasiequilibrium\nmodels of black hole-neutron star binaries. We solve the constraint equations\nof general relativity, decomposed in the conformal thin-sandwich formalism,\ntogether with the Euler equation for the neutron star matter. We take the\nsystem to be stationary in a corotating frame and thereby assume the presence\nof a helical Killing vector. We solve these coupled equations in the background\nmetric of a Kerr-Schild black hole, which accounts for the neutron star's black\nhole companion. In this paper we adopt a polytropic equation of state for the\nneutron star matter and assume large black hole--to--neutron star mass ratios.\nThese simplifications allow us to focus on the construction of quasiequilibrium\nneutron star models in the presence of strong-field, black hole companions. We\nsummarize the results of several code tests, compare with Newtonian models, and\nlocate the onset of tidal disruption in a fully relativistic framework.", "category": "gr-qc" }, { "text": "Periastron precession for an extremal spherically symmetric dilaton\n black hole: The purpose of this article is to obtain the periastron precession of a free\nparticle moving around an extremal spherically symmetric dilaton black hole. To\nget the formulae for the periastron precession we use the phase-plane analysis\nof the general relativistic equations of motion.", "category": "gr-qc" }, { "text": "The geometry of the Barbour-Bertotti theories II. The three body problem: We present a geometric approach to the three-body problem in the\nnon-relativistic context of the Barbour-Bertotti theories. The Riemannian\nmetric characterizing the dynamics is analyzed in detail in terms of the\nrelative separations. Consequences of a conformal symmetry are exploited and\nthe sectional curvatures of geometrically preferred surfaces are computed. The\ngeodesic motions are integrated. Line configurations, which lead to curvature\nsingularities for $N\\neq 3$, are investigated. None of the independent scalars\nformed from the metric and curvature tensor diverges there.", "category": "gr-qc" }, { "text": "Cosmology from an exponential dependence on the trace of the\n energy-momentum tensor -- Numerical approach and cosmological tests: In this paper, we present the cosmological scenario obtained from $f(R,T)$\ngravity by using an exponential dependence on the trace of the energy-momentum\ntensor. With a numerical approach applied to the equations of motion, we show\nseveral precise fits and the respective cosmological consequences. As a matter\nof completeness, we also analyzed cosmological scenarios where this new version\nof $f(R,T)$ is coupled with a real scalar field. In order to find analytical\ncosmological parameters, we used a slow-roll approximation for the evolution of\nthe scalar field. This approximation allowed us to derived the Hubble and the\ndeceleration parameters whose time evolutions describe the actual phase of\naccelerated expansion, and corroborate with our numerical investigations.\nTherefore, the analytical parameters unveil the viability of this proposal for\n$f(R,T)$ in the presence of an inflaton field.", "category": "gr-qc" }, { "text": "Stationary axisymmetric SU(2) Einstein-Yang-Mills fields with restricted\n circularity conditions are Abelian: In this paper we prove that in a stationary axisymmetric SU(2)\nEinstein-Yang-Mills theory the most reasonable circularity conditions that can\nbe considered for the Yang-Mills fields imply in fact that the field is of\nembedded Abelian type, or else that the metric is not asymptotically flat.", "category": "gr-qc" }, { "text": "Kinetic Scalar Curvature Extended $f(R)$ Gravity: In this work we study a modified version of vacuum $f(R)$ gravity with a\nkinetic term which consists of the first derivatives of the Ricci scalar. We\ndevelop the general formalism of this kinetic Ricci modified $f(R)$ gravity and\nwe emphasize on cosmological applications for a spatially flat cosmological\nbackground. By using the formalism of this theory, we investigate how it is\npossible to realize various cosmological scenarios. Also we demonstrate that\nthis theoretical framework can be treated as a reconstruction method, in the\ncontext of which it is possible to realize various exotic cosmologies for\nordinary Einstein-Hilbert action. Finally, we derive the scalar-tensor\ncounterpart theory of this kinetic Ricci modified $f(R)$ gravity, and we show\nthe mathematical equivalence of the two theories.", "category": "gr-qc" }, { "text": "Minimally modified gravity with an auxiliary constraint: a Hamiltonian\n construction: Working directly with a general Hamiltonian for the spacetime metric with the\n$3+1$ decomposition and keeping only the spatial covariance, we investigate the\npossibility of reducing the number of degrees of freedom by introducing an\nauxiliary constraint. The auxiliary constraint is considered as part of the\ndefinition of the theory. Through a general Hamiltonian analysis, we find the\nconditions for the Hamiltonian as well as for the auxiliary constraint, under\nwhich the theory propagates two tensorial degrees of freedom only. The class of\ntheories satisfying these conditions can be viewed as a new construction for\nthe type-II minimally modified gravity theories, which propagate the same\ndegrees of freedom of but are not equivalent to general relativity in the\nvacuum. We also illustrate our formalism by a concrete example, and derive the\ndispersion relation for the gravitational waves, which can be constrained by\nobservations.", "category": "gr-qc" }, { "text": "Mach's Principle and a Variable Speed of Light: Ernst Mach (1838-1916) suggested that the origin of gravitational interaction\ncould depend on the presence of all masses in the universe. A corresponding\nhypothesis of Sciama (1953) on the gravitational constant, c^2/G = \\sum\nm_i/r_i, is linked to Dicke's (1957) proposal of an electromagnetic origin of\ngravitation, a precursor of scalar-tensor-theories. In this an equivalent\ndescription in terms of a variable speed of light (VSL) is given, and the\nagreement with the four classical tests of general relativity is shown.\nMoreover, VSL opens the possibility to write the total energy of a particle as\nE=mc^2; this necessarily leads to the proportionality of inertial and\ngravitating mass, the equivalence principle. Furthermore, a formula for c\ndepending on the mass distribution is given that reproduces Newton's law of\ngravitation. This mass distribution allows to calculate a slightly variable\nterm that corresponds to the `constant' G. The present proposal may also supply\nan alternative explanation to the flatness problem and the horizon problem in\ncosmology.", "category": "gr-qc" }, { "text": "Renormalization, averaging, conservation laws and AdS (in)stability: We continue our analytic investigations of non-linear spherically symmetric\nperturbations around the anti-de Sitter background in gravity-scalar field\nsystems, and focus on conservation laws restricting the (perturbatively) slow\ndrift of energy between the different normal modes due to non-linearities. We\ndiscover two conservation laws in addition to the energy conservation\npreviously discussed in relation to AdS instability. A similar set of three\nconservation laws was previously noted for a self-interacting scalar field in a\nnon-dynamical AdS background, and we highlight the similarities of this system\nto the fully dynamical case of gravitational instability. The nature of these\nconservation laws is best understood through an appeal to averaging methods\nwhich allow one to derive an effective Lagrangian or Hamiltonian description of\nthe slow energy transfer between the normal modes. The conservation laws in\nquestion then follow from explicit symmetries of this averaged effective\ntheory.", "category": "gr-qc" }, { "text": "Measuring a cosmological distance-redshift relationship using only\n gravitational wave observations of binary neutron star coalescences: Detection of gravitational waves from the inspiral phase of binary neutron\nstar coalescence will allow us to measure the effects of the tidal coupling in\nsuch systems. These effects will be measurable using 3rd generation\ngravitational wave detectors, e.g. the Einstein Telescope, which will be\ncapable of detecting inspiralling binary neutron star systems out to redshift\nz=4. Tidal effects provide additional contributions to the phase evolution of\nthe gravitational wave signal that break a degeneracy between the system's mass\nparameters and redshift and thereby allow the simultaneous measurement of both\nthe effective distance and the redshift for individual sources. Using the\npopulation of O(10^3-10^7) detectable binary neutron star systems predicted for\nthe Einstein Telescope the luminosity distance--redshift relation can be probed\nindependently of the cosmological distance ladder and independently of\nelectromagnetic observations. We present the results of a Fisher information\nanalysis applied to waveforms assuming a subset of possible neutron star\nequations of state. We conclude that for our range of representative neutron\nstar equations of state the redshift of such systems can be determined to an\naccuracy of 8-40% for z<1 and 9-65% for 1