Symmetron Fields: Screening Long-Range Forces Through Local Symmetry Restoration

Abstract: We present a screening mechanism that allows a scalar field to mediate a long range ( Mpc) force of gravitational strength in the cosmos while satisfying local tests of gravity. The mechanism hinges on local symmetry restoration in the presence of matter. In regions of sufficiently high matter density, the field is drawn towards ϕ= 0 where its coupling to matter vanishes and the ϕ-> -ϕsymmetry is restored. In regions of low density, however, the symmetry is spontaneously broken, and the field couples to matter with gravitational strength. We predict deviations from general relativity in the solar system that are within reach of next-generation experiments, as well as astrophysically observable violations of the equivalence principle. The model can be distinguished experimentally from Brans-Dicke gravity, chameleon theories and brane-world modifications of gravity.

force
local symmetry restoration
screening mechanism
scalar field
symmetron mechanism


Aharonov-Bohm Radiation of Fermions

Abstract: We analyze Aharonov-Bohm radiation of charged fermions from oscillating solenoids and cosmic strings. We find that the angular pattern of the radiation has features that differ significantly from that for bosons. For example, fermionic radiation in the lowest harmonic is approximately isotropically distributed around an oscillating solenoid, whereas for bosons the radiation is dipolar. We also investigate the spin polarization of the emitted fermion-antifermion pair. Fermionic radiation from kinks and cusps on cosmic strings is shown to depend linearly on the ultraviolet cut-off, suggesting strong emission at an energy scale comparable to the string energy scale.

fermion
cosmic string
aharonov-bohm radiation


Multi-messenger constraints on the neutron-star equation of state and the Hubble constant

Abstract: Observations of neutron-star mergers based on distinct messengers, including gravitational waves and electromagnetic signals, can be used to study the behavior of matter denser than an atomic nucleus, and to measure the expansion rate of the Universe described by the Hubble constant. We perform a joint analysis of the gravitational-wave signal GW170817 with its electromagnetic counterparts AT2017gfo and GRB170817A, and the gravitational-wave signal GW190425, both originating from neutron-star mergers. We combine these with previous measurements of pulsars using X-ray and radio observations, and nuclear-theory computations using chiral effective field theory to constrain the neutron-star equation of state. We find that the radius of a 1.4 solar mass neutron star is 11.75^+0.86_-0.81 km at 90% confidence and the Hubble constant is 66.2^+4.4_-4.2 km Mpc^-1 s^-1 at 1σ uncertainty.

neutron-star equation
hubble constant
atomic nucleus


The spectral shapes of the fluxes of electrons and positrons and the average residence time of cosmic rays in the Galaxy

Abstract: The cosmic ray energy spectra encode very important information about the mechanisms that generate relativistic particles in the Milky Way, and about the properties of the Galaxy that control their propagation. Relativistic electrons and positrons traveling in interstellar space lose energy much more rapidly than more massive particles such as protons and nuclei, with a rate that grows quadratically with the particle energy E. One therefore expects that the effects of energy loss should leave observable signatures in the e^∓ spectra, in the form of softenings centered at the critical energy E^*. This quantity is determined by the condition that the total energy loss suffered by particles during their residence time in the Galaxy is of the same order of the initial energy. If the electrons and positrons are accelerated in discrete (quasi) point–like astrophysical objects, such as Supernova explosions or Pulsars, the stochastic nature of the sources should also leave observable signatures in the e^∓ energy spectra and angular distributions above a second (higher) critical energy E^†, determined by the condition that particles with E ≳ E^† can propagate only for a maximum distance shorter than the average separation between sources. In this work we discuss the theoretically expectations for the signatures of the energy loss effects on the electron and positron spectra, and compare these predictions with the existing observations. Recent measurements of the (e^- + e^+) flux have discovered the existence of a prominent spectral break at E ≃ 1 TeV. This spectral feature can perhaps be identified with the critical energy E^*. An alternative hypothesis is to assmume that E^* has a much lower value of order few GeV. Resolving this ambiguity is of great importance for our understanding of Galactic cosmic rays.


Pulsar glitches: The crust is not enough

Abstract: Pulsar glitches are traditionally viewed as a manifestation of vortex dynamics associated with a neutron superfluid reservoir confined to the inner crust of the star. In this Letter we show that the non-dissipative entrainment coupling between the neutron superfluid and the nuclear lattice leads to a less mobile crust superfluid, effectively reducing the moment of inertia associated with the angular momentum reservoir. Combining the latest observational data for prolific glitching pulsars with theoretical results for the crust entrainment we find that the required superfluid reservoir exceeds that available in the crust. This challenges our understanding of the glitch phenomenon, and we discuss possible resolutions to the problem.


Hunting Down the Best Model of Inflation with Bayesian Evidence

Abstract: We present the first calculation of the Bayesian evidence for different prototypical single field inflationary scenarios, including representative classes of small field and large field models. This approach allows us to compare inflationary models in a well-defined statistical way and to determine the current "best model of inflation". The calculation is performed numerically by interfacing the inflationary code FieldInf with MultiNest. We find that small field models are currently preferred, while large field models having a self-interacting potential of power p>4 are strongly disfavoured. The class of small field models as a whole has posterior odds of approximately 3:1 when compared with the large field class. The methodology and results presented in this article are an additional step toward the construction of a full numerical pipeline to constrain the physics of the early Universe with astrophysical observations. More accurate data (such as the Planck data) and the techniques introduced here should allow us to identify conclusively the best inflationary model.

bayesian evidence
inflationary model
cosmic microwave background
scalar power spectrum


Electron Acceleration by Multi-Island Coalescence

Abstract: Energetic electrons of up to tens of MeV are created during explosive phenomena in the solar corona. While many theoretical models consider magnetic reconnection as a possible way of generating energetic electrons, the precise roles of magnetic reconnection during acceleration and heating of electrons still remain unclear. Here we show from 2D particle-in-cell simulations that coalescence of magnetic islands that naturally form as a consequence of tearing mode instability and associated magnetic reconnection leads to efficient energization of electrons. The key process is the secondary magnetic reconnection at the merging points, or the `anti-reconnection', which is, in a sense, driven by the converging outflows from the initial magnetic reconnection regions. By following the trajectories of the most energetic electrons, we found a variety of different acceleration mechanisms but the energization at the anti-reconnection is found to be the most important process. We discuss possible applications to the energetic electrons observed in the solar flares. We anticipate our results to be a starting point for more sophisticated models of particle acceleration during the explosive energy release phenomena.

coalescence
reconnection
magnetic island
particle acceleration


Innermost stable circular orbits of spinning test particles in Schwarzschild and Kerr space-times

Abstract: We consider the motion of classical spinning test particles in Schwarzschild and Kerr metrics and investigate innermost stable circular orbits (ISCO). The main goal of this work is to find analytically the small-spin corrections for the parameters of ISCO (radius, total angular momentum, energy, orbital angular frequency) of spinning test particles in the case of vectors of black hole spin, particle spin and orbital angular momentum being collinear to each other. We analytically derive the small-spin linear corrections for arbitrary Kerr parameter a. The cases of Schwarzschild, slowly rotating and extreme Kerr black hole are considered in details. For a slowly rotating black hole the ISCO parameters are obtained up to quadratic in a and particle's spin s terms. From the formulae obtained it is seen that the spin-orbital coupling has attractive character when spin and angular momentum are parallel and repulsive when they are antiparallel. For the case of the extreme Kerr black hole with co-rotating particle we succeed to find the exact analytical solution for the limiting ISCO parameters for arbitrary spin. It has been shown that the limiting values of ISCO radius and frequency do not depend on the particle's spin while values of energy and total angular momentum depend on it. We have also considered circular orbits of arbitrary radius and have found small-spin linear corrections for the total angular momentum and energy at given radius. System of equations for numerical calculation of ISCO parameters for arbitrary a and s is also explicitly written.


Relaxation of the chiral imbalance and the generation of magnetic fields in magnetars

Abstract: The model for the generation of magnetic fields in a neutron star, based on the magnetic field instability caused by the electroweak interaction between electrons and nucleons, is developed. Using the methods of the quantum field theory, the helicity flip rate of electrons in their scattering off protons in dense matter of a neutron star is calculated. The influence of the electroweak interaction between electrons and background nucleons on the process of the helicity flip is studied. The kinetic equation for the evolution of the chital imbalance is derived. The obtained results are applied for the description of the magnetic fields evolution in magnetars.

chiral imbalance
magnetic field
field instability
spin-flip


Universality Class in Conformal Inflation

Abstract: We develop a new class of chaotic inflation models with spontaneously broken conformal invariance. Observational consequences of a broad class of such models are stable with respect to strong deformations of the scalar potential. This universality is a critical phenomenon near the point of enhanced symmetry, SO(1,1), in case of conformal inflation. It appears because of the exponential stretching of the moduli space and the resulting exponential flattening of scalar potentials upon switching from the Jordan frame to the Einstein frame in this class of models. This result resembles stretching and flattening of inhomogeneities during inflationary expansion. It has a simple interpretation in terms of velocity versus rapidity near the Kahler cone in the moduli space, similar to the light cone of special theory of relativity. This effect makes inflation possible even in the models with very steep potentials. We describe conformal and superconformal versions of this cosmological attractor mechanism.

conformal inflation
scalar potential
supergravity
conformal attractor


Introduction to dark matter experiments

Abstract: This is a set of four lectures presented at the Theoretical Advanced Study Institute (TASI-09) in June 2009. I provide an introduction to experiments designed to detect WIMP dark matter directly, focusing on building intuitive understanding of the characteristics of potential WIMP signals and the experimental techniques. After deriving the characteristics of potential signals in direct-detection experiments for standard WIMP models, I summarize the general experimental methods shared by most direct-detection experiments and review the advantages, challenges, and status of such searches (as of late 2009). Experiments are already probing SUSY models, with best limits on the spin-independent coupling below 10^-7 pb.

gravitational lensing
astrophysical evidence
universe


Exotic Compact Objects and How to Quench their Ergoregion Instability

Abstract: Gravitational-wave astronomy can give us access to the structure of black holes, potentially probing microscopic or even Planckian corrections at the horizon scale, as those predicted by some quantum-gravity models of exotic compact objects. A generic feature of these models is the replacement of the horizon by a reflective surface. Objects with these properties are prone to the so-called ergoregion instability when they spin sufficiently fast. We investigate in detail a simple model consisting of scalar perturbations of a Kerr geometry with a reflective surface near the horizon. The instability depends on the spin, on the compactness, and on the reflectivity at the surface. The instability time scale increases only logarithmically in the black-hole limit and, for a perfectly reflecting object, this is not enough to prevent the instability from occurring on dynamical time scales. However, we find that an absorption rate at the surface as small as 0.4 coefficient as large as | R|^2=0.996) is sufficient to quench the instability completely. Our results suggest that exotic compact objects are not necessarily ruled out by the ergoregion instability.

exotic compact object
black-hole spacetime
matter configuration
quantum theory


Gravitational waves from very massive stars collapsing to a black hole

Abstract: We compute gravitational waves emitted by the collapse of a rotating very massive star (VMS) core leading directly to a black hole in axisymmetric numerical-relativity simulations. The evolved rotating VMS is derived by a stellar evolution calculation and its initial mass and the final carbon-oxygen core mass are 320M_⊙ and ≈ 150M_⊙, respectively. We find that for the moderately rapidly rotating cases, the peak strain amplitude and the corresponding frequency of gravitational waves are ∼ 10^-22 and f ≈ 300–600 Hz for an event at the distance of D=50 Mpc. Such gravitational waves will be detectable only for D ≲ 10 Mpc by second generation detectors, advanced LIGO, advanced VIRGO, and KAGRA, even if the designed sensitivity for these detectors is achieved. However, third-generation detectors will be able to detect such gravitational waves for an event up to D ∼ 100 Mpc. The detection of the gravitational-wave signal will provide a potential opportunity for verifying the presence of VMSs with mass ≳ 300M_⊙ and their pair-unstable collapse in the universe.

massive star
black hole
initial mass
gravitational wave


Extra polarization states of cosmological gravitational waves in alternative theories of gravity

Abstract: Cosmological Gravitational Waves (GWs) are usually associated with the transverse-traceless part of the metric perturbations in the context of the theory of cosmological perturbations. These modes are just the usual polarizations `+' and `x' which appear in the general relativity theory. However, in the majority of the alternative theories of gravity, GWs can present more than these two polarization states. In this context, the Newman-Penrose formalism is particularly suitable for evaluating the number of non-null GW modes. In the present work we intend to take into account these extra polarization states for cosmological GWs in alternative theories of gravity. As an application, we derive the dynamical equations for cosmological GWs for two specific theories, namely, a general scalar-tensor theory which presents four polarization states and a massive bimetric theory which is in the most general case with six polarization states for GWs. The mathematical tool presented here is quite general, so it can be used to study cosmological perturbations in all metric theories of gravity.

gravity
scalar-tensor theory
extra polarization state


Buoyancy Instabilities in Galaxy Clusters: Convection Due to Adiabatic Cosmic Rays and Anisotropic Thermal Conduction

Abstract: Using a linear stability analysis and two and three-dimensional nonlinear simulations, we study the physics of buoyancy instabilities in a combined thermal and relativistic (cosmic ray) plasma, motivated by the application to clusters of galaxies. We argue that cosmic ray diffusion is likely to be slow compared to the buoyancy time on large length scales, so that cosmic rays are effectively adiabatic. If the cosmic ray pressure p_cr is ≳ 25 the thermal pressure, and the cosmic ray entropy (p_cr/ρ^4/3;ρis the thermal plasma density) decreases outwards, cosmic rays drive an adiabatic convective instability analogous to Schwarzschild convection in stars. Global simulations of galaxy cluster cores show that this instability saturates by reducing the cosmic ray entropy gradient and driving efficient convection and turbulent mixing. At larger radii in cluster cores, the thermal plasma is unstable to the heat flux-driven buoyancy instability (HBI), a convective instability generated by anisotropic thermal conduction and a background conductive heat flux. Cosmic-ray driven convection and the HBI may contribute to redistributing metals produced by Type 1a supernovae in clusters. Our calculations demonstrate that adiabatic simulations of galaxy clusters can artificially suppress the mixing of thermal and relativistic plasma; anisotropic thermal conduction allows more efficient mixing, which may contribute to cosmic rays being distributed throughout the cluster volume.

convection
anisotropic thermal conduction
cosmic-ray


Measuring our Peculiar Velocity by "Pre-deboosting" the CMB

Abstract: It was recently shown that our peculiar velocity βwith respect to the CMB induces mixing among multipoles and off-diagonal correlations at all scales which can be used as a measurement of β, which is independent of the standard measurement using the CMB temperature dipole. The proposed techniques rely however on a perturbative expansion which breaks down for ℓ≳1/(β) ≈800. Here we propose a technique which consists of deboosting the CMB temperature in the time-ordered data and show that it extends the validity of the perturbation analysis multipoles up to ℓ∼10000. We also obtain accurate fitting functions for the mixing between multipoles valid in a full non-linear treatment. Finally we forecast the achievable precision with which these correlations can be measured in a number of current and future CMB missions. We show that Planck could measure the velocity with a precision of around 60 km/s, ACTPol in 4 years around 40 km/s, while proposed future experiments could further shrink this error bar by over a factor of around 2.

peculiar velocity
cmb temperature
fitting function


The Hipparcos-Gaia Catalog of Accelerations: Gaia EDR3 Edition

Abstract: We present a cross-calibration of Hipparcos and Gaia EDR3 intended to identify astrometrically accelerating stars and to fit orbits to stars with faint, massive companions. The resulting catalog, the EDR3 edition of the Hipparcos-Gaia Catalog of Accelerations (HGCA), provides three proper motions with calibrated uncertainties on the EDR3 reference frame: the Hipparcos proper motion, the Gaia EDR3 proper motion, and the long-term proper motion given by the difference in position between Hipparcos and Gaia EDR3. Our approach is similar to that for the Gaia DR2 edition of the HGCA, but offers a factor of 3 improvement in precision thanks to the longer time baseline and improved data processing of Gaia EDR3. We again find that a 60/40 mixture of the two Hipparcos reductions outperforms either reduction individually, and we find strong evidence for locally variable frame rotations between all pairs of proper motion measurements. The substantial global frame rotation seen in DR2 proper motions has been removed in EDR3. We also correct for color- and magnitude-dependent frame rotations at a level of up to 50 μas/yr in Gaia EDR3. We calibrate the Gaia EDR3 uncertainties using a sample of radial velocity standard stars without binary companions; we find an error inflation factor (a ratio of total to formal uncertainty) of 1.37. This is substantially lower than the position dependent factor of 1.7 found for Gaia DR2 and reflects the improved data processing in EDR3. While the catalog should be used with caution, its proper motion residuals provide a powerful tool to measure the masses and orbits of faint, massive companions to nearby stars.

hipparcos-gaia catalog
accelerations
companion


Directional detection of dark matter in universal bound states

Abstract: It has been suggested that several small-scale structure anomalies in ΛCDM cosmology can be solved by strong self-interaction between dark matter particles. It was shown by Braaten and Hammer that the presence of a near threshold S-wave resonance can make the scattering cross section at nonrelativistic speeds come close to saturating the unitarity bound. This can result in the formation of a stable bound state of two asymmetric dark matter particles (we call the bound state as darkonium). Laha and Braaten studied the nuclear recoil energy spectrum in dark matter direct detection experiments due to this incident bound state. Here we study the angular recoil spectrum, and show that it is uniquely determined up to normalization by the S-wave scattering length. Observing this angular recoil spectrum in a dark matter directional detection experiment will uniquely determine many of the low-energy properties of dark matter independent of the underlying dark matter microphysics.

dark matter
matter particle
darkonium
recoil energy spectrum
directional detection


Fermi LAT Search for Internal Bremsstrahlung Signatures from Dark Matter Annihilation

Abstract: A commonly encountered obstacle in indirect searches for galactic dark matter is how to disentangle possible signals from astrophysical backgrounds. Given that such signals are most likely subdominant, the search for pronounced spectral features plays a key role for indirect detection experiments; monochromatic gamma-ray lines or similar features related to internal bremsstrahlung, in particular, provide smoking gun signatures. We perform a dedicated search for the latter in the data taken by the Fermi gamma-ray space telescope during its first 43 months. To this end, we use a new adaptive procedure to select optimal target regions that takes into account both standard and contracted dark matter profiles. The behaviour of our statistical method is tested by a subsampling analysis of the full sky data and found to reproduce the theoretical expectations very well. The limits on the dark matter annihilation cross-section that we derive are stronger than what can be obtained from the observation of dwarf galaxies and, at least for the model considered here, collider searches. While these limits are still not quite strong enough to probe annihilation rates expected for thermally produced dark matter, future prospects to do so are very good. In fact, we already find a weak indication, with a significance of 3.1σ(4.3σ) when (not) taking into account the look-elsewhere effect, for an internal bremsstrahlung-like signal that would correspond to a dark matter mass of ∼150 GeV; the same signal is also well fitted by a gamma-ray line at around 130 GeV. Although this would be a fascinating possibility, we caution that a much more dedicated analysis and additional data will be necessary to rule out or confirm this option.

internal bremsstrahlung
dark matter annihilation
fermi data
lepton


BBN And The CMB Constrain Light, Electromagnetically Coupled WIMPs

Abstract: (Abridged) In the presence of a light WIMP (< 30 MeV), there are degeneracies among the nature of the WIMP, its couplings to standard model particles, its mass m_chi, and the number of equivalent neutrinos beyond the standard model, Delta N_nu. These degeneracies cannot be broken by the CMB constraint on the effective number of neutrinos, N_eff. However, big bang nucleosynthesis (BBN) is affected by a light WIMP and equivalent neutrinos, so the combination of BBN and CMB can break some of the degeneracies. Here, BBN predictions for abundances in the presence of a light WIMP and equivalent neutrinos are explored, and estimates of their observationally determined relic abundances are used to limit m_chi, Delta N_nu, and the present Universe baryon density (Omega_B h^2). These constraints are explored for Majorana and Dirac fermion WIMPs, as well as for real and complex scalar WIMPs that couple to electrons, positrons, and photons. In a separate paper this analysis is repeated for WIMPs that couple only to the standard model neutrinos, and constraints for the two cases are contrasted. Without a light WIMP but allowing for equivalent neutrinos, the combined BBN and CMB constraints favor N_eff = 3.46 +/- 0.17, Omega_B h^2 = 0.0224 +/- 0.0003, and Delta N_nu = 0.40 +/- 0.17. In this case, standard BBN (Delta N_nu = 0) is disfavored at 98 presence of one sterile neutrino (Delta N_nu = 1) is disfavored at > 99 confidence. Allowing a light WIMP and Delta N_nu equivalent neutrinos together, the combined BBN and CMB data provide lower limits to the WIMP masses (> 0.5 - 5 MeV) that depend on the nature of the WIMP, favor m_chi 8 MeV slightly over no WIMP, and loosen constraints on equivalent neutrinos, Delta N_nu = 0.65+0.46-0.35. While Delta N_nu = 0 is still disfavored at 95 with a light WIMP, Delta N_nu = 1 is now allowed.

light wimp
standard model
neutrino
photon
bbn


Quark-Meson Coupling Model, Nuclear Matter Constraints and Neutron Star Properties

Abstract: We explore the equation of state for nuclear matter in the quark-meson coupling model, including full Fock terms. The comparison with phenomenological constraints can be used to restrict the few additional parameters appearing in the Fock terms which are not present at Hartree level. Because the model is based upon the in-medium modification of the quark structure of the bound hadrons, it can be applied without additional parameters to include hyperons and to calculate the equation of state of dense matter in beta-equilibrium. This leads naturally to a study of the properties of neutron stars, including their maximum mass, their radii and density profiles.

nuclear matter
neutron star
quark-meson coupling model


Strong Evidence for Gamma-ray Line Emission from the Inner Galaxy

Abstract: Using 3.7 years of -LAT data, we examine the diffuse 80-200 GeV emission in the inner Galaxy and find a resolved gamma-ray feature at ∼ 110-140 GeV. We model the spatial distribution of this emission with a ∼3 FWHM Gaussian, finding a best fit position 1.5 West of the Galactic Center. Even better fits are obtained for off-center Einasto and power-law profiles, which are preferred over the null (no line) hypothesis by 6.5σ (5.0σ/5.4σ after trials factor correction for one/two line case) assuming an NFW density profile centered at (ℓ, b)=(-1.5,0) with a power index α=1.2 . The energy spectrum of this structure is consistent with a single spectral line (at energy 127.0± 2.0 GeV with χ^2=4.48 for 4 d.o.f.). A pair of lines at 110.8± 4.4 GeV and 128.8± 2.7 GeV provides a marginally better fit (with χ^2=1.25 for 2 d.o.f.). The total luminosity of the structure is (3.2±0.6)× 10^35 erg/s, or (1.7±0.4)× 10^36 photons/sec. The energies in the two-line case are compatible with a 127.3 ± 2.7 GeV WIMP annihilating through γγ and γ Z (with χ^2=1.67 for 3 d.o.f.). We describe a possible change to the scan strategy that would accumulate S/N on spectral lines in the Galactic center 4 times as fast as the current survey strategy.

gamma-ray line
line emission
inner galaxy


Gravitational wave background from binary systems

Abstract: Basic aspects of the background of gravitational waves and its mathematical characterization are reviewed. The spectral energy density parameter Ω(f), commonly used as a quantifier of the background, is derived for an ensemble of many identical sources emitting at different times and locations. For such an ensemble, Ω(f) is generalized to account for the duration of the signals and of the observation, so that one can distinguish the resolvable and unresolvable parts of the background. The unresolvable part, often called confusion noise or stochastic background, is made by signals that cannot be either individually identified or subtracted out of the data. To account for the resolvability of the background, the overlap function is introduced. This function is a generalization of the duty cycle, which has been commonly used in the literature, in some cases leading to incorrect results. The spectra produced by binary systems (stellar binaries and massive black hole binaries) are presented over the frequencies of all existing and planned detectors. A semi-analytical formula for Ω(f) is derived in the case of stellar binaries (containing white dwarfs, neutron stars or stellar-mass black holes). Besides a realistic expectation of the level of background, upper and lower limits are given, to account for the uncertainties in some astrophysical parameters such as binary coalescence rates. One interesting result concerns all current and planned ground-based detectors (including the Einstein Telescope). In their frequency range, the background of binaries is resolvable and only sporadically present. In other words, there is no stochastic background of binaries for ground-based detectors.

binary system
black hole
white dwarf
gravitational wave background


The small scale power asymmetry in the cosmic microwave background

Abstract: We investigate the hemispherical power asymmetry in the cosmic microwave background on small angular scales. We find an anomalously high asymmetry in the multipole range l=601-2048, with a naive statistical significance of 6.5 sigma. However, we show that this extreme anomaly is simply a coincidence of three other effects, relativistic power modulation, edge effects from the mask applied, and inter-scale correlations. After correcting for all of these effects, the significance level drops to 1 sigma, i.e., there is no anomalous intrinsic asymmetry in the small angular scales. Using this null result, we derive a constraint on a potential dipolar modulation amplitude, A(k)<0.0045 on the 10 Mpc-scale, at 95 theoretical model attempting to explain the hemispherical asymmetry at large angular scales.

power asymmetry
cosmic microwave background
angular scale


Constraints on Primordial Black Holes From Big Bang Nucleosynthesis Revisited

Abstract: As space expands, the energy density in black holes increases relative to that of radiation, providing us with motivation to consider scenarios in which the early universe contained a significant abundance of such objects. In this study, we revisit the constraints on primordial black holes derived from measurements of the light element abundances. Black holes and their Hawking evaporation products can impact the era of Big Bang Nucleosynthesis (BBN) by altering the rate of expansion at the time of neutron-proton freeze-out, as well as by radiating mesons which can convert protons into neutrons and vice versa. Such black holes can thus enhance the primordial neutron-to-proton ratio, and increase the amount of helium that is ultimately produced. Additionally, the products of Hawking evaporation can break up helium nuclei, which both reduces the helium abundance and increases the abundance of primordial deuterium. Building upon previous work, we make use of modern deuterium and helium measurements to derive stringent constraints on black holes which evaporate in t_ evap∼ 10^-1 s to ∼ 10^13 s (corresponding to M ∼ 6× 10^8 g to ∼ 2 × 10^13 g, assuming Standard Model particle content). We also consider how physics beyond the Standard Model could impact these constraints. Due to the gravitational nature of Hawking evaporation, the rate at which a black hole evaporates, and the types of particles that are produced through this process, depend on the complete particle spectrum. Within this context, we discuss scenarios which feature a large number of decoupled degrees-of-freedom ( large hidden sectors), as well as models of TeV-scale supersymmetry.

black hole
big bang nucleosynthesis
early universe


Anisotropic Inflation from Charged Scalar Fields

Abstract: We consider models of inflation with U(1) gauge fields and charged scalar fields including symmetry breaking potential, chaotic inflation and hybrid inflation. We show that there exist attractor solutions where the anisotropies produced during inflation becomes comparable to the slow-roll parameters. In the models where the inflaton field is a charged scalar field the gauge field becomes highly oscillatory at the end of inflation ending inflation quickly. Furthermore, in charged hybrid inflation the onset of waterfall phase transition at the end of inflation is affected significantly by the evolution of the background gauge field. Rapid oscillations of the gauge field and its coupling to inflaton can have interesting effects on preheating and non-Gaussianities.

gauge field
inflaton field
anisotropic inflation


Inflection point inflation: WMAP constraints and a solution to the fine-tuning problem

Abstract: We consider observational constraints and fine-tuning issues in a renormalizable model of inflection point inflation, with two independent parameters. We derive constraints on the parameter space of this model arising from the WMAP 7-year power spectrum. It has previously been shown that it is possible to successfully embed this potential in the MSSM. Unfortunately, to do this requires severe fine-tuning. We address this issue by introducing a hybrid field to dynamically uplift the potential with a subsequent smooth phase transition to end inflation at the necessary point. Large parameter regions exist where this drastically reduces the fine-tuning required without ruining the viability of the model. A side effect of this mechanism is that it increases the width of the slow-roll region of the potential, thus also alleviating the problem of the fine-tuning of initial conditions. The MSSM embedding we study has been previously shown to be able to explain the smallness of the neutrino masses. The hybrid transition does not spoil this feature as there exist parameter regions where the fine-tuning parameter is as large as 10^-1 and the neutrino masses remain small.

mssm
smallness
inflection point inflation


Characterizing unknown systematics in large scale structure surveys

Abstract: Photometric large scale structure (LSS) surveys probe the largest volumes in the Universe, but are inevitably limited by systematic uncertainties. Imperfect photometric calibration leads to biases in our measurements of the density fields of LSS tracers such as galaxies and quasars, and as a result in cosmological parameter estimation. Earlier studies have proposed using cross-correlations between different redshift slices or cross-correlations between different surveys to reduce the effects of such systematics. In this paper we develop a method to characterize unknown systematics. We demonstrate that while we do not have sufficient information to correct for unknown systematics in the data, we can obtain an estimate of their magnitude. We define a parameter to estimate contamination from unknown systematics using cross-correlations between different redshift slices and propose discarding bins in the angular power spectrum that lie outside a certain contamination tolerance level. We show that this method improves estimates of the bias using simulated data and further apply it to photometric luminous red galaxies in the Sloan Digital Sky Survey as a case study.

unknown systematic
cross-correlation
angular power spectrum


Clustering of quintessence on horizon scales and its imprint on HI intensity mapping

Abstract: Quintessence can cluster only on horizon scales. What is the effect on the observed matter distribution? To answer this, we need a relativistic approach that goes beyond the standard Newtonian calculation and deals properly with large scales. Such an approach has recently been developed for the case when dark energy is vacuum energy, which does not cluster at all. We extend this relativistic analysis to deal with dynamical dark energy. Using three quintessence potentials as examples, we compute the angular power spectrum for the case of an HI intensity map survey. Compared to the concordance model with the same small-scale power at z=0, quintessence boosts the angular power by up to 15 redshifts. The difference is mainly due to the background evolution, driven mostly by the normalization of the power spectrum today. The dark energy perturbations make only a small contribution on the largest scales, and a negligible contribution on smaller scales. Ironically, the dark energy perturbations remove the false boost of large-scale power that arises if we impose the (unphysical) assumption that the dark energy is smooth.

quintessence
horizon scale
large scale
dark energy
gravity


Scale-dependent bias from primordial non-Gaussianity in general relativity

Abstract: In this note we examine the derivation of scale-dependent bias due to primordial non-Gaussianity of the local type in the context of general relativity. We justify the use of the Poisson equation in general relativistic perturbation theory and thus the derivation of scale-dependent bias as a test of primordial non-Gaussianity, using the spherical collapse model. The corollary is that the form of scale-dependent bias does not receive general relativistic corrections on scales larger than the Hubble radius. This leads to a formally divergent correlation function for biased tracers of the mass distribution which we discuss.

bias
primordial non-gaussianity
general relativity
spherical collapse model


Electrodynamics at non-zero temperature, chemical potential, and Bose condensate

Abstract: Electrodynamics of charged scalar bosons and spin 1/2 fermions is studied at non-zero temperature, chemical potentials, and possible Bose condensate of the charged scalars. Debye screening length, plasma frequency, and the photon dispersion relation are calculated. It is found that in presence of the condensate the time-time component of the photon polarization operator in the first order in electric charge squared acquires infrared singular parts proportional to inverse powers of the spatial photon momentum k.

non-zero temperature
bose condensate
fermion
electrodynamic


Circumstantial evidence for a soft nuclear symmetry energy at supra-saturation densities

Abstract: Within an isospin- and momentum-dependent hadronic transport model it is shown that the recent FOPI data on the π^-/π^+ ratio in central heavy-ion collisions at SIS/GSI energies (Willy Reisdorf et al., NPA 781, 459 (2007)) provide circumstantial evidence suggesting a rather soft nuclear symmetry energy at ρ≥ 2ρ_0 compared to the Akmal-Pandharipande-Ravenhall prediction. Some astrophysical implications and the need for further experimental confirmations are discussed.

nuclear symmetry energy
supra-saturation density
neutron-rich nucleonic matter


Atmospheric Dynamics on Terrestrial Planets: The Seasonal Response to changes in Orbital, Rotational and Radiative Timescales

Abstract: Thousands of exoplanets have been detected to date, and with future planned missions this tally will increase. Understanding the climate dependence on the planetary parameters is vital for the study of terrestrial exoplanet habitability. Using an idealized general circulation model with a seasonal cycle, we study the seasonal response of the surface temperature and Hadley circulation to changes in the orbital, rotational and radiative timescales. Analyzing the climate's seasonal response to variations in these timescales, we find a regime transition between planets controlled by the annual mean insolation to planets controlled by the seasonal variability depending on the relation between the length of the orbital period, obliquity and radiative timescale. Consequently, planets with obliquity greater than 54^∘ and short orbital period will have a minimum surface temperature at the equator. We also show that in specific configurations, mainly high atmospheric mass and short orbital periods, high obliquity planets can still have an equable climate. Based on the model results, we suggest an empirical power law for the ascending and descending branches of the Hadley circulation and its strength. These power laws show that the Hadley circulation becomes wider and stronger by increasing the obliquity and orbital period or by decreasing the atmospheric mass and rotation rate. Consistent with previous studies, we show that the rotation rate plays an essential role in dictating the width of the Hadley circulation.

planet
seasonal response
general circulation model


Correlating features in the primordial spectra

Abstract: Heavy fields coupled to the inflaton reduce the speed of sound in the effective theory of the adiabatic mode each time the background inflationary trajectory deviates from a geodesic. This can result in features in the primordial spectra. We compute the corresponding bispectrum and show that if a varying speed of sound induces features in the power spectrum, the change in the bispectrum is given by a simple formula involving the change in the power spectrum and its derivatives. In this manner, we provide a uniquely discriminable signature of a varying sound speed for the adiabatic mode during inflation that indicates the influence of heavy fields. We find that features in the bispectrum peak in the equilateral limit and, in particular, in the squeezed limit we find considerable enhancement entirely consistent with the single field consistency relation. From the perspective of the underlying effective theory, our results generalize to a wide variety of inflationary models where features are sourced by the time variation of background quantities. A positive detection of such correlated features would be unambiguous proof of the inflaton's nature as a single light scalar degree of freedom embedded in a theory that is UV completable.

primordial spectra
heavy field
adiabatic mode
power spectrum
curvature perturbation


Time-Sliced Perturbation Theory for Large Scale Structure I: General Formalism

Abstract: We present a new analytic approach to describe large scale structure formation in the mildly non-linear regime. The central object of the method is the time-dependent probability distribution function generating correlators of the cosmological observables at a given moment of time. Expanding the distribution function around the Gaussian weight we formulate a perturbative technique to calculate non-linear corrections to cosmological correlators, similar to the diagrammatic expansion in a three-dimensional Euclidean quantum field theory, with time playing the role of an external parameter. For the physically relevant case of cold dark matter in an Einstein–de Sitter universe, the time evolution of the distribution function can be found exactly and is encapsulated by a time-dependent coupling constant controlling the perturbative expansion. We show that all building blocks of the expansion are free from spurious infrared enhanced contributions that plague the standard cosmological perturbation theory. This paves the way towards the systematic resummation of infrared effects in large scale structure formation. We also argue that the approach proposed here provides a natural framework to account for the influence of short-scale dynamics on larger scales along the lines of effective field theory.

field theory
resummation
time-sliced perturbation theory
non-linear evolution


A Single Field Inflation Model with Large Local Non-Gaussianity

Abstract: A detection of large local form non-Gaussianity is considered to be able to rule out all single field inflation models. This statement is based on a single field consistency condition. Despite the awareness of some implicit assumptions in the derivation of this condition and the demonstration of corresponding examples that illustrate these caveats, to date there is still no explicit and self-consistent model which can serve as a counterexample to this statement. We present such a model in this Letter.

local non-gaussianity
non-attractor inflation
non-canonical kinetic term


Primordial fluctuations and non-Gaussianities in sidetracked inflation

Abstract: Heavy scalar fields can undergo an instability during inflation as a result of their kinetic couplings with the inflaton. This is known as the geometrical destabilization of inflation, as it relies on the effect of the negative curvature of the field-space manifold overcoming the stabilizing force of the potential. This instability can drive the system away from its original path in field space into a new inflationary attractor, a scenario that we dub sidetracked inflation. We study this second phase and its observable consequences in several classes of two-field models. We show that cosmological fluctuations exhibit varied behaviours depending on the potential and the field space geometry, and that they can be captured by single-field effective theories with either a modified dispersion relation, a reduced speed of sound, or an imaginary one — the latter case describing a transient tachyonic growth of the fluctuations. We also numerically calculate the bispectrum with the transport approach, finding large non-Gaussianities of equilateral and orthogonal shapes. In the hyperbolic geometry the potentials of our models present a pole at the boundary of the Poincaré disk and we discuss their relationships with α-attractors.

fluctuation
non-gaussianitie
inflation model


The anamorphic universe

Abstract: We introduce "anamorphic" cosmology, an approach for explaining the smoothness and flatness of the universe on large scales and the generation of a nearly scale-invariant spectrum of adiabatic density perturbations. The defining feature is a smoothing phase that acts like a contracting universe based on some Weyl frame-invariant criteria and an expanding universe based on other frame-invariant criteria. An advantage of the contracting aspects is that it is possible to avoid the multiverse and measure problems that arise in inflationary models. Unlike ekpyrotic models, anamorphic models can be constructed using only a single field and can generate a nearly scale-invariant spectrum of tensor perturbations. Anamorphic models also differ from pre-big bang and matter bounce models that do not explain the smoothness. We present some examples of cosmological models that incorporate an anamorphic smoothing phase.

universe
advantage
ijjas
early universe cosmology


Structure formation beyond shell-crossing: nonperturbative expansions and late-time attractors

Abstract: Structure formation in 1+1 dimensions is considered, with emphasis on the effects of shell-crossing. The breakdown of the perturbative expansion beyond shell-crossing is discussed, and it is shown, in a simple example, that the perturbative series can be extended to a transseries including nonperturbative terms. The latter converges to the exact result well beyond the range of validity of perturbation theory. The crucial role of the divergences induced by shell-crossing is discussed. They provide constraints on the structure of the transseries and act as a bridge between the perturbative and the nonperturbative sectors. Then, we show that the dynamics in the deep multistreaming regime is governed by attractors. In the case of simple initial conditions, these attractors coincide with the asymptotic configurations of the adhesion model, but in general they may differ. These results are applied to a cosmological setting, and an algorithm to build the attractor solution starting from the Zel'dovich approximation is developed. Finally, this algorithm is applied to the search of `haloes' and the results are compared with those obtained from the exact dynamical equations.

shell-crossing
attractor
nonperturbative term
structure formation


Geometrical destabilization, premature end of inflation and Bayesian model selection

Abstract: By means of Bayesian techniques, we study how a premature ending of inflation, motivated by geometrical destabilization, affects the observational evidences of typical inflationary models. Large field models are worsened, and inflection point potentials are drastically improved for a specific range of the field-space curvature characterizing the geometrical destabilization. For other models we observe shifts in the preferred values of the model parameters. For quartic hilltop models for instance, contrary to the standard case, we find preference for theoretically natural sub-Planckian hill widths. Eventually, the Bayesian ranking of models becomes substantially reordered with a premature end of inflation. Such a phenomenon also modifies the constraints on the reheating expansion history, which has to be properly accounted for since it determines the position of the observational window with respect to the end of inflation. Our results demonstrate how the interpretation of cosmological data in terms of fundamental physics is considerably modified in the presence of premature end of inflation mechanisms.

destabilization
premature end
bayesian technique
inflationary model
field space


The relativistic Feynman-Metropolis-Teller treatment at finite temperatures

Abstract: The Feynman-Metropolis-Teller treatment of compressed atoms has been recently generalized to relativistic regimes and applied to the description of static and rotating white dwarfs in general relativity. We present here the extension of this treatment to the case of finite temperatures and construct the corresponding equation of state (EOS) of the system; applicable in a wide regime of densities that includes both white dwarfs and neutron star outer crusts. We construct the mass-radius relation of white dwarfs at finite temperatures obeying this new EOS and apply it to the analysis of ultra low-mass white dwarfs with M≲ 0.2 M_⊙. In particular, we analyze the case of the white dwarf companion of PSR J1738+0333. The formulation is then extrapolated to compressed nuclear matter cores of stellar dimensions, systems with mass numbers A≈ (m_ Planck/m_n)^3 or mass M_ core≈ M_⊙, where m_ Planck and m_n are the Planck and the nucleon mass. For T ≪ m_e c^2/k_B ≈ 5.9× 10^9 K, a family of equilibrium configurations can be obtained with analytic solutions of the ultra-relativistic Thomas-Fermi equation at finite temperatures. Such configurations fulfill global but not local charge neutrality and have strong electric fields on the core surface. We find that the maximum electric field at the core surface is enhanced at finite temperatures with respect to the degenerate case.

finite temperature
white dwarf
mass-radius relation


Cosmological 3-point correlators from holography

Abstract: We investigate the non-Gaussianity of primordial cosmological perturbations using holographic methods. In particular, we derive holographic formulae that relate all cosmological 3-point correlation functions, including both scalar and tensor perturbations, to stress-energy correlation functions of a holographically dual three-dimensional quantum field theory. These results apply to general single scalar inflationary universes that at late times approach either de Sitter spacetime or accelerating power-law cosmologies. We further show that in Einstein gravity all 3-point functions involving tensors can be obtained from correlators containing only positive helicity gravitons, with the ratios of these to the correlators involving one negative helicity graviton being given by universal functions of momenta, irrespectively of the potential of the scalar field. As a by-product of this investigation, we obtain holographic formulae for the full 3-point function of the stress-energy tensor along general holographic RG flows. These results should have applications in a wider holographic context.

correlator
holographic formulae
inflationary universe
universe
dual qft


Generation of the Higgs Condensate and Its Decay after Inflation

Abstract: We investigate the dynamics of the Standard Model higgs with a minimal coupling to gravity during and after inflation. In the regime where the Standard Model vacuum is stable, we find that the higgs becomes a light spectator field after about 30 efolds of inflation, irrespectively of its initial value. Once the higgs has become light, its root-mean-square value h_* relaxes to equilibrium in about 85 efolds for the inflationary scale of H_*=10^4 GeV and in 20 efolds for H_*=10^10 GeV. The equilibrium value is given by h_* 0.36 lambda^-1/4H_*, where lambda=0.09 ... 0.0005 is the higgs self coupling at the scales H_*=10^4 ... 10^10 GeV. We show that the main decay channel of the higgs condensate after inflation is the resonant production of Standard Model gauge bosons. For a set of parameters we find that a significant part of the condensate has decayed in between 340 and 630 Hubble times after the onset of higgs oscillations, depending on H_* in a non-trivial way. The higgs perturbations correspond to isocurvature modes during inflation but they could generate significant adiabatic perturbations at a later stage for example through a modulation of the reheating stage. However, this requires that the inflaton(s) decay no later than a few hundred Hubble times after the onset of higgs oscillations.

higgs condensate
gauge boson
non-perturbative production
new physics


CARPool: fast, accurate computation of large-scale structure statistics by pairing costly and cheap cosmological simulations

Abstract: To exploit the power of next-generation large-scale structure surveys, ensembles of numerical simulations are necessary to give accurate theoretical predictions of the statistics of observables. High-fidelity simulations come at a towering computational cost. Therefore, approximate but fast simulations, surrogates, are widely used to gain speed at the price of introducing model error. We propose a general method that exploits the correlation between simulations and surrogates to compute fast, reduced-variance statistics of large-scale structure observables without model error at the cost of only a few simulations. We call this approach Convergence Acceleration by Regression and Pooling (CARPool). In numerical experiments with intentionally minimal tuning, we apply CARPool to a handful of GADGET-III N-body simulations paired with surrogates computed using COmoving Lagrangian Acceleration (COLA). We find ∼ 100-fold variance reduction even in the non-linear regime, up to k_max≈ 1.2 h Mpc^-1 for the matter power spectrum. CARPool realises similar improvements for the matter bispectrum. In the nearly linear regime CARPool attains far larger sample variance reductions. By comparing to the 15,000 simulations from the Quijote suite, we verify that the CARPool estimates are unbiased, as guaranteed by construction, even though the surrogate misses the simulation truth by up to 60% at high k. Furthermore, even with a fully configuration-space statistic like the non-linear matter density probability density function, CARPool achieves unbiased variance reduction factors of up to ∼ 10, without any further tuning. Conversely, CARPool can be used to remove model error from ensembles of fast surrogates by combining them with a few high-accuracy simulations.

simulation
theoretical prediction
carpool


Abstract: The standard cosmological model successfully describes many observations from widely different epochs of the Universe, from primordial nucleosynthesis to the accelerating expansion of the present day. However, as the basic cosmological parameters of the model are being determined with increasing and unprecedented precision, it is not guaranteed that the same model will fit more precise observations from widely different cosmic epochs. Discrepancies developing between observations at early and late cosmological time may require an expansion of the standard model, and may lead to the discovery of new physics. The workshop "Tensions between the Early and the Late Universe" was held at the Kavli Institute for Theoretical Physics on July 15-17 2019 (More details of the workshop (including on-line presentations) are given at the website: https://www.kitp.ucsb.edu/activities/enervac-c19) to evaluate increasing evidence for these discrepancies, primarily in the value of the Hubble constant as well as ideas recently proposed to explain this tension. Multiple new observational results for the Hubble constant were presented in the time frame of the workshop using different probes: Cepheids, strong lensing time delays, tip of the red giant branch (TRGB), megamasers, Oxygen-rich Miras and surface brightness fluctuations (SBF) resulting in a set of six new ones in the last several months. Here we present the summary plot of the meeting that shows combining any three independent approaches to measure H_0 in the late universe yields tension with the early Universe values between 4.0σ and 5.8σ. This shows that the discrepancy does not appear to be dependent on the use of any one method, team, or source. Theoretical ideas to explain the discrepancy focused on new physics in the decade of expansion preceding recombination as the most plausible. This is a brief summary of the workshop.

late universe
hubble constant
crisis
corresponding difference


BlackHoleCam: fundamental physics of the Galactic center

Abstract: Einstein's General Theory of Relativity (GR) successfully describes gravity. The most fundamental predictions of GR are black holes (BHs), but in spite of many convincing BH candidates in the Universe, there is no conclusive experimental proof of their existence using astronomical observations in the electromagnetic spectrum. Are BHs real astrophysical objects? Does GR hold in its most extreme limit or are alternatives needed? The prime target to address these fundamental questions is in the center of our own Galaxy, which hosts the closest and best-constrained supermassive BH candidate in the Universe, Sagittarius A* (Sgr A*). Three different types of experiments hold the promise to test GR in a strong-field regime using observations of Sgr A* with new-generation instruments. The first experiment aims to image the relativistic plasma emission which surrounds the event horizon and forms a "shadow" cast against the background, whose predicted size ( 50 microarcseconds) can now be resolved by upcoming VLBI experiments at mm-waves such as the Event Horizon Telescope (EHT). The second experiment aims to monitor stars orbiting Sgr A* with the upcoming near-infrared interferometer GRAVITY at the Very Large Telescope (VLT). The third experiment aims to time a radio pulsar in tight orbit about Sgr A* using radio telescopes (including the Atacama Large Millimeter Array or ALMA). The BlackHoleCam project exploits the synergy between these three different techniques and aims to measure the main BH parameters with sufficient precision to provide fundamental tests of GR and probe the spacetime around a BH in any metric theory of gravity. Here, we review our current knowledge of the physical properties of Sgr A* as well as the current status of such experimental efforts towards imaging the event horizon, measuring stellar orbits, and timing pulsars around Sgr A*.

gravity
black hole
event horizon telescope


Growth Histories in Bimetric Massive Gravity

Abstract: We perform cosmological perturbation theory in Hassan-Rosen bimetric gravity for general homogeneous and isotropic backgrounds. In the de Sitter approximation, we obtain decoupled sets of massless and massive scalar gravitational fluctuations. Matter perturbations then evolve like in Einstein gravity. We perturb the future de Sitter regime by the ratio of matter to dark energy, producing quasi-de Sitter space. In this more general setting the massive and massless fluctuations mix. We argue that in the quasi-de Sitter regime, the growth of structure in bimetric gravity differs from that of Einstein gravity.

gravity
perturbation theory
cosmological solution
phenomenological advantage


Notes on the Missing Satellites Problem

Abstract: The Missing Satellites Problem (MSP) broadly refers to the overabundance of predicted Cold Dark Matter (CDM) subhalos compared to satellite galaxies known to exist in the Local Group. The most popular interpretation of the MSP is that the smallest dark matter halos in the universe are extremely inefficient at forming stars. The question from that standpoint is to identify the feedback source that makes small halos dark and to identify any obvious mass scale where the truncation in the efficiency of galaxy formation occurs. Among the most exciting developments in near-field cosmology in recent years is the discovery of a new population satellite galaxies orbiting the Milky Way and M31. Wide field, resolved star surveys have more than doubled the dwarf satellite count in less than a decade, revealing a population of ultrafaint galaxies that are less luminous that some star clusters. For the first time, there are empirical reasons to believe that there really are missing satellite galaxies in the Local Group, lurking just beyond our ability to detect them, or simply inhabiting a region of the sky that has yet to have been surveyed. Both kinematic studies and completeness-correction studies seem to point to a characteristic potential well depth for satellite subhalos that is quite close to the mass scale where photoionization and atomic cooling should limit galaxy formation. Among the more pressing problems associated with this interpretation is to understand the selection biases that limit our ability to detect the lowest mass galaxies. The least massive satellite halos are likely to host stealth galaxies with very-low surface brightness and this may be an important limitation in the hunt for low-mass fossils from the epoch of reionization.

missing satellites problem
satellite galaxy
milky way


Galaxy imaging surveys as spin-sensitive detector for cosmological colliders

Abstract: Galaxy imaging surveys provide us with information on both the galaxy distribution and their shapes. In this paper, we systematically investigate the sensitivity of galaxy shapes to new physics in the initial conditions. For this purpose, we decompose the galaxy shape function into spin components, and compute the contributions to each spin component from both intrinsic alignment and weak lensing. We then consider the angular-dependent primordial non-Gaussianity, which is generated by a non-zero integer spin particle when active during inflation, and show that a galaxy imaging survey essentially functions as a spin-sensitive detector of such particles in the early universe. We also perform a forecast of the PNG generated from a higher spin particle, considering a Rubin Observatory LSST-like galaxy survey.

spin-sensitive detector
collider
galaxy imaging survey


The electromagnetic counterparts of compact binary mergers

Abstract: Mergers of binaries consisting of two neutron stars, or a black hole and a neutron star, offer a unique opportunity to study a range of physical and astrophysical processes using two different and almost orthogonal probes - gravitational waves (GW) and electromagnetic (EM) emission. The GW signal probes the binary and the physical processes that take place during the last stages of the merger, while the EM emission provides clues to the material that is thrown out following the merger. The accurate localization, which only the EM emission can provide, also indicates the astrophysical setting in which the merger took place. In addition, the combination of the two signals provides constraints on the nature of gravity and on the expansion rate of the Universe. The first detection of a binary neutron star merger by the LIGO-Virgo collaboration, GW170817, initiated the era of multi-messenger GW-EM astrophysics and demonstrated the great promise it holds. The event produced an unprecedented data set, and although it was only a single event, it provided remarkable results that revolutionized our knowledge of neutron star mergers. GW170817 is especially exciting since we know that it is not one of a kind and that many more events will be detected during the next decade. In this review, I summarize, first, the theory of EM emission from compact binary mergers, highlighting the unique information that the combined GW-EM detection provides. I then describe the entire set of GW and EM observations of GW170817, and summarize the range of insights that it offers. This includes clues about the role that similar events play in the r-process elements budget of the Universe, the neutron star equation of state, the properties of the relativistic outflow that followed the merger, and the connection between neutron star binary mergers and short gamma-ray bursts.

compact binary merger
neutron star
gravitational wave


What a Two Solar Mass Neutron Star Really Means

Abstract: The determination of neutron star masses is reviewed in light of a new measurement of 1.97 M_⊙ for PSR J1614-2230 and an estimate of 2.4 M_⊙ for the black widow pulsar. Using a simple analytic model related to the so-called maximally compact equation of state, model-independent upper limits to thermodynamic properties in neutron stars, such as energy density, pressure, baryon number density and chemical potential, are established which depend upon the neutron star maximum mass. Using the largest well-measured neutron star mass, 1.97 M_⊙, it is possible to show that the energy density can never exceed about 2 GeV, the pressure about 1.3 GeV, and the baryon chemical potential about 2.1 GeV. Further, if quark matter comprises a significant component of neutron star cores, these limits are reduced to 1.3 GeV, 0.9 GeV, and 1.5 GeV, respectively. We also find that the maximum binding energy of any neutron star is about 25 properties and astrophysical constraints additionally imply an upper limit to the neutron star maximum mass of about 2.4 M_⊙. A measured mass of 2.4 M⊙ would be incompatible with hybrid star models containing significant proportions of exotica in the form of hyperons, Bose condensates or quark matter.


Pearson's random walk in the space of the CMB phases: evidence for parity asymmetry

Abstract: The temperature fluctuations of the Cosmic Microwave Background (CMB) are supposed to be distributed randomly in both magnitude and phase, following to the simplest model of inflation. In this paper, we look at the odd and even multipoles of the spherical harmonic decomposition of the CMB, and the different characteristics of these, giving rise to a parity asymmetry. We compare the even and odd multipoles in the CMB power spectrum, and also the even and odd mean angles. We find for the multipoles of the power spectrum, that there is power excess in odd multipoles, compared to even ones, meaning that we have a parity asymmetry. Further, for the phases, we present a random walk for the mean angles, and find a significant separation for even/odd mean angles, especially so for galactic coordinates. This is further tested and confirmed with a directional parity test, comparing the parity asymmetry in galactic and ecliptic coordinates.

random walk
parity asymmetry
odd multipole
cmb power spectrum


The observational evidence for horizons: from echoes to precision gravitational-wave physics

Abstract: The existence of black holes and of spacetime singularities is a fundamental issue in science. Despite this, observations supporting their existence are scarce, and their interpretation unclear. We overview how strong a case for black holes has been made in the last few decades, and how well observations adjust to this paradigm. Unsurprisingly, we conclude that observational evidence for black holes is impossible to come by. However, just like Popper's black swan, alternatives can be ruled out or confirmed to exist with a single observation. These observations are within reach. In the next few years and decades, we will enter the era of precision gravitational-wave physics with more sensitive detectors. Just as accelerators require larger and larger energies to probe smaller and smaller scales, more sensitive gravitational-wave detectors will be probing regions closer and closer to the horizon, potentially reaching Planck scales and beyond. What may be there, lurking?

horizon
echo
black hole
compact horizonless object


Two-integral distribution functions in axisymmetric galaxies: implications for dark matter searches

Abstract: We address the problem of reconstructing the phase-space distribution function for an extended collisionless system, with known density profile and in equilibrium within an axisymmetric gravitational potential. Assuming that it depends on only two integrals of motion, namely the energy and the component of the angular momentum along the axis of symmetry L_z, there is a one-to-one correspondence between the density profile and the component of the distribution function that is even in L_z, as well as between the weighted azimuthal velocity profile and the odd component. This inversion procedure was originally proposed by Lynden-Bell and later refined in its numerical implementation by Hunter Qian; after overcoming a technical difficulty, we apply it here for the first time in presence of a strongly flattened component, as a novel approach of extracting the phase-space distribution function for dark matter particles in the halo of spiral galaxies. We compare results obtained for realistic axisymmetric models to those in the spherical symmetric limit as assumed in previous analyses, showing the rather severe shortcomings in the latter. We then apply the scheme to the Milky Way and discuss the implications for the direct dark matter searches. In particular, we reinterpret the null results of the Xenon1T experiment for spin-(in)dependent interactions and make predictions for the annual modulation of the signal for a set of axisymmetric models, including a self-consistently defined co-rotating halo.

distribution function
galaxy
dark matter search
phase-space distribution function


Waveform of gravitational waves in the general parity-violating gravities

Abstract: As an extension of our previous work [J.Qiao, T.Zhu, W.Zhao A.Wang, arXiv:1909.03815], in this article, we calculate the effects of parity violation on gravitational-wave (GW) waveforms during their propagation in the most general parity-violating gravities, including Chern-Simons modified gravity, ghost-free scalar-tensor gravity, symmetric teleparallel equivalence of GR theory, Hořava-Lifshitz gravity and so on. For this purpose, we consider the GWs generated by the coalescence of compact binaries and concentrate on the imprints of the parity violation in the propagation of GWs. With a unified description of GW in the theories of parity-violating gravity, we study the effects of velocity and amplitude birefringence on the GW waveforms. Decomposing the GWs into the circular polarization modes, the two birefringence effects exactly correspond to the modifications in phase and amplitude of GW waveforms respectively. We find that, for each circular polarization mode, the amplitude, phase and velocity of GW can be modified by both the parity-violating terms and parity-conserving terms in gravity. Therefore, in order to test the parity symmetry in gravity, we should compare the difference between two circular polarization modes, rather than measuring an individual mode. Combining two circular modes, we obtain the GW waveforms in the Fourier domain, and obtain the deviations from those in General Relativity. The GW waveforms derived in this paper are also applicable to the theories of parity-conserving gravity, which have the modified dispersion relations (e.g. massive gravity, double special relativity theory, extra-dimensional theories, etc), or/and have the modified friction terms (e.g. nonlocal gravity, gravitational theory with time-dependent Planck mass, etc).


Warm Dark Matter as a solution to the small scale crisis: new constraints from high redshift Lyman-alpha forest data

Abstract: We present updated constraints on the free-streaming of warm dark matter (WDM) particles derived from an analysis of the Lya flux power spectrum measured from high-resolution spectra of 25 z > 4 quasars obtained with the Keck High Resolution Echelle Spectrometer (HIRES) and the Magellan Inamori Kyocera Echelle (MIKE) spectrograph. We utilize a new suite of high-resolution hydrodynamical simulations that explore WDM masses of 1, 2 and 4 keV (assuming the WDM consists of thermal relics), along with different physically motivated thermal histories. We carefully address different sources of systematic error that may affect our final results and perform an analysis of the Lya flux power with conservative error estimates. By using a method that samples the multi-dimensional astrophysical and cosmological parameter space, we obtain a lower limit mwdm > 3.3 keV (2sigma) for warm dark matter particles in the form of early decoupled thermal relics. Adding the Sloan Digital Sky Survey (SDSS) Lya flux power spectrum does not improve this limit. Thermal relics of masses 1 keV, 2 keV and 2.5 keV are disfavoured by the data at about the 9sigma, 4sigma and 3sigma C.L., respectively. Our analysis disfavours WDM models where there is a suppression in the linear matter power spectrum at (non-linear) scales corresponding to k=10h/Mpc which deviates more than 10 Given this limit, the corresponding "free-streaming mass" below which the mass function may be suppressed is 2x10^8 Msun/h. There is thus very little room for a contribution of the free-streaming of WDM to the solution of what has been termed the small scale crisis of cold dark matter.

thermal relic
warm dark matter
sterile neutrino


Coincident General Relativity

Abstract: The metric-affine variational principle is applied to generate teleparallel and symmetric teleparallel theories of gravity. From the latter is discovered an exceptional class which is consistent with a vanishing affine connection. Based on this remarkable property, this work proposes a simpler geometrical formulation of General Relativity that is oblivious to the affine spacetime structure, thus fundamentally depriving gravity of any inertial character. The resulting theory is described by the Hilbert action purged from the boundary term and is more robustly underpinned by the spin-2 field theory, where an extra symmetry is now manifest, possibly related to the double copy structure of the gravity amplitudes. This construction also provides a novel starting point for modified gravity theories, and the paper presents new and simple generalisations where analytical self-accelerating cosmological solutions arise naturally in the early and late time universe.

exceptional class
coincident general relativity
non-metricity
curvature
stegr


The maximum sizes of large scale structures in alternative theories of gravity

Abstract: The maximum size of a cosmic structure is given by the maximum turnaround radius – the scale where the attraction due to its mass is balanced by the repulsion due to dark energy. We derive generic formulae for the estimation of the maximum turnaround radius in any theory of gravity obeying the Einstein equivalence principle, in two situations: on a spherically symmetric spacetime and on a perturbed Friedman-Robertson-Walker spacetime. We show that the two formulae agree. As an application of our formula, we calculate the maximum turnaround radius in the case of the Brans-Dicke theory of gravity. We find that for this theory, such maximum sizes always lie above the value, by a factor 1 + 1/3ω, where ω≫ 1 is the Brans-Dicke parameter, implying consistency of the theory with current data.

maximum size
alternative theory
gravity
einstein equivalence principle


Quantum Primordial Standard Clocks

Abstract: In this paper, we point out and study a generic type of signals existing in the primordial universe models, which can be used to model-independently distinguish the inflation scenario from alternatives. These signals are generated by massive fields that function as standard clocks. The role of massive fields as standard clocks has been realized in previous works. Although the existence of such massive fields is generic, the previous realizations require sharp features to classically excite the oscillations of the massive clock fields. Here, we point out that the quantum fluctuations of massive fields can actually serve the same purpose as the standard clocks. We show that they are also able to directly record the defining property of the scenario type, namely, the scale factor of the primordial universe as a function of time a(t), but through shape-dependent oscillatory features in non-Gaussianities. Since quantum fluctuating massive fields exist in any realistic primordial universe models, these quantum primordial standard clock signals are present in any inflation models, and should exist quite generally in alternative-to-inflation scenarios as well. However, the amplitude of such signals is very model-dependent.

standard clock
universe
massive field
particle physics
field inflation


Constraints on Earth-mass primordial black holes from OGLE 5-year microlensing events

Abstract: We constrain the abundance of primordial black holes (PBH) using 2622 microlensing events obtained from 5-years observations of stars in the Galactic bulge by the Optical Gravitational Lensing Experiment (OGLE). The majority of microlensing events display a single or at least continuous population that has a peak around the light curve timescale t_ E≃ 20 days and a wide distribution over the range t_ E≃ [1, 300] days, while the data also indicates a second population of 6 ultrashort-timescale events in t_ E≃ [0.1,0.3] days, which are advocated to be due to free-floating planets. We confirm that the main population of OGLE events can be well modeled by microlensing due to brown dwarfs, main sequence stars and stellar remnants (white dwarfs and neutron stars) in the standard Galactic bulge and disk models for their spatial and velocity distributions. Using the dark matter (DM) model for the Milky Way (MW) halo relative to the Galactic bulge/disk models, we obtain the tightest upper bound on the PBH abundance in the mass range M_ PBH≃[10^-6,10^-3]M_⊙ (Earth-Jupiter mass range), if we employ null hypothesis that the OGLE data does not contain any PBH microlensing event. More interestingly, we also show that Earth-mass PBHs can well reproduce the 6 ultrashort-timescale events, without the need of free-floating planets, if the mass fraction of PBH to DM is at a per cent level, which is consistent with other constraints such as the microlensing search for Andromeda galaxy (M31) and the longer timescale OGLE events. Our result gives a hint of PBH existence, and can be confirmed or falsified by microlensing search for stars in M31, because M31 is towards the MW halo direction and should therefore contain a much less number of free-floating planets, even if exist, than the direction to the MW center.

primordial black hole
microlensing event
mass scale


Implicit large eddy simulations of global solar convection: effects of numerical resolution in non-rotating and rotating cases

Abstract: Simulating deep solar convection and its coupled mean-field motions is a formidable challenge where few observational results constrain models that suffer from the non-physical influence of the grid resolution. We present hydrodynamic global Implicit Large-Eddy simulations (ILES) of deep solar convection performed with the EULAG-MHD code, and explore the effects of grid resolution on the properties of rotating and non-rotating convection. The results, based on low-order moments and turbulent spectra reveal that convergence could be achieved in non-rotating simulations provided sufficient resolution in the radial direction. The flow is highly anisotropic, with the energy contained in horizontal divergent motions exceeding by more than three orders of magnitude their radial counterpart. By contrast, in rotating simulations the largest energy is in the toroidal part of the horizontal motions. As the grid resolution increases, the turbulent correlations change in such a way that a solar-like differential rotation, obtained in the simulation with the coarser grid, transitions to the anti-solar differential rotation. The reason for this change is the contribution of the effective viscosity to the balance of the forces driving large-scale flows. As the effective viscosity decreases, the angular momentum balance improves, yet the force balance in the meridional direction lessens, favoring a strong meridional flow that advects angular momentum towards the poles. The results suggest that obtaining the correct distribution of angular momentum may not be a mere issue of numerical resolution. Accounting for additional physics, such as magnetism or the near-surface shear layer, may be necessary in simulating the solar interior.

solar convection
numerical resolution
near-surface shear layer


Generalized Galileon cosmology

Abstract: We study the cosmology of a generalized Galileon field ϕ with five covariant Lagrangians in which ϕ is replaced by general scalar functions f_i(ϕ) (i=1,...,5). For these theories, the equations of motion remain at second-order in time derivatives. We restrict the functional forms of f_i(ϕ) from the demand to obtain de Sitter solutions responsible for dark energy. There are two possible choices for power-law functions f_i(ϕ), depending on whether the coupling F(ϕ) with the Ricci scalar R is independent of ϕ or depends on ϕ. The former corresponds to the covariant Galileon theory that respects the Galilean symmetry in the Minkowski space-time. For generalized Galileon theories we derive the conditions for the avoidance of ghosts and Laplacian instabilities associated with scalar and tensor perturbations as well as the condition for the stability of de Sitter solutions. We also carry out detailed analytic and numerical study for the cosmological dynamics in those theories.

cosmology
covariant galileon theory
tensor perturbation


The Average Star Formation Histories of Galaxies in Dark Matter Halos from z=0-8

Abstract: We present a robust method to constrain average galaxy star formation rates, star formation histories, and the intracluster light as a function of halo mass. Our results are consistent with observed galaxy stellar mass functions, specific star formation rates, and cosmic star formation rates from z=0 to z=8. We consider the effects of a wide range of uncertainties on our results, including those affecting stellar masses, star formation rates, and the halo mass function at the heart of our analysis. As they are relevant to our method, we also present new calibrations of the dark matter halo mass function, halo mass accretion histories, and halo-subhalo merger rates out to z=8. We also provide new compilations of cosmic and specific star formation rates; more recent measurements are now consistent with the buildup of the cosmic stellar mass density at all redshifts. Implications of our work include: halos near 10^12 Msun are the most efficient at forming stars at all redshifts, the baryon conversion efficiency of massive halos drops markedly after z 2.5 (consistent with theories of cold-mode accretion), the ICL for massive galaxies is expected to be significant out to at least z 1-1.5, and dwarf galaxies at low redshifts have higher stellar mass to halo mass ratios than previous expectations and form later than in most theoretical models. Finally, we provide new fitting formulae for star formation histories that are more accurate than the standard declining tau model. Our approach places a wide variety of observations relating to the star formation history of galaxies into a self-consistent framework based on the modern understanding of structure formation in LCDM. Constraints on the stellar mass-halo mass relationship and star formation rates are available for download at http://www.peterbehroozi.com/data.html .


Axionic extension of the Einstein-aether theory

Abstract: We extend the Einstein-aether theory to take into account the interaction between a pseudoscalar field, which describes the axionic dark matter, and a time-like dynamic unit vector field, which characterizes the velocity of the aether motion. The Lagrangian of the Einstein-aether-axion theory includes cross-terms based on the axion field and its gradient four-vector, on the covariant derivative of the aether velocity four-vector, and on the Riemann tensor and its convolutions. We follow the principles of the Effective Field theory, and include into the Lagrangian of interactions all possible terms up to the second order in the covariant derivative. Interpretation of new couplings is given in terms of irreducible parts of the covariant derivative of the aether velocity, namely, the acceleration four-vector, the shear and vorticity tensors, and the expansion scalar. A spatially isotropic and homogeneous cosmological model with dynamic unit vector field and axionic dark matter is considered as an application of the established theory; new exact solutions are discussed, which describe models with a Big Rip, Pseudo Rip and de Sitter-type asymptotic behavior.

einstein-aether theory
pseudoscalar field
expansion scalar


Spectrum of Galactic Cosmic Rays Accelerated in Supernova Remnants

Abstract: The spectra of high-energy protons and nuclei accelerated by supernova remnant shocks are calculated taking into account magnetic field amplification and Alfvenic drift both upstream and downstream of the shock for different types of supernova remnants during their evolution. The maximum energy of accelerated particles may reach 5·10^18 eV for Fe ions in Type IIb SNRs. The calculated energy spectrum of cosmic rays after propagation through the Galaxy is in good agreement with the spectrum measured at the Earth.

cosmic ray
supernova remnant
diffusive shock acceleration


A peculiar hard X-ray counterpart of a Galactic fast radio burst

Abstract: Fast radio bursts are bright, millisecond-scale radio flashes of yet unknown physical origin. Recently, their extragalactic nature has been demonstrated and an increasing number of the sources have been found to repeat. Young, highly magnetized, isolated neutron stars - magnetars - have been suggested as the most promising candidates for fast radio burst progenitors owing to their energetics and high X-ray flaring activity. Here we report the detection with the Konus-Wind of a hard X-ray event of April 28, 2020, temporarily coincident with a bright, two-peak radio burst from the Galactic magnetar SGR 1935+2154 with properties remarkably similar to those of fast radio bursts. We show that two peaks of the double-peaked X-ray burst coincide in time with the radio peaks, confirming that the X-ray and radio emission most likely have a common origin. Thus, this is the first simultaneous detection of a fast radio burst from a Galactic magnetar and its high-energy counterpart. The total energy emitted in X-rays in this burst is typical of bright short magnetar bursts, but an unusual hardness of its energy spectrum strongly distinguish the April 28 event among multiple "ordinary" flares detected from SGR 1935+2154 previously. This, and a recent non-detection of radio emission from about one hundred typical soft bursts from SGR 1935+2154 favors the idea that bright, FRB-like magnetar signals are associated with rare, hard-spectrum X-ray bursts, which implied rate (∼ 0.04 yr^-1 magnetar^-1) appears consistent with the rate estimate of SGR 1935+2154-like radio bursts (0.007 - 0.04 yr^-1 magnetar^-1).

hard x-ray counterpart
radio burst
galactic magnetar


Cosmic Bounces and Cyclic Universes

Abstract: Cosmological models involving a bounce from a contracting to an expanding universe can address the standard cosmological puzzles and generate "primordial" density perturbations without the need for inflation. Some such models, in particular the ekpyrotic and cyclic models that we focus on, fit rather naturally into string theory. We discuss a number of topics related to these models: the reasoning that leads to the ekpyrotic phase, the predictions for upcoming observations, the differences between singular and non-singular models of the bounce as well as the predictive and explanatory power offered by these models.


Peculiar motions, accelerated expansion and the cosmological axis

Abstract: Peculiar velocities change the expansion rate of any observer moving relative to the smooth Hubble flow. As a result, observers in a galaxy like our Milky Way can experience accelerated expansion within a globally decelerating universe, even when the drift velocities are small. The effect is local, but the affected scales can be large enough to give the false impression that the whole cosmos has recently entered an accelerating phase. Generally, peculiar velocities are also associated with dipole-like anisotropies, triggered by the fact that they introduce a preferred spatial direction. This implies that observers experiencing locally accelerated expansion, as a result of their own drift motion, may also find that the acceleration is maximised in one direction and minimised in the opposite. We argue that, typically, such a dipole anisotropy should be relatively small and the axis should probably lie fairly close to the one seen in the spectrum of the Cosmic Microwave Background.

peculiar velocity
anisotropy
drift motion


Dynamical Chern-Simons Modified Gravity I: Spinning Black Holes in the Slow-Rotation Approximation

Abstract: The low-energy limit of string theory contains an anomaly-canceling correction to the Einstein-Hilbert action, which defines an effective theory: Chern-Simons (CS) modified gravity. The CS correction consists of the product of a scalar field with the Pontryagin density, where the former can be treated as a background field (non-dynamical formulation) or as an evolving field (dynamical formulation). Many solutions of general relativity persist in the modified theory; a notable exception is the Kerr metric, which has sparked a search for rotating black hole solutions. Here, for the first time, we find a solution describing a rotating black hole within the dynamical framework, and in the small-coupling/slow-rotation limit. The solution is axisymmetric and stationary, constituting a deformation of the Kerr metric with dipole scalar "hair," whose effect on geodesic motion is to weaken the frame-dragging effect and shift the location of the inner-most stable circular orbit outwards (inwards) relative to Kerr for co-rotating (counter-rotating) geodesics. We further show that the correction to the metric scales inversely with the fourth power of the radial distance to the black hole, suggesting it will escape any meaningful bounds from weak-field experiments. For example, using binary pulsar data we can only place an initial bound on the magnitude of the dynamical coupling constant of ξ^1/4≲ 10^4km. More stringent bounds will require observations of inherently strong-field phenomena.

gravity
slow-rotation approximation
black hole solution


Measuring the bulk Lorentz factors of gamma-ray bursts with Fermi

Abstract: Gamma-ray bursts (GRBs) are powered by ultra-relativistic jets. Usually a minimum value of the Lorentz factor of the relativistic bulk motion is obtained based on the argument that the observed high energy photons (≫ MeV) can escape without suffering from absorption due to pair production. The exact value, rather than a lower limit, of the Lorentz factor can be obtained if the spectral cutoff due to such absorption is detected. With the good spectral coverage of the Large Area Telescope (LAT) on Fermi, measurements of such cutoff become possible, and two cases (GRB 090926A and GRB 100724B) have been reported to have high-energy cutoffs or breaks. We systematically search for such high energy spectral cutoffs/breaks from the LAT and the Gamma-ray burst monitor (GBM) observations of the prompt emission of GRBs detected since August 2011. Six more GRBs are found to have cutoff-like spectral feature at energies of ∼10-500 MeV. Assuming that these cutoffs are caused by pair-production absorption within the source, the bulk Lorentz factors of these GRBs are obtained. We further find that the Lorentz factors are correlated with the isotropic gamma-ray luminosity of the bursts, indicating that more powerful GRB jets move faster.

bulk lorentz factor
gamma-ray burst
fermi
high energy


Supersonic Relative Velocity between Dark Matter and Baryons: A Review

Abstract: In this review we summarize the effect of relative supersonic velocities between gas and dark matter created right after recombination on astrophysics and cosmology. The relative velocities formally introduce a second order effect on the standard results and thus have been neglected in the framework of linear theory. However, when properly considered, the velocities yield a non-perturbative contribution to the growth of structures which is then inherited by the majority of cosmic signals coming from redshifts above z 10, and in certain cases may even propagate to various low-redshift observables such as the Baryon Acoustic Oscillations measured from the distribution of galaxies. At higher redshifts, the supersonic velocities have thus strong impact affecting the abundance of first star forming halos in an inhomogeneous way, hindering creation of the first stars, leaving traces in the redshifted 21-cm signal of neutral hydrogen, as well as having other important contributions at high redshifts all of which we review in this manuscript.


Nonlocal Models of Cosmic Acceleration

Abstract: I review a class of nonlocally modified gravity models which were proposed to explain the current phase of cosmic acceleration without dark energy. Among the topics considered are deriving causal and conserved field equations, adjusting the model to make it support a given expansion history, why these models do not require an elaborate screening mechanism to evade solar system tests, degrees of freedom and kinetic stability, and the negative verdict of structure formation. Although these simple models are not consistent with data on the growth of cosmic structures many of their features are likely to carry over to more complicated models which are in better agreement with the data.

cosmic acceleration
field equation
infrared graviton
cosmology


Inflation, Cosmic Perturbations and Non-Gaussianities

Abstract: We review the theory of inflationary perturbations. Perturbations at both linear and nonlinear orders are reviewed. We also review a variety of inflation models, emphasizing their signatures on cosmic perturbations.

inflation model
non-gaussianitie
bispectrum
early universe
observational constraint


Scientific Data Mining in Astronomy

Abstract: We describe the application of data mining algorithms to research problems in astronomy. We posit that data mining has always been fundamental to astronomical research, since data mining is the basis of evidence-based discovery, including classification, clustering, and novelty discovery. These algorithms represent a major set of computational tools for discovery in large databases, which will be increasingly essential in the era of data-intensive astronomy. Historical examples of data mining in astronomy are reviewed, followed by a discussion of one of the largest data-producing projects anticipated for the coming decade: the Large Synoptic Survey Telescope (LSST). To facilitate data-driven discoveries in astronomy, we envision a new data-oriented research paradigm for astronomy and astrophysics – astroinformatics. Astroinformatics is described as both a research approach and an educational imperative for modern data-intensive astronomy. An important application area for large time-domain sky surveys (such as LSST) is the rapid identification, characterization, and classification of real-time sky events (including moving objects, photometrically variable objects, and the appearance of transients). We describe one possible implementation of a classification broker for such events, which incorporates several astroinformatics techniques: user annotation, semantic tagging, metadata markup, heterogeneous data integration, and distributed data mining. Examples of these types of collaborative classification and discovery approaches within other science disciplines are presented.

astronomy
classification
data-intensive astronomy


Zonal flow regimes in rotating anelastic spherical shells: an application to giant planets

Abstract: The surface zonal winds observed in the giant planets form a complex jet pattern with alternating prograde and retrograde direction. While the main equatorial band is prograde on the gas giants, both ice giants have a pronounced retrograde equatorial jet. We use three-dimensional numerical models of compressible convection in rotating spherical shells to explore the properties of zonal flows in different regimes where either rotation or buoyancy dominates the force balance. We conduct a systematic parameter study to quantify the dependence of zonal flows on the background density stratification and the driving of convection. We find that the direction of the equatorial zonal wind is controlled by the ratio of buoyancy and Coriolis force. The prograde equatorial band maintained by Reynolds stresses is found in the rotation-dominated regime. In cases where buoyancy dominates Coriolis force, the angular momentum per unit mass is homogenised and the equatorial band is retrograde, reminiscent to those observed in the ice giants. In this regime, the amplitude of the zonal jets depends on the background density contrast with strongly stratified models producing stronger jets than comparable weakly stratified cases. Furthermore, our results can help to explain the transition between solar-like and "anti-solar" differential rotations found in anelastic models of stellar convection zones. In the strongly stratified cases, we find that the leading order force balance can significantly vary with depth (rotation-dominated inside and buoyancy-dominated in a thin surface layer). This so-called "transitional regime" has a visible signature in the main equatorial jet which shows a pronounced dimple where flow amplitudes notably decay towards the equator. A similar dimple is observed on Jupiter, which suggests that convection in the planet interior could possibly operate in this regime.