bluuebunny/crossref_metadata_2025 huggingface dataset:
On Fractional-Order QFT Controllers
Authors: Nataraj P. S. V. , Tharewal Sachin
Published: March 2007

We propose the synthesis of robust fractional-order controllers using the principles of quantitative feedback theory (QFT). The resulting controllers are called as fractional-order QFT controllers. To demonstrate the synthesis method, we synthesize proportional-integral-derivative (PID) and more general types of fractional-order QFT controllers for a fractional-order plant, a DC motor, and a multistage flash desalination process.
Coupled Deformation Modes in the Large Deformation Finite-Element Analysis: Problem Definition
Authors: Hussein Bassam A. , Sugiyama Hiroyuki , Shabana Ahmed A.
Published: March 2007

In the classical formulations of beam problems, the beam cross section is assumed to remain rigid when the beam deforms. In Euler–Bernoulli beam theory, the rigid cross section remains perpendicular to the beam centerline; while in the more general Timoshenko beam theory the rigid cross section is permitted to rotate due to the shear deformation, and as a result, the cross section can have an arbitrary rotation with respect to the beam centerline. In more general beam models as the ones based on the absolute nodal coordinate formulation (ANCF), the cross section is allowed to deform and it is no longer treated as a rigid surface. These more general models lead to new geometric terms that do not appear in the classical formulations of beams. Some of these geometric terms are the result of the coupling between the deformation of the cross section and other modes of deformations such as bending and they lead to a new set of modes referred to in this paper as the ANCF-coupled deformation modes. The effect of the ANCF-coupled deformation modes can be significant in the case of very flexible structures. In this investigation, three different large deformation dynamic beam models are discussed and compared in order to investigate the effect of the ANCF-coupled deformation modes. The three methods differ in the way the beam elastic forces are calculated. The first method is based on a general continuum mechanics approach that leads to a model that includes the ANCF-coupled deformation modes; while the second method is based on the elastic line approach that systematically eliminates these modes. The ANCF-coupled deformation modes eliminated in the elastic line approach are identified and the effect of such deformation modes on the efficiency and accuracy of the numerical solution is discussed. The third large deformation beam model discussed in this investigation is based on the Hellinger–Reissner principle that can be used to eliminate the shear locking encountered in some beam models. Numerical examples are presented in order to demonstrate the use and compare the results of the three different beam formulations. It is shown that while the effect of the ANCF-coupled deformation modes is not significant in very stiff and moderately stiff structures, the effect of these modes can not be neglected in the case of very flexible structures.
Linearized Kerr and spinning massive bodies: An electrodynamics analogy
Authors: Franklin J. , Baker P. T.
Published: March 2007

We discuss the correspondence between spinning, charged spherical sources in electrodynamics and spinning, massive spherical sources in linearized general relativity and show that the form of the potentials and equations of motion are similar in the two cases in the slow motion limit. This similarity allows us to interpret the Kerr metric in analogy with a spinning sphere in electrodynamics and aids in understanding linearized general relativity, where the “forces” are effective and come from the intrinsic curvature of space-time.
A Data Analysis for the Inverse Square Law
Authors: Downie Russell
Published: March 2007

When first encountered, inverse square laws can damage the confidence of beginning physics students whose maths skills are uncertain. Engaging in practical exercises that help them work with the idea can be a great help. We have used the following apparatus for a number of years and our students understand and enjoy the exercise.
Two Kinds of Electron?
Authors: Miller Franklin
Published: March 2007

In the 1930s physicists were confronted by two conflicting values for the charge of the electron as measured with great precision by two different methods. Could it be that there are two kinds of (negative) electrons, differing in charge by a fraction of a percent? The experiments were reconciled when a small systematic error in one of the experiments was identified and taken into account using elementary methods. This is physics at its best.
Galilean Relativity and the Work-Kinetic Energy Theorem
Authors: Tefft Brandon J. , Tefft James A.
Published: March 2007

As the topic of relativity is developed in a first-year physics class, there seems to be a tendency to move as quickly as possible to the fascinating ideas set forth in Einstein's special theory of relativity.1 In this paper we linger a little with the Galilean side of relativity and discuss an intriguing problem and its solution to illustrate a sometimes omitted issue in relativity. Using only ideas from Galilean relativity, we will consider the covariance of the relationship between work and kinetic energy as we move from one inertial reference frame to another.
Why Shiny Metals Are Poor Emitters of Radiation
Authors: Bryan Ronald
Published: March 2007

Students often wonder why polished metals are poor emitters of radiation when they are so shiny. Here I present a simple model that helps to explain this apparent paradox.
A Simple Experiment for Determining the Elastic Constant of a Fine Wire
Authors: Freeman W. Larry , Freda Ronald F.
Published: March 2007

Many general physics laboratories involve the use of springs to demonstrate Hooke's law, and much ado is made about how this can be used as a model for describing the elastic characteristics of materials at the molecular or atomic level. In recent years, the proliferation of computers, and appropriate sensors, have made it possible to demonstrate this on a small scale without the necessity of purchasing or fabricating specialized equipment. This paper describes an experiment that uses these new resources to determine the elongation of a very fine wire as a function of stretching force. A graphical presentation clearly demonstrates the range of validity of Hooke's law and also allows one to see the onset of plastic deformation without catastrophic failure of the wire. The experiment is easy to perform with off-the-shelf equipment and may be readily incorporated into a standard Hooke's law laboratory exercise.
Physics Lab Renovation 101
Authors: Ramlo Susan
Published: March 2007

If you have an older physics laboratory and/or classroom space, you've probably dreamed about how to make the space more adaptable to today's technology and the best teaching and learning practices. Yet, few articles exist in the literature to give us direction for this type of renovation, thus creating the potential of misspending funds that are difficult to obtain. This paper discusses how I determined to renovate my existing introductory laboratory/classroom space and how these changes improved the teaching and learning environment as well as accessibility to the physically handicapped.
Having Fun with Error Analysis
Authors: Siegel Peter
Published: March 2007

We present a fun activity that can be used to introduce students to error analysis: the M&M game. Students are told to estimate the number of individual candies plus uncertainty in a bag of M&M's. The winner is the group whose estimate brackets the actual number with the smallest uncertainty. The exercise produces enthusiastic discussions and serves as a good “mixer” for the first day of a laboratory class.
Why You Should Measure Your Students' Reasoning Ability
Authors: Coletta Vincent P. , Phillips Jeffrey A. , Steinert Jeffrey J.
Published: March 2007

Many teachers administer a force concept test such as the Force Concept Inventory1,2 (FCI) to their students in an effort to evaluate and improve their instructional practices. It has been commonly assumed that looking at class normalized gains allows teachers to compare their courses with other courses. In this paper we present evidence to suggest that the use of class normalized gains alone may not provide a complete picture. We argue that student reasoning ability should also be assessed before between-course comparisons can be made. Assessment of reasoning ability is also useful in identifying students who are at risk. In the following we shall concentrate on the FCI, but we think our conclusions probably apply to physics concept tests generally.
Measuring the Thickness of a Transparent Ring with a Laser
Authors: Leung Alfred F.
Published: March 2007

There seems to be no reasonable way to measure the thickness of a narrow-mouth glass bottle. One can measure the outer and inner diameters of the mouth with a ruler or a pair of calipers and then calculate the thickness. However, this measurement might be interfered with by the threads at the mouth. Furthermore, it is uncertain whether the thickness at the mouth is the same as that of the body. An optical method1 has been proposed. It requires the alignment of an incident laser beam tangent to the bottle and the measurement of the angle of laser light refracted out of the bottle. Here, another method for measuring of thickness is presented. It is first tested on a transparent ring, because the thickness of the ring can be easily measured. The method is then applied to measure the thickness of narrow-mouth glass bottles.
Oscillations in a Linearly Stratified Salt Solution
Authors: Heavers Richard M.
Published: March 2007

Our physics students like to watch a ball bouncing underwater. They do this by dropping a weighted plastic ball into a 1000-ml cylinder filled with a linearly stratified salt-water solution at room temperature. The ball oscillates and comes to rest at about mid-depth. Its motion is analogous to the damped vertical oscillations of a mass hanging from a spring. Our apparatus for producing precise linear salt gradients is simple, inexpensive (cost about $50), and is easy to use. A refractometer is used to measure salt content (or density) of samples taken at various depths. Here we specify fresh- and salt-water volumes for making density gradients suitable for a class demonstration or a three-hour laboratory class.
Make a Mystery Circuit with a Bar Light Fixture
Authors: Lietz Martha
Published: March 2007

Teachers have been building mystery circuits or so-called “black box circuits” to use as a demonstration with their students for years.1 This paper presents an easy way to make simple mystery circuits using inexpensive light fixtures (see Fig. 1) available at almost any home improvement store.2 In a black box circuit, only the lightbulbs are visible and the wiring in these circuits is hidden from the students. The students are then presented with the challenge of deducing the nature of the electrical connections hidden by the black box. The students may remove one or more lightbulbs from the circuit to watch how the brightness of the other bulbs changes and, from this, deduce which bulbs are connected in series and which are connected in parallel.
Steamy Optics: A System for Demonstrating Geometric and Physical Optics
Authors: Craig Matthew , Johnson Ryan , Schultz Sara
Published: March 2007

A simple apparatus consisting of a plastic box with transparent sides filled with fog from an ultrasonic humidifier is very effective for demonstrating the passage of light through optical elements. There are several other methods for making the light passing through an optical system visible, including an apparatus that generates fog using liquid nitrogen.1 Logiurato et al.2 have described a setup that employs a mist maker available from scientific supply companies to produce the fog. The demonstration described here is useful because the initial equipment costs are low, and the necessary items may all be easily obtained locally. The vapor is generated using only tap water.
Three or Four Golden Rules of Lecture
Authors: Dubson Michael
Published: March 2007

• Editor's Note: Each year, AAPT recognizes excellence in teaching by presenting awards at the pre-college and undergraduate level to teachers who have been nominated by their peers for their dedication to teaching. Michael Dubson, the 2006 recipient of the Excellence in Undergraduate Teaching Award, shares his insights and guidelines for effective instruction.
Invariant and Noninvariant Natural Killer T Cells Exert Opposite Regulatory Functions on the Immune Response during Murine Schistosomiasis
Authors: Mallevaey Thierry , Fontaine Josette , Breuilh Laetitia , Paget Christophe , Castro-Keller Alexandre , Vendeville Catherine , Capron Monique , Leite-de-Moraes Maria , Trottein François , Faveeuw Christelle
Published: March 2007

CD1d-restricted natural killer T (NKT) cells represent a heterogeneous population of innate memory immune cells expressing both NK and T-cell markers distributed into two major subsets, i.e., invariant NKT (iNKT) cells, which express exclusively an invariant T-cell receptor (TCR) α chain (Vα14Jα18 in mice), and non-iNKT cells, which express more diverse TCRs. NKT cells quickly produce Th1- and/or Th2-type cytokines following stimulation with glycolipid antigen (Ag) and, through this property, play potent immunoregulatory roles in autoimmune diseases, cancer, and infection. No study has addressed the role of NKT cells in metazoan parasite infections so far. We show that during murine schistosomiasis, the apparent frequency of both iNKT cells and non-iNKT cells decreased in the spleen as early as 3 weeks postinfection (p.i.) and that both populations expressed a greater amount of the activation marker CD69 at 6 weeks p.i., suggesting an activated phenotype. Two different NKT-cell-deficient mouse models, namely, TCR Jα18−/−(exclusively deficient in iNKT cells) and CD1d−/−(deficient in both iNKT and non-iNKT cells) mice, were used to explore the implication of these subsets in infection. We show that whereas both iNKT and non-iNKT cells do not have a major impact on the immune response during the early phase (1 and 4 weeks) of infection, they exert important, although opposite, effects on the immune response during the acute phase of the disease (7 and 12 weeks), after schistosome egg production. Indeed, iNKT cells contribute to Th1 cell differentiation whereas non-iNKT cells might be mostly implicated in Th2 cell differentiation in response to parasite Ag. Our findings suggest, for the first time, that helminths activate both iNKT and non-iNKT cells in vivo, enabling them to differentially influence the Th1/Th2 balance of the immune response.
NRAMP13′ Untranslated Region Polymorphisms Are Not Associated with Natural Resistance toBrucella abortusin Cattle
Authors: Paixão Tatiane A. , Poester Fernando P. , Carvalho Neta Alcina V. , Borges Álan M. , Lage Andrey P. , Santos Renato L.
Published: March 2007

TheNRAMP1gene encodes a divalent cation transporter, located in the phagolysosomal membrane of macrophages, that has been associated with resistance to intracellular pathogens. In cattle, natural resistance against brucellosis has been associated with polymorphisms at the 3′ untranslated region (3′UTR) of theNRAMP1gene, which are detectable by single-strand conformational analysis (SSCA). This study aimed to evaluate the association betweenNRAMP13′UTR polymorphisms and resistance against bovine brucellosis in experimental and natural infections. In experimentally infected pregnant cows, abortion occurred in 42.1% of cows with a resistant genotype (SSCAr;n= 19) and in 43.1% of those with a susceptible genotype (SSCAs;n= 23). Furthermore, no association between intensity of pathological changes and genotype was detected. In a farm with a very high prevalence of bovine brucellosis, the percentages of strains of the SSCArgenotype were 86 and 84% in serologically positive (n= 64) and negative (n= 36) cows, respectively. Therefore, no association was found between theNRAMP1-resistant allele and the resistant phenotype in either experimental or naturally occurring brucellosis. To further support these results, bacterial intracellular survival was assessed in bovine monocyte-derived macrophages from cattle with either the resistant or susceptible genotype. In agreement with our previous results, no difference was observed in the rates of intracellular survival ofB. abortuswithin macrophages from cattle with susceptible or resistant genotypes. Taken together, these results indicate that these polymorphisms at theNRAMP13′UTR do not affect resistance againstB. abortusin cattle and that they are therefore not suitable markers of natural resistance against bovine brucellosis.
Identification of In Vivo-Expressed Immunogenic Proteins by Serological Proteome Analysis of the
Bacillus anthracis
Secretome
Authors: Chitlaru Theodor , Gat Orit , Grosfeld Haim , Inbar Itzhak , Gozlan Yael , Shafferman Avigdor
Published: March 2007

In a previous comparative proteomic study of
Bacillus anthracis
examining the influence of the virulence plasmids and of various growth conditions on the composition of the bacterial secretome, we identified 64 abundantly expressed proteins (T. Chitlaru, O. Gat, Y. Gozlan, N. Ariel, and A. Shafferman, J. Bacteriol.
188
:3551-3571, 2006). Using a battery of sera from
B. anthracis
-infected animals, in the present study we demonstrated that 49 of these proteins are immunogenic. Thirty-eight
B. anthracis
immunogens are documented in this study for the first time. The relative immunogenicities of the 49 secreted proteins appear to span a >10,000-fold range. The proteins eliciting the highest humoral response in the course of infection include, in addition to the well-established immunogens protective antigen (PA), Sap, and EA1, GroEL (BA0267), AhpC (BA0345), MntA (BA3189), HtrA (BA3660), 2,3-cyclic nucleotide diesterase (BA4346), collagen adhesin (BAS5205), an alanine amidase (BA0898), and an endopeptidase (BA1952), as well as three proteins having unknown functions (BA0796, BA0799, and BA0307). Of these 14 highly potent secreted immunogens, 11 are known to be associated with virulence and pathogenicity in
B. anthracis
or in other bacterial pathogens. Combining the results reported here with the results of a similar study of the membranal proteome of
B. anthracis
(T. Chitlaru, N. Ariel, A. Zvi, M. Lion, B. Velan, A. Shafferman, and E. Elhanany, Proteomics
4
:677-691, 2004) and the results obtained in a functional genomic search for immunogens (O. Gat, H. Grosfeld, N. Ariel, I. Inbar, G. Zaide, Y. Broder, A. Zvi, T. Chitlaru, Z. Altboum, D. Stein, S. Cohen, and A. Shafferman, Infect. Immun.
74
:3987-4001, 2006), we generated a list of 84 in vivo-expressed immunogens for future evaluation for vaccine development, diagnostics, and/or therapeutic intervention. In a preliminary study, the efficacies of eight immunogens following DNA immunization of guinea pigs were compared to the efficacy of a PA DNA vaccine. All eight immunogens induced specific high antibody titers comparable to the titers elicited by PA; however, unlike PA, none of them provided protection against a lethal challenge (50 50% lethal doses) of virulent
B. anthracis
strain Vollum spores.
THE REPRESENTATION CATEGORY OF THE WORONOWICZ QUANTUM GROUP SμU(d) AS A BRAIDED TENSOR C*-CATEGORY
Authors: PINZARI CLAUDIA
Published: March 2007

An abstract characterization of the representation category of the Woronowicz twisted SU(d) group is given, generalizing analogous results known in the classical case.
NON-ALGEBRAIC LIMIT CYCLES FOR PARAMETRIZED PLANAR POLYNOMIAL SYSTEMS
Authors: AL-DOSARY KHALIL I. T.
Published: March 2007

In this paper, we determine conditions for planar systems of the form [Formula: see text] where a, b and c are real constants, to possess non-algebraic limit cycles. This is done as an application of a former theorem gives description of the existence of the non-algebraic limit cycles of the family of systems: [Formula: see text] where Pn(x,y), Qn(x,y) and Rm(x,y) are homogeneous polynomials of degrees n, n and m respectively with n < m and n is odd, m is even. The tool for proving these results is based on a method developed in [7].
VACUUM ENERGY: MYTHS AND REALITY
Authors: VOLOVIK G. E.
Published: March 2007

We discuss the main myths related to the vacuum energy and cosmological constant, such as: "unbearable lightness of space–time"; the dominating contribution of zero-point energy of quantum fields to the vacuum energy; non-zero vacuum energy of the false vacuum; dependence of the vacuum energy on the overall shift of energy; the absolute value of energy only has significance for gravity; the vacuum energy depends on the vacuum content; cosmological constant changes after the phase transition; zero-point energy of the vacuum between the plates in Casimir effect must gravitate, that is why the zero-point energy in the vacuum outside the plates must also gravitate; etc. All these and some other conjectures appear to be wrong when one considers the thermodynamics of the ground state of the quantum many-body system, which mimics macroscopic thermodynamics of quantum vacuum. In particular, in spite of the ultraviolet divergence of the zero-point energy, the natural value of the vacuum energy is comparable with the observed dark energy. That is why the vacuum energy is the plausible candidate for the dark energy.
ON THE POSSIBILITY OF QUANTUM GRAVITY EFFECTS AT ASTROPHYSICAL SCALES
Authors: REUTER MARTIN , WEYER HOLGER
Published: March 2007

The nonperturbative renormalization group flow of quantum Einstein gravity (QEG) is reviewed. It is argued that at large distances there could be strong renormalization effects, including a scale dependence of Newton's constant, which mimic the presence of dark matter at galactic and cosmological scales.
WHY DOES GRAVITY IGNORE THE VACUUM ENERGY?
Authors: PADMANABHAN T.
Published: March 2007

The equations of motion for matter fields are invariant under the shift of the matter Lagrangian by a constant. Such a shift changes the energy–momentum tensor of matter by [Formula: see text]. In the conventional approach, gravity breaks this symmetry and the gravitational field equations are not invariant under such a shift of the energy–momentum tensor. We argue that until this symmetry is restored, one cannot obtain a satisfactory solution to the cosmological constant problem. We describe an alternative perspective to gravity in which the gravitational field equations are [Gab - κTab]nanb = 0 for all null vectors na. This is obviously invariant under the change [Formula: see text] and restores the symmetry under shifting the matter Lagrangian by a constant. These equations are equivalent to Gab = κTab + Cgab, where C is now an integration constant so that the role of the cosmological constant is very different in this approach. The cosmological constant now arises as an integration constant, somewhat like the mass M in the Schwarzschild metric, the value of which can be chosen depending on the physical context. These equations can be obtained from a variational principle which uses the null surfaces of space–time as local Rindler horizons and can be given a thermodynamic interpretation. This approach turns out to be quite general and can encompass even the higher order corrections to Einstein's gravity and suggests a principle to determine the form of these corrections in a systematic manner.
PROBING COLD DARK MATTER CUSPS BY GRAVITATIONAL LENSING
Authors: ONEMLI V. K.
Published: March 2007

I elaborate on my prediction that an indirect detection of cold dark matter (CDM) may be possible by observing the gravitational lensing effects of the CDM cusp caustics at cosmological distances. Cusps in the distribution of CDM are plentiful once density perturbations enter the nonlinear regime. Caustic Ring Model of galactic halos provides a well-defined density profile and a geometry near the cusps of the caustic rings. I calculate the lensing effects in this model. As a point-like background source passes behind a cusp of a cosmological foreground halo, the magnification in its image may be detected by present instruments. Depending on the strength of detected effect and the time scale of brightness change, it may even be possible to discriminate between the CDM candidates: axions and weakly interacting massive particles.
RELATIVISTIC EFFECTS IN EXTRASOLAR PLANETARY SYSTEMS
Authors: ADAMS FRED C. , LAUGHLIN GREGORY
Published: March 2007

This paper considers general relativistic (GR) effects in currently observed extrasolar planetary systems. Although GR corrections are small, they can compete with secular interactions in these systems and thereby play an important role. Specifically, some of the observed multiple planet systems are close to secular resonance, where the dynamics is extremely sensitive to GR corrections, and these systems can be used as laboratories to test general relativity. For the three-planet solar system Upsilon Andromedae, secular interaction theory implies an 80% probability of finding the system with its observed orbital elements if GR is correct, compared with only a 2% probability in the absence of GR. In the future, tighter constraints can be obtained with increased temporal coverage.
A NEW SCALE IN THE SKY
Authors: GASPERINI M.
Published: March 2007

The existence of a new ultraviolet scale Λ = gMP for effective theories with gravity and U(1) gauge fields has recently been conjectured as a possible criterion for distinguishing parts of the swampland from the string landscape. Here we discuss a possible phenomenological signature of this scale, for electromagnetic fields, in astrophysical observations.
DARK ENERGY IN MOTION
Authors: MAROTO ANTONIO L.
Published: March 2007

Recent large-scale peculiar velocity surveys suggest that large matter volumes could be moving with appreciable velocity with respect to the CMB rest frame. If confirmed, such results could conflict with the Cosmological Principle according to which the matter and CMB rest frames should converge on very large scales. In this work, we explore the possibility that such large-scale bulk flows are due, not to the motion of matter with respect to the CMB, but to the flow of dark energy with respect to matter. Indeed, when dark energy is moving, the usual definition of the CMB rest frame as that in which the CMB dipole vanishes is not appropriate. We find instead that the dipole vanishes for observers at rest with respect to the cosmic center of mass, i.e. in motion with respect to the background radiation.
ULTRACOLD QUANTUM GASES AS PROBES OF THE UNRUH EFFECT
Authors: CROWELL LAWRENCE B.
Published: March 2007

A high accelerated ultracold quantum gas should be heated by the thermal vacuum of the Unruh effect. This essay discusses possible experimental designs for detecting the Unruh effect with ultracold quantum bosonic gases.
FROM DE SITTER TO DE SITTER: A NON-SINGULAR INFLATIONARY UNIVERSE DRIVEN BY VACUUM
Authors: CARNEIRO SAULO
Published: March 2007

A semi-classical analysis of vacuum energy in the expanding space–time suggests that the cosmological term decays with time, with a concomitant matter production. For early times we find, in Planck units, Λ ≈ H4, where H is the Hubble parameter. The corresponding cosmological solution has no initial singularity, existing since an infinite past. During an infinitely long period we have a quasi-de Sitter, inflationary universe, with H ≈ 1. However, at a given time, the expansion undertakes a phase transition, with H and Λ decreasing to nearly zero in a few Planck times, producing a huge amount of radiation. On the other hand, the late-time scenario is similar to the standard model, with the radiation phase followed by a dust era, which tends asymptotically to a de Sitter universe, with vacuum dominating again.
FERMION CONDENSATION IN A RELATIVISTIC DEGENERATE STAR: ARRESTED COLLAPSE AND MACROSCOPIC EQUILIBRIUM
Authors: SILVERMAN M. P.
Published: March 2007

Fermionic Cooper pairing leading to the BCS-type hadronic superfluidity is believed to account for periodic variations ("glitches") and subsequent slow relaxation in spin rates of neutron stars. Under appropriate conditions, however, fermions can also form a Bose–Einstein condensate of composite bosons. Both types of behavior have recently been observed in tabletop experiments with ultra-cold fermionic atomic gases. Since the behavior is universal (i.e., independent of atomic potential) when the modulus of the scattering length greatly exceeds the separation between particles, one can expect analogous processes to occur within the supradense matter of neutron stars. In this paper, I show how neutron condensation to a Bose–Einstein condensate, in conjunction with relativistically exact expressions for fermion energy and degeneracy pressure and the relations for thermodynamic equilibrium in a spherically symmetric space–time with Schwarzschild metric, leads to stable macroscopic equilibrium states of stars of finite density, irrespective of mass.
GRAVITY'S IMMUNITY FROM VACUUM: THE HOLOGRAPHIC STRUCTURE OF SEMICLASSICAL ACTION
Authors: PADMANABHAN T.
Published: March 2007

Principle of equivalence, general covariance and the demand that the variation of the action functional should be well defined lead to a generic Lagrangian for semiclassical gravity of the form L = Qabcd Rabcd with ∇b Qabcd = 0. The expansion of Qabcd in terms of the derivatives of the metric tensor determines the structure of the theory uniquely. The zeroth order term gives the Einstein–Hilbert action and the first-order correction is given by the Gauss–Bonnet action. Remarkably, any such Lagrangian can be decomposed into surface and bulk terms which are related holographically. The equations of motion can be obtained purely from a surface term in the gravity sector and hence gravity does not respond to the changes in the bulk vacuum energy density.
THE "DARK SIDE" OF GRAVITATIONAL EXPERIMENTS
Authors: HOYLE CHARLES D.
Published: March 2007

Theoretical speculations about the quantum nature of the gravitational interaction have motivated many recent experiments. But perhaps the most profound and puzzling questions that these investigations address surround the observed cosmic acceleration, or Dark Energy. This mysterious substance comprises roughly 2/3 of the energy density of the universe. Current gravitational experiments may soon have the sensitivity to detect subtle clues that will reveal the mechanism behind the cosmic acceleration. On the laboratory scale, short-range tests of the Newtonian inverse-square law (ISL) provide the most sensitive measurements of gravity at the Dark Energy length scale, λ d = (ħc/ρ d )1/4 ≈ 85 μm, where ρ d ≈ 3.8 keV / cm 3 is the observed Dark Energy density. This length scale may also have fundamental significance that could be related to the "size" of the graviton. At the University of Washington, we are conducting the world's most sensitive, short-range test of the Newtonian ISL.
EVADING THE ASTROPHYSICAL LIMITS ON LIGHT PSEUDOSCALARS
Authors: JAIN PANKAJ , MANDAL SUBHAYAN
Published: March 2007

We study the possibility of evading astrophysical bounds on light pseudoscalars. We argue that the solar bounds can be evaded if we have a sufficiently strong self-coupling of the pseudoscalars. The required couplings do not conflict with any known experimental bounds. We show that it is possible to find a coupling range such that the results of the recent PVLAS experiment are not in conflict with any astrophysical bounds.
KALUZA–KLEIN REDUCTION OF CONFORMALLY FLAT SPACES
Authors: GRUMILLER D. , JACKIW R.
Published: March 2007

Kaluza–Klein reduction of conformally flat spaces is considered for arbitrary dimensions. The corresponding equations are particularly elegant for the reduction from four to three dimensions. Assuming circular symmetry leads to explicit solutions which also arise from specific two-dimensional dilaton gravity actions.
KILLING HORIZONS KILL HORIZON DEGREES
Authors: BERGAMIN L. , GRUMILLER D.
Published: March 2007

Frequently, it is argued that the microstates responsible for the Bekenstein–Hawking entropy should arise from some physical degrees of freedom located near or on the black hole horizon. In this essay, we elucidate that instead entropy may emerge from the conversion of physical degrees of freedom, attached to a generic boundary, into unobservable gauge degrees of freedom attached to the horizon. By constructing the reduced phase space, it can be demonstrated that such a transmutation indeed takes place for a large class of black holes, including Schwarzschild.
QUANTUM DETERMINISM FROM QUANTUM GENERAL COVARIANCE
Authors: NIKOLIĆ HRVOJE
Published: March 2007

The requirement of general covariance of quantum field theory (QFT) naturally leads to quantization based on the manifestly covariant De Donder–Weyl formalism. To recover the standard noncovariant formalism without violating covariance, fields need to depend on time in a specific deterministic manner. This deterministic evolution of quantum fields is recognized as a covariant version of the Bohmian hidden-variable interpretation of QFT.
IS THE RINDLER HORIZON ENERGY NONVANISHING?
Authors: CULETU HRISTU
Published: March 2007

A nonvanishing value for the Rindler horizon energy is proposed, by an analogy with the "near horizon" Schwarzschild metric. We show that the Rindler horizon energy is given by the same formula E = α/2 obtained by Padmanabhan for the Schwarzschild space–time, where α is the gravitational radius.
FUNDAMENTAL SPATIO-TEMPORAL DECOHERENCE: A KEY TO SOLVING THE CONCEPTUAL PROBLEMS OF BLACK HOLES, COSMOLOGY AND QUANTUM MECHANICS
Authors: GAMBINI RODOLFO , PORTO RAFAEL A. , PULLIN JORGE
Published: March 2007

Unitarity is a pillar of quantum theory. Nevertheless, it is also a source of several of its conceptual problems. We note that in a world where measurements are relational, as is the case in gravitation, quantum mechanics exhibits a fundamental level of loss of coherence. This can be the key to solving, among others, the puzzles posed by the black hole information paradox, the formation of inhomogeneities in cosmology and the measurement problem in quantum mechanics.
BLACK HOLE BREMSSTRAHLUNG: CAN IT BE AN EFFICIENT SOURCE OF GRAVITATIONAL WAVES?
Authors: DE OLIVEIRA H. P. , DAMIÃO SOARES I. , TONINI E. V.
Published: March 2007

We propose a new source of gravitational waves in the context of full nonlinear General Relativity: the black hole bremsstrahlung. A black hole initially in uniform motion is decelerated along its direction of motion; as a consequence of the deceleration, a radiative transfer process is set up through which the black hole loses its kinetic energy along with part of its rest mass by the emission of gravitational waves. Depending on the initial velocity and the strength of the deceleration, this process can correspond to a high power output in the initial pulse of gravitational radiation emitted.
SUPERMASSIVE BLACK HOLES OR BOSON STARS? HAIR COUNTING WITH GRAVITATIONAL WAVE DETECTORS
Authors: BERTI EMANUELE , CARDOSO VITOR
Published: March 2007

The evidence for supermassive Kerr black holes in galactic centers is strong and growing, but only the detection of gravitational waves will convincingly rule out other possibilities to explain the observations. The Kerr space–time is completely specified by the first two multipole moments: mass and angular momentum. This is usually referred to as the "no-hair theorem," but it is really a "two-hair" theorem. If general relativity is the correct theory of gravity, the most plausible alternative to a supermassive Kerr black hole is a rotating boson star. Numerical calculations indicate that the space–time of rotating boson stars is determined by the first three multipole moments ("three-hair theorem"). The Laser Interferometer Space Antenna (LISA) could accurately measure the oscillation frequencies of these supermassive objects. We propose to use these measurements to "count their hair," unambiguously determining their nature and properties.
HOW TO BYPASS BIRKHOFF THROUGH EXTRA DIMENSIONS: A SIMPLE FRAMEWORK FOR INVESTIGATING THE GRAVITATIONAL COLLAPSE IN VACUUM
Authors: BIZOŃ PIOTR , SCHMIDT BERND G.
Published: March 2007

It is fair to say that our current mathematical understanding of the dynamics of gravitational collapse to a black hole is limited to the spherically symmetric situation and, in fact, even in this case much remains to be learned. The reason is that Einstein's equations become tractable only if they are reduced to a (1 + 1)-dimensional system of partial differential equations. Owing to this technical obstacle, very little is known about the collapse of pure gravitational waves because by Birkhoff's theorem there is no spherical collapse in vacuum. In this essay, we describe a new cohomogeneity-two symmetry reduction of the vacuum Einstein equations in five and higher odd dimensions which evades Birkhoff's theorem and admits time-dependent asymptotically flat solutions. We argue that this model provides an attractive (1 + 1)-dimensional geometric setting for investigating the dynamics of gravitational collapse in vacuum.
BLACK HOLE IN A MODEL WITH DILATON AND MONOPOLE FIELDS
Authors: KYRIAKOPOULOS E.
Published: March 2007

We present an exact black hole solution in a model having besides gravity a dilaton and a monopole field. The solution has three free parameters, which can be identified with the monopole charge, the ADM mass, and the dilaton charge. The metric is asymptotically flat and has two horizons and irremovable singularity only at r = 0. The dilaton field is singular only at r = 0. The dominant and the strong energy condition are satisfied outside and on the external horizon. According to a formulation of the no hair conjecture, the solution is "hairy." Also the well-known GHS-GM solution is obtained from our solution for certain values of its parameters.
HOW MANY UNIVERSES DO THERE NEED TO BE?
Authors: SCOTT DOUGLAS , ZIBIN J. P.
Published: March 2007

In the simplest cosmological models consistent with General Relativity, the total volume of the Universe is either finite or infinite, depending on whether or not the spatial curvature is positive. Current data suggest that the curvature is very close to flat, implying that one can place a lower limit on the total volume. In a Universe of finite age, the "particle horizon" defines the patch of the Universe which is observable to us. Based on today's best-fit cosmological parameters it is possible to constrain the number of observable Universe sized patches, NU. Specifically, using the new Wilkinson Microwave Anisotropy Probe (WMAP) data, we can say that there are at least 21 patches out there the same volume as ours, at 95% confidence. Moreover, even if the precision of our cosmological measurements continues to increase, density perturbations at the particle horizon size limit us to never knowing that there are more than about 105 patches out there.
A TRAPPED DIPOLAR BEC INTERFEROMETRY TEST OF E = mc2
Authors: PINTO FABRIZIO
Published: March 2007

It is shown that, by arranging the atoms in a Bose–Einstein condensate gas within an optical lattice, it is possible to directly verify experimentally that the van der Waals potential energy contributes an effective change to the gravitational mass of the atoms trapped in the array sites equal to that predicted by Einstein's mass–energy equivalency equation, E = mgc2. Some original results of electrostatics and quantum mechanics in curved space are discussed, and strategies to amplify and observe these effects are quantitatively evaluated to illustrate the feasibility of the novel test proposed herein.
IS THE EQUIVALENCE PRINCIPLE DOOMED FOREVER TO DANTE'S INFERNO ON ACCOUNT OF QUANTUM MECHANICS?
Authors: ACCIOLY ANTONIO , ALDROVANDI RUBEN , PASZKO RICARDO
Published: March 2007

It is commonly assumed that the equivalence principle can coexist without conflict with quantum mechanics. We shall argue here that, contrary to popular belief, this principle does not hold in quantum mechanics. We illustrate this point by computing the second-order correction for the scattering of a massive scalar boson by a weak gravitational field, treated as an external field. The resulting cross-section turns out to be mass-dependent. A way out of this dilemma would be, perhaps, to consider gravitation without the equivalence principle. At first sight, this seems to be a too much drastic attitude toward general relativity. Fortunately, the teleparallel version of general relativity — a description of the gravitational interaction by a force similar to the Lorentz force of electromagnetism and that, of course, dispenses with the equivalence principle — is equivalent to general relativity, thus providing a consistent theory for gravitation in the absence of the aforementioned principle.
TOWERS OF GRAVITATIONAL THEORIES
Authors: GOLDBERGER WALTER D. , ROTHSTEIN IRA Z.
Published: March 2007

In this essay, we introduce a theoretical framework designed to describe black hole dynamics. The difficulties in understanding such dynamics stems from the proliferation of scales involved when one attempts to simultaneously describe all of the relevant dynamical degrees of freedom. These range from the modes that describe the black hole horizon, which are responsible for dissipative effects, to the long wavelength gravitational radiation that drains mechanical energy from macroscopic black hole bound states. We approach the problem from a Wilsonian point of view, by building a tower of theories of gravity each of which is valid at different scales. The methodology leads to multiple new results in diverse topics including phase transitions of Kaluza–Klein black holes and the interactions of spinning black hole in non-relativistic orbits. Moreover, our methods tie together speculative ideas regarding dualities for black hole horizons to real physical measurements in gravitational wave detectors.
RECONSTRUCTING DARK ENERGY
Authors: SAHNI VARUN , STAROBINSKY ALEXEI
Published: March 2007

This review summarizes recent attempts to reconstruct the expansion history of the universe and to probe the nature of dark energy. Reconstruction methods can be broadly classified into parametric and non-parametric approaches. It is encouraging that, even with the limited observational data currently available, different approaches give consistent results for the reconstruction of the Hubble parameter H(z) and the effective equation of state w(z) of dark energy. Model independent reconstruction using current data allows for modest evolution of dark energy density with redshift. However, a cosmological constant (= dark energy with a constant energy density) remains an excellent fit to the data. Some pitfalls to be guarded against during cosmological reconstruction are summarized and future directions for the model independent reconstruction of dark energy are explored.
COSMOLOGICAL ACCELERATION FROM STRUCTURE FORMATION
Authors: RÄSÄNEN SYKSY
Published: March 2007

We discuss the Buchert equations, which describe the average expansion of an inhomogeneous dust universe. In the limit of small perturbations, they reduce to the Friedmann–Robertson–Walker equations. However, when the universe is very inhomogeneous, the behavior can be qualitatively different from the FRW case. In particular, the average expansion rate can accelerate even though the local expansion rate decelerates everywhere. We clarify the physical meaning of this paradoxical feature with a simple toy model, and demonstrate how acceleration is intimately connected with gravitational collapse. This provides a link to structure formation, which in turn has a preferred time around the era when acceleration has been observed to start.
STRING THEORY, QUANTUM MECHANICS AND NONCOMMUTATIVE GEOMETRY: A NEW PERSPECTIVE ON THE GRAVITATIONAL DYNAMICS OF D0-BRANES
Authors: SINGH T. P.
Published: March 2007

We do not know the symmetries underlying string theory. Furthermore, there must exist an inherently quantum, and space–time independent, formulation of this theory. Independent of string theory, there should exist a description of quantum mechanics which does not refer to a classical space–time manifold. We propose such a formulation of quantum mechanics, based on noncommutative geometry. This description reduces to standard quantum mechanics, whenever an external classical space–time is available. However, near the Planck energy scale, self-gravity effects modify the Schrödinger equation to the non-linear Doebner–Goldin equation. Remarkably, this non-linear equation also arises in the quantum dynamics of D0-branes. This suggests that the noncommutative quantum dynamics proposed here is actually the quantum gravitational dynamics of D0-branes, and that automorphism invariance is a symmetry of string theory.
COSMOLOGY: A BIRD'S EYE VIEW
Authors: COLEY ALAN A. , HERVIK SIGBJØRN , LIM WOEI CHET
Published: March 2007

We discuss the difference in views of the universe as seen by two different observers. While one of the observers follows a geodesic congruence defined by the geometry of the cosmological model, the other observer follows the fluid flow lines of a perfect fluid with a linear equation of state. We point out that the information these observers collect regarding the state of the universe can be radically different; while one observes a non-inflating, ever-expanding, ever-lasting universe, the other observer can experience a dynamical behavior reminiscent of that of quintessence or even that of a phantom cosmology leading to a "big rip" singularity within finite time (but without the need for exotic forms of matter).
THE INERTIA OF HEAT AND ITS ROLE IN THE DYNAMICS OF DISSIPATIVE COLLAPSE
Authors: HERRERA L.
Published: March 2007

The decreasing of the inertial mass density, established in the past for dissipative fluids, is found to be produced by the "inertial" term of the transport equation. Once the transport equation is coupled to the dynamical equation, one finds that the contribution of the inertial term diminishes the effective inertial mass and the "gravitational" force term, by the same factor. An intuitive picture and prospective applications of this result to astrophysical scenarios are discussed.
INFORMATION RECOVERY FROM BLACK HOLES
Authors: BALASUBRAMANIAN VIJAY , MAROLF DONALD , ROZALI MOSHE
Published: March 2007

We argue that if black hole entropy arises from a finite number of underlying quantum states, then any particular such state can be identified from infinity. The finite density of states implies a discrete energy spectrum, and, in general, such spectra are non-degenerate except as determined by symmetries. Therefore, knowledge of the precise energy, and of other commuting conserved charges, determines the quantum state. In a gravitating theory, all conserved charges including the energy are given by boundary terms that can be measured at infinity. Thus, within any theory of quantum gravity, no information can be lost in black holes with a finite number of states. However, identifying the state of a black hole from infinity requires measurements with Planck scale precision. Hence, observers with insufficient resolution will experience information loss.
DARK MATTER, AND ITS DARKNESS
Authors: AHLUWALIA-KHALILOVA D. V.
Published: March 2007

Assuming the validity of the general relativistic description of gravitation on astrophysical and cosmological length scales, we analytically infer that the Friedmann–Robertson–Walker cosmology with Einsteinian cosmological constant, and a vanishing spatial curvature constant, unambiguously requires a significant amount of dark matter. This requirement is consistent with other indications for dark matter. The same space–time symmetries that underlie the freely falling frames of Einsteinian gravity also provide symmetries which, for the spin one half representation space, furnish a novel construct that carries extremely limited interactions with respect to the terrestrial detectors made of the standard model material. Both the "luminous" and "dark" matter turn out to be residents of the same representation space but they derive their respective "luminosity" and "darkness" from either belonging to the sector with (CPT)2 = +𝟙, or to the sector with (CPT)2 = -𝟙.