Physical Review D (Particles, Fields, Gravitation, and Cosmology) -- 15 April 2007

Orbital precession due to central-force perturbations

Published 5 April 2007 (7 pages)

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We calculate the perifocus precession of Keplerian orbits under the influence of arbitrary central-force perturbations. Our result is in the form of a one-dimensional integral that is straightforward to evaluate numerically. We demonstrate the effectiveness of our formula for the case of the Yukawa potential. We obtain analytic results for potentials of the form V(r)=alpharⁿ and V(r)=alphaln(r/lambda) in terms of the hypergeometric function 2F1((1/2)-(n/2),1-(n/2);2;e²), where e is the eccentricity. Our results reproduce the known general relativistic (n=-3), constant force (n=1), and cosmological constant (n=2) precession formulas. Planetary perihelion precessions are often used to constrain the sizes of hypothetical new weak forces—our results allow for more precise, and often stronger, constraints on such proposed new forces.

Theoretical approach to thermal noise caused by an inhomogeneously distributed loss: Physical insight by the advanced modal expansion

Kazuhiro Yamamoto, Masaki Ando, Keita Kawabe, and Kimio Tsubono

Published 17 April 2007 (9 pages)

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We modified the modal expansion, which is the traditional method used to calculate thermal noise. This advanced modal expansion provides physical insight about the discrepancy between the actual thermal noise caused by inhomogeneously distributed loss and the traditional modal expansion. This discrepancy comes from correlations between the thermal fluctuations of the resonant modes. The thermal-noise spectra estimated by the advanced modal expansion are consistent with the results of measurements of thermal fluctuations caused by inhomogeneous losses.

Quantum variational measurement and the optical lever intracavity topology of gravitational-wave detectors

F. Ya. Khalili

Published 19 April 2007 (14 pages)

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The intracavity topologies of laser gravitational-wave detectors proposed several years ago are the promising way to obtain sensitivity of these devices significantly better than the Standard Quantum Limit (SQL). In essence, the intracavity detector is a two-stage device where the end mirrors displacement created by the gravitational wave is transferred to the displacement of an additional local mirror by means of the optical rigidity. The local mirror positions have to be monitored by an additional local meter. It is evident that the local meter precision defines the sensitivity of the detector. To overcome the SQL, the quantum variational measurement can be used in the local meter. In this method a frequency-dependent correlation between the meter backaction noise and measurement noise is introduced, which allows us to eliminate the backaction noise component from the meter output signal. This correlation is created by means of an additional filter cavity. In this article the sensitivity limitations of this scheme imposed by the optical losses both in the local meter itself and in the filter cavity are estimated. It is shown that the main sensitivity limitation stems from the filter cavity losses. In order to overcome it, it is necessary to increase the filter cavity length. In a preliminary prototype experiment, an approximate 10 m long filter cavity can be used to obtain sensitivity approximately 2–3 times better than the SQL. For future Quantum Non-Demolition (QND) gravitational-wave detectors with sensitivity about 10 times better than the SQL, the filter cavity length should be within kilometer range.

Diagonalization of the length sensing matrix of a dual recycled laser interferometer gravitational wave antenna

Shuichi Sato, Seiji Kawamura, Keiko Kokeyama, Fumiko Kawazoe, and Kentaro Somiya

Published 26 April 2007 (9 pages)

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Next generation gravitational wave antennas employ resonant sideband extraction (RSE) interferometers with Fabry-Perot cavities in the arms as an optical configuration. In order to realize stable, robust control of the detector system, it is a key issue to extract appropriate control signals for longitudinal degrees of freedom of the complex coupled-cavity system. In this paper, a novel length sensing and control scheme is proposed for the tuned RSE interferometer that is both simple and efficient. The sensing matrix can be well diagonalized, owing to a simple allocation of two rf modulations and to a macroscopic displacement of the cavity mirrors, which cause a detuning of the rf modulation sidebands.

Dissecting the Cygnus region with TeV gamma rays and neutrinos

John F. Beacom and Matthew D. Kistler

Published 5 April 2007 (7 pages)

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Recent Milagro observations of the Cygnus region have revealed both diffuse TeV gamma-ray emission and a bright and extended TeV source, MGRO J2019+37, which seems to lack an obvious counterpart at other wavelengths. Additional study of this curious object also promises to provide important clues concerning one of the Milky Way's most active environments. We point out some of the principal facts involved by following three modes of attack. First, to gain insight into this mysterious source, we consider its relation to known objects in both the Cygnus region and the rest of the Galaxy. Second, we find that a simple hadronic model can easily accommodate Milagro's flux measurement (which is at a single energy), as well as other existing observations spanning nearly 7 orders of magnitude in gamma-ray energy. Third, since a hadronic gamma-ray spectrum necessitates an accompanying TeV neutrino flux, we show that IceCube observations may provide the first direct evidence of a Galactic cosmic-ray accelerator.

Self-induced decoherence in dense neutrino gases

Georg G. Raffelt and Günter Sigl

Published 16 April 2007 (15 pages)

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Dense neutrino gases exhibit collective oscillations where “self-maintained coherence” is a characteristic feature, i.e., neutrinos of different energies oscillate with the same frequency. In a nonisotropic gas, however, the flux term of the neutrino-neutrino interaction has the opposite effect of causing kinematical decoherence of neutrinos propagating in different directions, an effect that is at the origin of the “multiangle behavior” of neutrinos streaming off a supernova core. We cast the equations of motion in a form where the role of the flux term is manifest. We study in detail the symmetric case of equal neutrino and antineutrino densities where the evolution consists of collective pair conversions (“bipolar oscillations”). A gas of this sort is unstable in that an infinitesimal anisotropy is enough to trigger a runaway towards flavor equipartition. The “self-maintained coherence” of a perfectly isotropic gas gives way to “self-induced decoherence.”

Dark energy and the mass of galaxy clusters

Cosimo Bambi

Published 16 April 2007 (5 pages)

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Up to now, dark energy evidences are based on the dynamics of the universe on very large scales, above 1 Gpc. Assuming it continues to behave like a cosmological constant Lambda on much smaller scales, I discuss its effects on the motion of nonrelativistic test particles in a weak gravitational field, and I propose a way to detect evidences of Lambda[not-equal]0 at the scale of about 1 Mpc: the main ingredient is the measurement of galaxy cluster masses.

Enhanced cosmological GRB rates and implications for cosmogenic neutrinos

Hasan Yüksel and Matthew D. Kistler

Published 17 April 2007 (11 pages)

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Gamma-ray bursts, which are among the most violent events in the Universe, are one of the few viable candidates to produce ultra high-energy cosmic rays. Recently, observations have revealed that GRBs generally originate from metal-poor, low-luminosity galaxies and do not directly trace cosmic star formation, as might have been assumed from their association with core-collapse supernovae. Several implications follow from these findings. The redshift distribution of observed GRBs is expected to peak at higher redshift (compared to cosmic star formation), which is supported by the mean redshift of the Swift GRB sample, <z>~3. If GRBs are, in fact, the source of the observed UHECR, then cosmic-ray production would evolve with redshift in a stronger fashion than has been previously suggested. This necessarily leads, through the GZK process, to an enhancement in the flux of cosmogenic neutrinos, providing a near-term approach for testing the gamma-ray burst-cosmic-ray connection with ongoing and proposed UHE neutrino experiments.

Gravitational lensing from compact bodies: Analytical results for strong and weak deflection limits

Paolo Amore, Mayra Cervantes, Arturo De Pace, and Francisco M. Fernández

Published 18 April 2007 (12 pages)

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We develop a nonperturbative method that yields analytical expressions for the deflection angle of light in a general static and spherically symmetric metric. The method works by introducing into the problem an artificial parameter, called delta, and by performing an expansion in this parameter to a given order. The results obtained are analytical and nonperturbative because they do not correspond to a polynomial expression in the physical parameters. Already to first order in delta the analytical formulas obtained using our method provide at the same time accurate approximations both at large distances (weak deflection limit) and at distances close to the photon sphere (strong deflection limit). We have applied our technique to different metrics and verified that the error is at most 0.5% for all regimes. We have also proposed an alternative approach which provides simpler formulas, although with larger errors.

Galactic antiproton spectrum at high energies: Background expectation versus exotic contributions

Torsten Bringmann and Pierre Salati

Published 27 April 2007 (16 pages)

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A new generation of upcoming space-based experiments will soon start to probe the spectrum of cosmic-ray antiparticles with an unprecedented accuracy and, in particular, will open up a window to energies much higher than those accessible so far. It is thus timely to carefully investigate the expected antiparticle fluxes at high energies. Here, we perform such an analysis for the case of antiprotons. We consider both standard sources as the collision of other cosmic rays with interstellar matter, as well as exotic contributions from dark matter annihilations in the galactic halo. Up to energies well above 100 GeV, we find that the background flux in antiprotons is almost uniquely determined by the existing low-energy data on various cosmic-ray species; for even higher energies, however, the uncertainties in the parameters of the underlying propagation model eventually become significant. We also show that if the dark matter is composed of particles with masses at the TeV scale, which is naturally expected in extra-dimensional models as well as in certain parameter regions of supersymmetric models, the annihilation flux can become comparable to—or even dominate—the antiproton background at the high energies considered here.

Unveiling hidden patterns in CMB anisotropy maps

Tuhin Ghosh, Amir Hajian, and Tarun Souradeep

Published 27 April 2007 (8 pages)

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Bianchi VIIh models have been recently proposed to explain potential anomalies in the CMB anisotropy as observed by WMAP. We investigate the violation of statistical isotropy due to an embedded Bianchi VIIh template in the CMB anisotropy maps to determine whether the existence of a hidden Bianchi template in the WMAP data is consistent with the previous null detection of the bipolar power spectrum in the WMAP first-year maps. We argue that, although correcting the WMAP maps for the Bianchi template may explain some features in the WMAP data, it may cause other anomalies such as preferred directions leading to detectable levels of violation of statistical isotropy in the Bianchi corrected maps. We compute the bipolar power spectrum for the low density Bianchi VIIh models embedded in the background CMB anisotropy maps with the power spectrum that have been shown in recent literature to best fit the first-year WMAP data. By examining the statistical isotropy of these maps, we put a limit of (((sigma)/H))0<=2.77×10^-10(99% C.L.) on the shear parameter in Bianchi VIIh models.

Stellar collapse dynamics with neutrino flavor changing neutral currents

Philip S. Amanik and George M. Fuller

Published 30 April 2007 (12 pages)

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We perform one-zone simulations of the infall epoch of a presupernova stellar core in the presence of neutrino flavor changing scattering interactions. Our calculations give a self-consistent assessment of the relationship between flavor changing rates and the reduction in electron fraction and redistribution of initial electron lepton number among the neutrino flavors. We discuss and include in our calculations subnuclear density medium corrections for flavor changing scattering coherence factors. We find that flavor changing couplings epsilon>3×10^-4 in either the nue<-->nuµ or nue<-->nutau channels result in a dynamically significant reduction in the core electron fraction relatively soon after neutrino trapping and well before the core reaches nuclear matter density.

Non-Gaussian covariance of CMB B modes of polarization and parameter degradation

Chao Li, Tristan L. Smith, and Asantha Cooray

Published 5 April 2007 (12 pages)

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The B-mode polarization lensing signal is a useful probe of the neutrino mass and to a lesser extent the dark energy equation of state as the signal depends on the integrated mass power spectrum between us and the last scattering surface. This lensing B-mode signal, however, is non-Gaussian and the resulting non-Gaussian covariance to the power spectrum could impact cosmological parameter measurements, as correlations between B-mode bins are at a level of 0.1. On the other hand, for temperature and E-mode polarization power spectra, the non-Gaussian covariance is not significant, where we find correlations at the 10^-5 level even for adjacent bins. When the power spectrum is estimated with roughly 5 uniformly spaced bins from l=5 to l=100 and 13 logarithmic uniformly spaced bins from l=100 to l=2000, the resulting degradation on neutrino mass and dark energy equation of state is about a factor of 2 to 3 when compared to the case where statistics are simply considered to be Gaussian. If we increase the total number of bins between l=5 and l=2000 to be about 100, we find that the non-Gaussianities only make a minor difference with less than a few percent correction to uncertainties of most cosmological parameters determined from the data. For Planck, the resulting constraints on the sum of the neutrino masses is sigmaSigmamnu~0.2 eV and on the dark energy equation of state parameter we find that sigmaw~0.5. A post-Planck experiment can improve the neutrino mass measurement by a factor of 3 to 4.

Imprints of a primordial preferred direction on the microwave background

Lotty Ackerman, Sean M. Carroll, and Mark B. Wise

Published 5 April 2007 (7 pages)

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Rotational invariance is a well-established feature of low-energy physics. Violations of this symmetry must be extremely small today, but could have been larger in earlier epochs. In this paper we examine the consequences of a small breaking of rotational invariance during the inflationary era when the primordial density fluctuations were generated. Assuming that a fixed-norm vector picked out a preferred-direction during the inflationary era, we explore the imprint it would leave on the cosmic microwave background anisotropy, and provide explicit formulas for the expected amplitudes <almal[sup [prime]]m[sup [prime]]^*> of the spherical-harmonic coefficients. We suggest that it is natural to expect that the imprint on the primordial power spectrum of a preferred spatial direction is approximately scale-invariant, and examine a simple model in which this is true.

Dynamical behavior of generic quintessence potentials: Constraints on key dark energy observables

Dragan Huterer and Hiranya V. Peiris

Published 5 April 2007 (22 pages)

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We perform a comprehensive study of a class of dark energy models—scalar field models where the effective potential can be described by a polynomial series—exploring their dynamical behavior using the method of flow equations that has previously been applied to inflationary models. Using supernova, baryon oscillation, cosmic microwave background (CMB) and Hubble constant data, and an implicit theoretical prior imposed by the scalar field dynamics, we find that the LambdaCDM model provides an excellent fit to the data. Constraints on the generic scalar field potential parameters are presented, along with the reconstructed w(z) histories consistent with the data and the theoretical prior. We propose and pursue computationally feasible algorithms to obtain estimates of the principal components of the equation of state, as well as parameters w0 and wa. Further, we use the Monte Carlo Markov chain machinery to simulate future data based on the Joint Dark Energy Mission, Planck, and baryon acoustic oscillation surveys and find that the inverse area figure of merit improves nearly by an order of magnitude. Therefore, most scalar field models that are currently consistent with data can be potentially ruled out by future experiments. We also comment on the classification of dark energy models into thawing and freezing in light of the more diverse evolution histories allowed by this general class of potentials.

Conditions for the cosmological viability of f(R) dark energy models

Luca Amendola, Radouane Gannouji, David Polarski, and Shinji Tsujikawa

Published 6 April 2007 (22 pages)

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We derive the conditions under which dark energy models whose Lagrangian densities f are written in terms of the Ricci scalar R are cosmologically viable. We show that the cosmological behavior of f(R) models can be understood by a geometrical approach consisting of studying the m(r) curve on the (r,m) plane, where m[equivalent]Rf,RR/f,R and r[equivalent]-Rf,R/f with f,R[equivalent]df/dR. This allows us to classify the f(R) models into four general classes, depending on the existence of a standard matter epoch and on the final accelerated stage. The existence of a viable matter-dominated epoch prior to a late-time acceleration requires that the variable m satisfies the conditions m(r)[approximate]+0 and dm/dr>-1 at r[approximate]-1. For the existence of a viable late-time acceleration we require instead either (i) m=-r-1, (sqrt(3)-1)/2<m<=1 and dm/dr<-1 or (ii) 0<m<=1 at r=-2. These conditions identify two regions in the (r,m) space, one for the matter era and the other for the acceleration. Only models with an m(r) curve that connects these regions and satisfies the requirements above lead to an acceptable cosmology. The models of type f(R)=alphaR^-n and f=R+alphaR^-n do not satisfy these conditions for any n>0 and n<-1 and are thus cosmologically unacceptable. Similar conclusions can be reached for many other examples discussed in the text. In most cases the standard matter era is replaced by a cosmic expansion with scale factor a[proportional]t^1/2. We also find that f(R) models can have a strongly phantom attractor but in this case there is no acceptable matter era.

Fine-tuning free paradigm of two-measures theory: k-essence, absence of initial singularity of the curvature, and inflation with graceful exit to the zero cosmological constant state

E. I. Guendelman and A. B. Kaganovich

Published 9 April 2007 (31 pages)

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The dilaton-gravity sector of the two-measures field theory (TMT) is explored in detail in the context of spatially flat Friedman-Robertson-Walker (FRW) cosmology. The model possesses scale invariance which is spontaneously broken due to the intrinsic features of the TMT dynamics. The dilaton phi dependence of the effective Lagrangian appears only as a result of the spontaneous breakdown of the scale invariance. If no fine-tuning is made, the effective phi-Lagrangian p(phi,X) depends quadratically upon the kinetic term X. Hence TMT represents an explicit example of the effective k-essence resulting from first principles without any exotic term in the underlying action intended for obtaining this result. Depending of the choice of regions in the parameter space (but without fine-tuning), TMT exhibits different possible outputs for cosmological dynamics: (a) Absence of initial singularity of the curvature while its time derivative is singular. This is a sort of sudden singularities studied by Barrow on purely kinematic grounds. (b) Power law inflation in the subsequent stage of evolution. Depending on the region in the parameter space the inflation ends with a graceful exit either into the state with zero cosmological constant (CC) or into the state driven by both a small CC and the field phi with a quintessencelike potential. (c) Possibility of resolution of the old CC problem. From the point of view of TMT, it becomes clear why the old CC problem cannot be solved (without fine-tuning) in conventional field theories. (d) TMT enables two ways for achieving small CC without fine-tuning of dimensionful parameters: either by a seesaw type mechanism or due to a correspondence principle between TMT and conventional field theories (i.e. theories with only the measure of integration sqrt(-g) in the action). (e) There is a wide range of the parameters such that in the late time universe: the equation of state w=p/rho<-1; w asymptotically (as t-->[infinity]) approaches -1 from below; rho approaches a constant, the smallness of which does not require fine-tuning of dimensionful parameters.

Consequences of dark matter-dark energy interaction on cosmological parameters derived from type Ia supernova data

Luca Amendola, Gabriela Camargo Campos, and Rogerio Rosenfeld

Published 10 April 2007 (7 pages)

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Models where the dark matter component of the Universe interacts with the dark energy field have been proposed as a solution to the cosmic coincidence problem, since in the attractor regime both dark energy and dark matter scale in the same way. In these models the mass of the cold dark matter particles is a function of the dark energy field responsible for the present acceleration of the Universe, and different scenarios can be parametrized by how the mass of the cold dark matter particles evolves with time. In this article we study the impact of a constant coupling delta between dark energy and dark matter on the determination of a redshift dependent dark energy equation of state wDE(z) and on the dark matter density today from SNIa data. We derive an analytical expression for the luminosity distance in this case. In particular, we show that the presence of such a coupling increases the tension between the cosmic microwave background data from the analysis of the shift parameter in models with constant wDE and SNIa data for realistic values of the present dark matter density fraction. Thus, an independent measurement of the present dark matter density can place constraints on models with interacting dark energy.

Dark energy and the MSSM

Philippe Brax and Jérôme Martin

Published 12 April 2007 (17 pages)

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We consider the coupling of quintessence to observable matter in supergravity and study the dynamics of both supersymmetry breaking and quintessence in this context. We investigate how the quintessence potential is modified by supersymmetry breaking and analyze the structure of the soft supersymmetry-breaking terms. We pay attention to their dependence on the quintessence field and to the electroweak symmetry breaking, i.e. the pattern of fermion masses at low energy within the minimal supersymmetric standard model (MSSM) coupled to quintessence. In particular, we compute explicitly how the fermion masses generated through the Higgs mechanism depend on the quintessence field for a general model of quintessence. Fifth force and equivalence principle violations are potentially present as the vacuum expectation values of the Higgs bosons become quintessence field dependent. We emphasize that equivalence principle violations are a generic consequence of the fact that, in the MSSM, the fermions couple differently to the two Higgs doublets. Finally, we also discuss how the scaling of the cold dark and baryonic matter energy density is modified and comment on the possible variation of the gauge coupling constants, among which is the fine structure constant, and of the proton-electron mass ratio.

Search for gravitational waves in the CMB after WMAP3: Foreground confusion and the optimal frequency coverage for foreground minimization

Alexandre Amblard, Asantha Cooray, and Manoj Kaplinghat

Published 12 April 2007 (13 pages)

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B-modes of cosmic microwave background (CMB) polarization can be created by a primordial gravitational wave background. If this background was created by inflation, then the amplitude of the polarization signal is proportional to the energy density of the universe during inflation. The primordial signal will be contaminated by polarized foregrounds including dust and synchrotron emission within the galaxy. In light of the WMAP polarization maps, we consider the ability of several hypothetical CMB polarization experiments to separate primordial CMB B-mode signal from galactic foregrounds. We also study the optimization of a CMB experiment with a fixed number of detectors in the focal plane to determine how the detectors should be distributed in different frequency bands to minimize foreground confusion. We show that the optimal configuration requires observations in at least 5 channels spread over the frequency range between 30 and 500 GHz with substantial coverage around 150 GHz. If a low-resolution space experiment using 1000 detectors to reach a noise level of about 1000 nK² concentrates on roughly 66% of the sky with the least foreground contamination, the minimum detectable level of the tensor-to-scalar ratio would be about 0.002 at the 99% confidence level for an optical depth of 0.1.

Reconstruction of interacting dark energy models from parametrizations

R. Rosenfeld

Published 12 April 2007 (6 pages)

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Models with interacting dark energy can alleviate the cosmic coincidence problem by allowing dark matter and dark energy to evolve in a similar fashion. At a fundamental level, these models are specified by choosing a functional form for the scalar potential and for the interaction term. However, in order to compare to observational data it is usually more convenient to use parametrizations of the dark energy equation of state and the evolution of the dark matter energy density. Once the relevant parameters are fitted, it is important to obtain the shape of the fundamental functions. In this paper I show how to reconstruct the scalar potential and the scalar interaction with dark matter from general parametrizations. I give a few examples and show that it is possible for the effective equation of state for the scalar field to cross the phantom barrier when interactions are allowed. I analyze the uncertainties in the reconstructed potential arising from foreseen errors in the estimation of fit parameters and point out that a Yukawa-like linear interaction results from a simple parametrization of the coupling.

Using the cluster mass function from weak lensing to constrain neutrino masses

Jostein R. Kristiansen, Øystein Elgarøy, and Håkon Dahle

Published 13 April 2007 (7 pages)

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We discuss the variation of cosmological upper bounds on Mnu, the sum of the neutrino masses, with the choice of data sets included in the analysis, pointing out a few oddities not easily seen when all data sets are combined. For example, the effect of applying different priors varies significantly depending on whether we use the power spectrum from the 2dFGRS or SDSS galaxy survey. A conservative neutrino mass limit of Mnu<1.43 eV (95% C.L.) is obtained by combining the WMAP 3 yr data with the cluster mass function measured by weak gravitational lensing. This limit has the virtue of not making any assumptions about the bias of luminous matter with respect to the dark matter, and is in this sense (and this sense only) bias-free.

Connecting low energy leptonic CP violation to leptogenesis

S. Pascoli, S. T. Petcov, and Antonio Riotto

Published 13 April 2007 (5 pages)

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It was commonly thought that the observation of low energy leptonic CP-violating phases would not automatically imply the existence of a baryon asymmetry in the leptogenesis scenario. This conclusion does not generically hold when the issue of flavor is relevant and properly taken into account in leptogenesis. We illustrate this point with various examples studying the correlation between the baryon asymmetry and the CP-violating asymmetry in neutrino oscillations and the effective Majorana mass in neutrinoless double beta decay.

Inflation, reheating, and dark matter

Víctor H. Cárdenas

Published 13 April 2007 (4 pages)

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In a recent paper, Liddle and Urena-Lopez suggested that to have a unified model of inflation and dark matter is imperative to have a proper reheating process where part of the inflaton field remains. In this paper I propose a model where this is possible. I found that incorporating the effect of plasma masses generated by the inflaton products enables us to stop the process. A numerical estimated model is presented.

Causal field theory with an infinite speed of sound

Niayesh Afshordi, Daniel J. H. Chung, and Ghazal Geshnizjani

Published 13 April 2007 (10 pages)

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We introduce a model of scalar field dark energy, Cuscuton, which can be realized as the incompressible (or infinite speed of sound) limit of a scalar field theory with a noncanonical kinetic term (or k-essence). Even though perturbations of Cuscuton propagate superluminally, we show that they have a locally degenerate phase space volume (or zero entropy), implying that they cannot carry any microscopic information, and thus the theory is causal. Even coupling to ordinary scalar fields cannot lead to superluminal signal propagation. Furthermore, we show that the family of constant field hypersurfaces is the family of constant mean curvature hypersurfaces, which are the analogs of soap films (or soap bubbles) in Euclidian space. This enables us to find the most general solution in 1+1 dimensions, whose properties motivate conjectures for global degeneracy of the phase space in higher dimensions. Finally, we show that the Cuscuton action can model the continuum limit of the evolution of a field with discrete degrees of freedom and argue why it is protected against quantum corrections at low energies. While this paper mainly focuses on interesting features of Cuscuton in a Minkowski space-time, a companion paper examines cosmology with Cuscuton dark energy.

Vacuum energy and spectral function sum rules

Alexander Y. Kamenshchik, Alessandro Tronconi, Gian Paolo Vacca, and Giovanni Venturi

Published 13 April 2007 (11 pages)

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We reformulate the problem of the cancellation of the ultraviolet divergencies of the vacuum energy, particularly important at the cosmological level, in terms of a saturation of spectral function sum rules which leads to a set of conditions on the spectrum of the fundamental theory. We specialize the approach to both Minkowski and de Sitter space-times and investigate some examples.

Statefinder diagnostic for the modified polytropic Cardassian universe

Ze-Long Yi and Tong-Jie Zhang

Published 16 April 2007 (9 pages)

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We apply the statefinder diagnostic to the modified polytropic Cardassian universe in this work. We find that the statefinder diagnostic is quite effective to distinguish Cardassian models from a series of other cosmological models. The s-r plane is used to classify the modified polytropic Cardassian models into six cases. The evolutionary trajectories in the s-r plane for the cases with different n and beta reveal different evolutionary properties of the universe. In addition, we combine the observational H(z) data, the cosmic microwave background data, and the baryonic acoustic oscillation data to make a joint analysis. We find that case 2 can be excluded at the 68.3% confidence level and any case is consistent with the observations at the 95.4% confidence level.

Property of the spectrum of large-scale magnetic fields from inflation

Kazuharu Bamba

Published 16 April 2007 (5 pages)

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The property of the spectrum of large-scale magnetic fields generated due to the breaking of the conformal invariance of the Maxwell theory through some mechanism in inflationary cosmology is studied. It is shown that the spectrum of the generated magnetic fields should not be perfectly scale-invariant but be slightly red so that the amplitude of large-scale magnetic fields can be stronger than ~10^-12 G at the present time. This analysis is performed by assuming the absence of amplification due to the late-time action of some dynamo (or similar) mechanism.

Systematic errors in cosmic microwave background interferometry

Emory F. Bunn

Published 16 April 2007 (18 pages)

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Cosmic microwave background (CMB) polarization observations will require superb control of systematic errors in order to achieve their full scientific potential, particularly in the case of attempts to detect the B modes that may provide a window on inflation. Interferometry may be a promising way to achieve these goals. This paper presents a formalism for characterizing the effects of a variety of systematic errors on interferometric CMB polarization observations, with particular emphasis on estimates of the B-mode power spectrum. The most severe errors are those that couple the temperature anisotropy signal to polarization; such errors include cross talk within detectors, misalignment of polarizers, and cross polarization. In a B mode experiment, the next most serious category of errors are those that mix E and B modes, such as gain fluctuations, pointing errors, and beam shape errors. The paper also indicates which sources of error may cause circular polarization (e.g., from foregrounds) to contaminate the cosmologically interesting linear polarization channels, and conversely whether monitoring of the circular-polarization channels may yield useful information about the errors themselves. For all the sources of error considered, estimates of the level of control that will be required for both E and B mode experiments are provided. Simulations of a mock experiment are presented to illustrate the results. Both experiments that interfere linear polarizations and those that interfere circular polarizations are considered. The fact that circular experiments simultaneously measure both linear polarization Stokes parameters in each baseline mitigates some sources of error.

Cosmological perturbations in warm inflationary models with viscous pressure

Sergio del Campo, Ramón Herrera, and Diego Pavón

Published 17 April 2007 (8 pages)

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Scalar and tensorial cosmological perturbations generated in warm inflationary scenarios whose matter-radiation fluid is endowed with a viscous pressure are considered. Recent observational data from the Wilkinson Microwave Anisotropy Probe experiment are employed to restrict the parameters of the model. Although the effect of this pressure on the matter power spectrum is of the order of a few percent, it may be detected in future experiments.

Indirect detection of little Higgs dark matter

Maxim Perelstein and Andrew Spray

Published 17 April 2007 (7 pages)

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Little Higgs models with T parity contain an attractive dark matter candidate, the heavy photon. We compute the cross section of the heavy photon annihilation into Z-photon pairs, which turns out to be substantially higher than the previously computed cross section for the two photon final state. Unfortunately, even with this enhancement, the monochromatic photon flux from galactic heavy photon annihilation is unlikely to be detectable by GLAST or the currently operating atmospheric Cerenkov telescopes. We also compute the flux of high-energy neutrinos from the annihilation of the heavy photons captured by the Sun and the Earth. The maximum flux of upward-going muons due to such neutrinos is about 1 yr^-1 km^-2.

On what scale should inflationary observables be constrained?

Marina Cortês, Andrew R. Liddle, and Pia Mukherjee

Published 18 April 2007 (9 pages)

083520 Full Text: PDF (473 kB) | Buy Article

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We examine the choice of scale at which constraints on inflationary observables are presented. We describe an implementation of the hierarchy of inflationary consistency equations which ensures that they remain enforced on different scales, and then seek to optimize the scale for presentation of constraints on marginalized inflationary parameters from WMAP3 data. For models with spectral index running, we find a strong variation of the constraints through the range of observational scales available, and optimize by finding the scale which decorrelates constraints on the spectral index nS and the running. This scale is k=0.017 Mpc^-1, and gives a reduction by a factor of more than four in the allowed parameter area in the nS–r plane (r being the tensor-to-scalar ratio) relative to k=0.002 Mpc^-1. These optimized constraints are similar to those obtained in the no-running case. We also extend the analysis to a larger compilation of data, finding essentially the same conclusions.

Constraining the variation of G by cosmic microwave background anisotropies

K. C. Chan and M.-C. Chu

Published 18 April 2007 (7 pages)

083521 Full Text: PDF (500 kB) | Buy Article

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We use the Cosmic Microwave Background Anisotropies (CMBA) power spectra to constrain the cosmological variation of gravitational constant G. It is found that the sensitivity of CMBA to the variation of G is enhanced when G is required to converge to its present value. The variations of G from the CMB decoupling epoch z~1000 to the present time are modeled by a step function and a linear function of scale factor a respectively, and the corresponding 95% confidence intervals for G/G0 are [0.95, 1.05] and [0.89, 1.13], G0 being the present value. The CMBA constraint is unique in the sense that it entails the range of redshift from z[approximate]1000 to 0.

What is the criterion for a strong first order electroweak phase transition in singlet models?

Amine Ahriche

Published 18 April 2007 (9 pages)

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It is widely believed that the existence of singlet scalars in some standard model extensions can easily make the electroweak phase transition strongly first order, which is needed for the electroweak baryogenesis scenario. In this paper, we will examine the strength of the electroweak phase transition in the simplest extension of the standard model with a real singlet using the sphaleron energy at the critical temperature. We find that the phase transition is stronger by adding a singlet; and also that the criterion for a strong phase transition Omega(Tc)/Tc>~1, where Omega=(upsilon²+(x-x0)²)^1/2 and x (x0) is the singlet vev in the broken (symmetric) phase, is not valid for models containing singlets, even though often used in the literature. The usual condition upsilonc/Tc>~1 is more meaningful, and it is satisfied for a large part of the parameter space for physically allowed Higgs masses.

Energy conditions and cosmic acceleration

J. Santos, J. S. Alcaniz, N. Pires, and M. J. Rebouças

Published 23 April 2007 (6 pages)

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In general relativity, the energy conditions are invoked to restrict general energy-momentum tensors Tµnu in different frameworks, and to derive general results that hold in a variety of general contexts on physical grounds. We show that in the standard Friedmann-Lemaître-Robertson-Walker (FLRW) approach, where the equation of state of the cosmological fluid is unknown, the energy conditions provide model-independent bounds on the behavior of the distance modulus of cosmic sources as a function of the redshift for any spatial curvature. We use the most recent type Ia supernovae (SNe Ia) observations, which include the new Hubble Space Telescope SNe Ia events, to carry out a model-independent analysis of the energy conditions violation in the context of the standard cosmology. We show that both the null (NEC), weak (WEC), and dominant (DEC) conditions, which are associated with the existence of the so-called phantom fields, seem to have been violated only recently (z<~0.2), whereas the condition for attractive gravity, i.e., the strong energy condition (SEC) was first violated billions of years ago, at z>~1.

Radiation can never again dominate matter in a vacuum dominated universe

Lawrence M. Krauss and Robert J. Scherrer

Published 25 April 2007 (4 pages)

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+ -	Show Abstract We demonstrate that in a vacuum-energy-dominated expansion phase, surprisingly neither the decay of matter nor matter-antimatter annihilation into relativistic particles can ever cause radiation to once again dominate over matter in the future history of the Universe.

Efficient cosmological parameter estimation with Hamiltonian Monte Carlo technique

Amir Hajian

Published 30 April 2007 (11 pages)

083525 Full Text: PDF (505 kB) | Buy Article

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Traditional Markov Chain Monte Carlo methods suffer from low acceptance rate, slow mixing, and low efficiency in high dimensions. Hamiltonian Monte Carlo resolves this issue by avoiding the random walk. Hamiltonian Monte Carlo (HMC) is a Markov Chain Monte Carlo (MCMC) technique built upon the basic principle of Hamiltonian mechanics. Hamiltonian dynamics allows the chain to move along trajectories of constant energy, taking large jumps in the parameter space with relatively inexpensive computations. This new technique improves the acceptance rate by a factor of 4 while reducing the correlations and boosts up the efficiency by almost a factor of D in a D-dimensional parameter space. Therefore shorter chains will be needed for a reliable parameter estimation comparing to a traditional MCMC chain yielding the same performance. Besides that, the HMC is well suited for sampling from non-Gaussian and curved distributions which are very hard to sample from using the traditional MCMC methods. The method is very simple to code and can be easily plugged into standard parameter estimation codes such as CosmoMC. In this paper we demonstrate how the HMC can be efficiently used in cosmological parameter estimation. Also we discuss possible ways of getting good estimates of the derivatives of (the log of) posterior which is needed for HMC.

Precise constraints on the dark matter content of Milky Way dwarf galaxies for gamma-ray experiments

Louis E. Strigari, Savvas M. Koushiappas, James S. Bullock, and Manoj Kaplinghat

Published 30 April 2007 (13 pages)

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We examine the prospects for detecting gamma-rays from dark matter annihilation in the six most promising dwarf spheroidal (dSph) satellite galaxies of the Milky Way. We use recently measured velocity dispersion profiles to provide a systematic investigation of the dark matter mass distribution of each galaxy, and show that the uncertainty in the gamma-ray flux from mass modeling is less than a factor of ~5 for each dSph if we assume a smooth Navarro-Frenk-White (NFW) profile. We show that Ursa Minor and Draco are the most promising dSphs for gamma-ray detection with GLAST and other planned observatories. For each dSph, we investigate the flux enhancement resulting from halo substructure, and show that the enhancement factor relative to a smooth halo flux cannot be greater than about 100. This enhancement depends very weakly on the lower mass cutoff scale of the substructure mass function. While the amplitude of the expected flux from each dSph depends sensitively on the dark matter model, we show that the flux ratios between the six Sphs are known to within a factor of about 10. The flux ratios are also relatively insensitive to the current theoretical range of cold dark matter halo central slopes and substructure fractions.

Higgs effective potential in the littlest Higgs model at the one-loop level

Antonio Dobado, Lourdes Tabares-Cheluci, and Siannah Peñaranda

Published 30 April 2007 (12 pages)

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In this work we compute the contributions to the Higgs effective potential coming from the fermion and gauge boson sectors at the one-loop level in the context of the SU(5)/SO(5) littlest Higgs (LH) model using a cutoff Lambda and including all finite parts. We consider both, the (SU(2)×U(1))1×(SU(2)×U(1))2 and the (SU(2)×U(1))1×(SU(2)×U(1)) gauge group versions of the LH model. We also show that the Goldstone bosons present in the model do not contribute to the effective potential at the one-loop level. Finally, by neglecting the contribution of higher dimensional operators, we discuss the restrictions that the new one-loop contributions set on the parameter space of the LH model and the need to include higher loop corrections to the Higgs potential.

Quantum causal histories in the light of quantum information

Etera R. Livine and Daniel R. Terno

Published 2 April 2007 (8 pages)

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We use techniques of quantum information theory to analyze the quantum causal histories approach to quantum gravity. While it is consistent to introduce closed timelike curves (CTCs), they cannot generically carry independent degrees of freedom. Moreover, if the effective dynamics of the chronology-respecting part of the system is linear, it should be completely decoupled from the CTCs. In the absence of a CTC, not all causal structures admit the introduction of quantum mechanics. It is possible for those and only those causal structures that can be represented as quantum computational networks. Dynamics of the subsystems should not be unitary or even completely positive. However, we show that other commonly made assumptions ensure the complete positivity of the reduced dynamics.

Coupled bulk and brane fields about a de Sitter brane

Antonio Cardoso, Kazuya Koyama, Andrew Mennim, Sanjeev S. Seahra, and David Wands

Published 3 April 2007 (16 pages)

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We consider the evolution of a bulk scalar field in anti-de Sitter (AdS) spacetime linearly coupled to a scalar field on a de Sitter boundary brane. We present results of a spectral analysis of the system, and find that the model can exhibit both bound and continuum resonant modes. We find that zero, one, or two bound states may exist, depending upon the masses of the brane and bulk fields relative to the Hubble length and the AdS curvature scale and the coupling strength. In all cases, we find a critical coupling above which there exists a tachyonic bound state. We show how the 5-dimensional spectral results can be interpreted in terms of a 4-dimensional effective theory for the bound states. We find excellent agreement between our analytic results and the results of a new numerical code developed to model the evolution of bulk fields coupled to degrees of freedom on a moving brane. This code can be used to model the behavior of brane-world cosmological perturbations in scenarios for which no analytic results are known.

Thermodynamic behavior of the Friedmann equation at the apparent horizon of the FRW universe

M. Akbar and Rong-Gen Cai

Published 3 April 2007 (9 pages)

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It is shown that the differential form of Friedmann equation of a FRW universe can be rewritten as the first law of thermodynamics dE=TdS+WdV at apparent horizon, where E=rhoV is the total energy of matter inside the apparent horizon, V is the volume inside the apparent horizon, W=(rho-P)/2 is the work density, rho and P are energy density and pressure of matter in the universe, respectively. From the thermodynamic identity one can derive that the apparent horizon r-tildeA has associated entropy S=A/4G and temperature T=kappa/2pi in Einstein general relativity, where A is the area of apparent horizon and kappa is the surface gravity at apparent horizon of FRW universe. We extend our procedure to the Gauss-Bonnet gravity and more general Lovelock gravity and show that the differential form of Friedmann equations in these gravities can also be written as dE=TdS+WdV at the apparent horizon of FRW universe with entropy S being given by expression previously known via black hole thermodynamics.

Late time tails of the massive vector field in a black hole background

R. A. Konoplya, A. Zhidenko, and C. Molina

Published 4 April 2007 (9 pages)

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We investigate the late-time behavior of the massive vector field in the background of the Schwarzschild and Schwarzschild–de Sitter black holes. For Schwarzschild black hole, at intermediately late times the massive vector field is represented by three functions with different decay law Psi0~t^{-([script-l]+3/2)}sinmt, Psi1~t^{-([script-l]+5/2)}sinmt, Psi2~t^{-([script-l]+1/2)}sinmt, while at asymptotically late times the decay law Psi~t^-5/6sin(mt) is universal and does not depend on the multipole number [script-l]. Together with a previous study of massive scalar and Dirac fields where the same asymptotically late-time decay law was found, it means that the asymptotically late-time decay law ~t^-5/6sin(mt) does not depend also on the spin of the field under consideration. For Schwarzschild–de Sitter black holes it is observed in two different regimes in the late-time decay of perturbations: nonoscillatory exponential damping for small values of m and oscillatory quasinormal mode decay for high enough m. Numerical and analytical results are found for these quasinormal frequencies.

Quasiequilibrium black hole-neutron star binaries in general relativity

Keisuke Taniguchi, Thomas W. Baumgarte, Joshua A. Faber, and Stuart L. Shapiro

Published 4 April 2007 (17 pages)

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We construct quasiequilibrium sequences of black hole-neutron star binaries in general relativity. We solve Einstein's constraint equations in the conformal thin-sandwich formalism, subject to black hole boundary conditions imposed on the surface of an excised sphere, together with the relativistic equations of hydrostatic equilibrium. In contrast to our previous calculations we adopt a flat spatial background geometry and do not assume extreme mass ratios. We adopt a Gamma=2 polytropic equation of state and focus on irrotational neutron star configurations as well as approximately nonspinning black holes. We present numerical results for ratios of the black hole's irreducible mass to the neutron star's ADM mass in isolation of Mirr^BH/MADM,0^NS=1, 2, 3, 5, and 10. We consider neutron stars of baryon rest mass MB^NS/MB^max=83% and 56%, where MB^max is the maximum allowed rest mass of a spherical star in isolation for our equation of state. For these sequences, we locate the onset of tidal disruption and, in cases with sufficiently large mass ratios and neutron star compactions, the innermost stable circular orbit. We compare with previous results for black hole-neutron star binaries and find excellent agreement with third-order post-Newtonian results, especially for large binary separations. We also use our results to estimate the energy spectrum of the outgoing gravitational radiation emitted during the inspiral phase for these binaries.

Statistical description of rotating Kaluza-Klein black holes

Roberto Emparan and Alessandro Maccarrone

Published 5 April 2007 (11 pages)

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We extend the recent microscopic analysis of extremal dyonic Kaluza-Klein (D0-D6) black holes to cover the regime of fast rotation in addition to slow rotation. Fastly rotating black holes, in contrast to slow ones, have nonzero angular velocity and possess ergospheres, so they are more similar to the Kerr black hole. The D-brane model reproduces their entropy exactly, but the mass gets renormalized from weak to strong coupling, in agreement with recent macroscopic analyses of rotating attractors. We discuss how the existence of the ergosphere and superradiance manifest themselves within the microscopic model. In addition, we show in full generality how Myers-Perry black holes are obtained as a limit of Kaluza-Klein black holes, and discuss the slow and fast rotation regimes and superradiance in this context.

Post-Newtonian expansion for Gauss-Bonnet gravity

Thomas Sotiriou and Enrico Barausse

Published 5 April 2007 (5 pages)

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+ -	Show Abstract The Parametrized Post-Newtonian expansion of gravitational theories with a scalar field coupled to the Gauss-Bonnet invariant is performed and confrontation of such theories with solar system experiments is discussed.

A tale of two superpotentials: Stability and instability in designer gravity

Aaron J. Amsel, Thomas Hertog, Stefan Hollands, and Donald Marolf

Published 5 April 2007 (11 pages)

084008 Full Text: PDF (266 kB) | Buy Article
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We investigate the stability of asymptotically anti-de Sitter gravity coupled to tachyonic scalar fields with mass at or slightly above the Breitenlohner-Freedman bound. The boundary conditions in these “designer gravity” theories are defined in terms of an arbitrary function W. Previous work had suggested that the energy in designer gravity is bounded below if (i) W has a global minimum and (ii) the scalar potential admits a superpotential P. More recently, however, certain solutions were found (numerically) to violate the proposed energy bound. We resolve the discrepancy by observing that a given scalar potential can admit two possible branches of the corresponding superpotential, P±. When there is a P- branch, we rigorously prove a lower bound on the energy; the P+ branch alone is not sufficient. Our numerical investigations (i) confirm this picture, (ii) confirm other critical aspects of the (complicated) proofs, and (iii) suggest that the existence of P- may in fact be necessary (as well as sufficient) for the energy of a designer gravity theory to be bounded below.

Quantum field backreaction corrections and remnant stable evaporating Schwarzschild-de Sitter dynamical black hole

Hossein Ghaffarnejad

Published 5 April 2007 (8 pages)

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Time-dependent backreaction corrections of the renormalized expectation value of the stress-energy tensor operator of a massless quantum scalar field, minimally coupled, in the two-dimensional spherically symmetric nondilatonic Schwarzschild-de Sitter dynamical black hole metric, is applied to determine the final state of its thermal radiation. Renormalization theory in the two-dimensional analog of a generally curved space-time reduces to a trace anomaly defined in terms of the Ricci scalar. So the regularized stress-energy tensor, in close relation to the work by Christensen and Fulling, may be obtained by the nonlocal contribution of the trace anomaly and some suitable parameters. Linear-order, time-dependent solutions of the metric backreaction equations, in close relation to the work by Bousso et al., show that the spherically symmetric nondilatonic evaporating Schwarzschild-de Sitter dynamical black hole final state, reduces to a set of remnant, stable, mini black holes where their metric should be described in terms of some permissible discrete Eddington-Finkelstein advance times. Also the results of this article are confirmed by the consequences of time-independent backreaction solutions which were derived recently by the author and may propose a new approach for quantization of gravity, in which the cosmological constant and a distinguished observer have a key role.

Cosmology of f(R) gravity in the metric variational approach

Baojiu Li and John D. Barrow

Published 5 April 2007 (13 pages)

084010 Full Text: PDF (3565 kB) | Buy Article

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We consider the cosmologies that arise in a subclass of f(R) gravity with f(R)=R+µ²ⁿ⁺²/(-R)ⁿ and n[is-an-element-of](-1,0) in the metric (as opposed to the Palatini) variational approach to deriving the gravitational field equations. The calculations of the isotropic and homogeneous cosmological models are undertaken in the Jordan frame and at both the background and the perturbation levels. For the former, we also discuss the connection to the Einstein frame in which the extra degree of freedom in the theory is associated with a scalar field sharing some of the properties of a “chameleon” field. For the latter, we derive the cosmological perturbation equations in general theories of f(R) gravity in covariant form and implement them numerically to calculate the cosmic microwave background (CMB) temperature and matter power spectra of the cosmological model. The CMB power is shown to reduce at low l's, and the matter power spectrum is almost scale independent at small scales, thus having a similar shape to that in standard general relativity. These are in stark contrast with what was found in the Palatini f(R) gravity, where the CMB power is largely amplified at low l's and the matter spectrum is strongly scale dependent at small scales. These features make the present model more adaptable than that arising from the Palatini f(R) field equations, and none of the data on background evolution, CMB power spectrum, or matter power spectrum currently rule it out.

Cylindrical gravitational waves in expanding universes: Models for waves from compact sources

Robert H. Gowdy and B. Douglas Edmonds

Published 5 April 2007 (4 pages)

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New boundary conditions are imposed on the familiar cylindrical gravitational wave vacuum spacetimes. The new spacetime family represents cylindrical waves in a flat expanding (Kasner) universe. Space sections are flat and nonconical where the waves have not reached and wave amplitudes fall off more rapidly than they do in Einstein-Rosen solutions, permitting a more regular null inifinity.

Effects of QCD phase transition on gravitational radiation from two-dimensional collapse and bounce of massive stars

Nobutoshi Yasutake, Kei Kotake, Masa-aki Hashimoto, and Shoichi Yamada

Published 5 April 2007 (12 pages)

084012 Full Text: PDF (805 kB) | Buy Article

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We perform two-dimensional, magneto-hydrodynamical core-collapse simulations of massive stars accompanying the QCD phase transition. We study how the phase-transition affects the gravitational waveforms near the epoch of core-bounce. As for initial models, we change the strength of rotation and magnetic fields. Particularly, the degree of differential rotation in the iron core (Fe-core) is changed parametrically. As for the microphysics, we adopt a phenomenological equation of state above the saturation density, including two parameters to change the hardness before the transition. We assume the first order phase transition, where the conversion of bulk nuclear matter to a chirally symmetric quark-gluon phase is described by the MIT bag model. Based on these computations, we find that the phase transition can make the maximum amplitudes larger up to ~10 percents than the ones without the phase transition. On the other hand, when the degree of the differential rotation becomes larger, the maximum amplitudes become smaller up to ~10 percents owing to the phase transition. We find that even extremely strong magnetic fields ~10¹⁷ G in the protoneutron star do not affect these results.

Spacetimes with longitudinal and angular magnetic fields in third order Lovelock gravity

M. H. Dehghani and N. Bostani

Published 6 April 2007 (8 pages)

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We obtain two new classes of magnetic solutions in third order Lovelock gravity. The first class of solutions yields an (n+1)-dimensional spacetime with a longitudinal magnetic field generated by a static source. We generalize this class of solutions to the case of spinning magnetic strings with one or more rotation parameters. These solutions have no curvature singularity and no horizons, but have a conic geometry. For the spinning string, when one or more rotation parameters are nonzero, the string has a net electric charge which is proportional to the magnitude of the rotation parameters, while the static string has no net electric charge. The second class of solutions yields a spacetime with an angular magnetic field. These solutions have no curvature singularity, no horizon, and no conical singularity. Although the second class of solutions may be made electrically charged by a boost transformation, the transformed solutions do not present new spacetimes. Finally, we use the counterterm method in third order Lovelock gravity and compute the conserved quantities of these spacetimes.

Renormalization group running of Newton's constant G: The static isotropic case

H. W. Hamber and R. M. Williams

Published 6 April 2007 (19 pages)

084014 Full Text: PDF (307 kB) | Buy Article

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Corrections are computed to the classical static isotropic solution of general relativity, arising from nonperturbative quantum gravity effects. A slow rise of the effective gravitational coupling with distance is shown to involve a genuinely nonperturbative scale, closely connected with the gravitational vacuum condensate, and thereby related to the observed effective cosmological constant. We argue that in contrast to phenomenological approaches, the underlying functional integral formulation of the theory severely constrains possible scenarios for the renormalization group evolution of couplings. The general analysis is extended here to a set of covariant nonlocal effective field equations, intended to incorporate the full scale dependence of G, and examined in the case of the static isotropic metric. We find that the existence of vacuum solutions to the effective field equations in general severely restricts the possible values of the scaling exponent nu.

Gravitational radiation from collapsing magnetized dust

Hajime Sotani, Shijun Yoshida, and Kostas D. Kokkotas

Published 6 April 2007 (20 pages)

084015 Full Text: PDF (1248 kB) | Buy Article

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In this article we study the influence of magnetic fields on the axial gravitational waves emitted during the collapse of a homogeneous dust sphere. We found that while the energy emitted depends weakly on the initial matter perturbations it has strong dependence on the strength and the distribution of the magnetic field perturbations. The gravitational wave output of such a collapse can be up to an order of magnitude larger or smaller calling for detailed numerical 3D studies of collapsing magnetized configurations.

Cylindrically symmetric solitons in Einstein-Yang-Mills theory

Dmitri V. Gal'tsov and Evgeny A. Davydov

Published 9 April 2007 (17 pages)

084016 Full Text: PDF (484 kB) | Buy Article

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Recently new Einstein-Yang-Mills (EYM) soliton solutions were presented which describe superconducting strings with Kasner asymptotics [D. V. Gal'tsov, E. A. Davydov, and M. S. Volkov, hep-th/0610183.]. Here we study the static cylindrically symmetric SU(2) EYM system in more detail. The ansatz for the gauge field corresponds to superposition of the azimuthal Bphi and the longitudinal Bz components of the color magnetic field. We derive sum rules relating data on the symmetry axis to asymptotic data and show that generic asymptotic structure of regular solutions is Kasner. Solutions starting with vacuum data on the axis generically are divergent. Regular solutions correspond to some bifurcation manifold in the space of parameters which has the low-energy limiting point corresponding to string solutions in flat space (with the divergent total energy) and the high-curvature point where gravity is crucial. Some analytical results are presented for the low-energy limit, and numerical bifurcation curves are constructed in the gravitating case. Depending on the parameters, the solution looks like a straight string or a pair of straight and circular strings. The existence of such nonlinear superposition of two strings becomes possible due to self-interaction terms in the Yang-Mills action which suppress contribution of the circular string near the polar axis.

Lepton asymmetry in the primordial gravitational wave spectrum

Kiyotomo Ichiki, Masahide Yamaguchi, and Jun'Ichi Yokoyama

Published 9 April 2007 (8 pages)

084017 Full Text: PDF (303 kB) | Buy Article

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Effects of neutrino free streaming are evaluated on the primordial spectrum of gravitational radiation taking both neutrino chemical potential and masses into account. The former or the lepton asymmetry induces two competitive effects, namely, to increase anisotropic stress, which damps the gravitational wave more, and to delay the matter-radiation equality time, which reduces the damping. The latter effect is more prominent and a large lepton asymmetry would reduce the damping. We may thereby be able to measure the magnitude of lepton asymmetry from the primordial gravitational wave spectrum.

Quantum-corrected ultraextremal horizons and the validity of the WKB approximation in the massless limit

Arkady A. Popov and O. B. Zaslavskii

Published 12 April 2007 (8 pages)

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We consider quantum backreaction of the quantized scalar field with an arbitrary mass and curvature coupling on ultraextremal horizons. The problem is distinguished in that (in contrast to nonextremal or extremal black holes) the WKB approximation remains valid near r+ (which is the radius of the horizon) even in the massless limit. We examine the behavior of the stress-energy tensor of the quantized field near r+ and show that quantum-corrected objects under discussion do exist. In the limit of the large mass our results agree with previous ones known in literature.

Isolated, slowly evolving, and dynamical trapping horizons: Geometry and mechanics from surface deformations

Ivan Booth and Stephen Fairhurst

Published 12 April 2007 (23 pages)

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We study the geometry and dynamics of both isolated and dynamical trapping horizons by considering the allowed variations of their foliating two-surfaces. This provides a common framework that may be used to consider both their possible evolutions and their deformations as well as derive the well-known flux laws. Using this framework, we unify much of what is already known about these objects as well as derive some new results. In particular we characterize and study the “almost isolated” trapping horizons known as slowly evolving horizons. It is for these horizons that a dynamical first law holds and this is analogous and closely related to the Hawking-Hartle formula for event horizons.

Gravitational Larmor formula in higher dimensions

Vitor Cardoso, Marco Cavaglià, and Jun-Qi Guo

Published 12 April 2007 (4 pages)

084020 Full Text: PDF (93 kB) | Buy Article

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The Larmor formula for scalar and gravitational radiation from a pointlike particle is derived in any even higher-dimensional flat spacetime. General expressions for the field in the wave zone and the energy flux are obtained in closed form. The explicit results in four and six dimensions are used to illustrate the effect of extra dimensions on linear and uniform circular motion. Prospects for detection of bulk gravitational radiation are briefly discussed.

Quasinormal modes and stability criterion of dilatonic black holes in 1+1 and 4+1 dimensions

Ramón Becar, Samuel Lepe, and Joel Saavedra

Published 12 April 2007 (7 pages)

084021 Full Text: PDF (293 kB) | Buy Article

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We study the stability of black holes that are solutions of the dilaton gravity derived from string-theoretical models in two and five dimensions under scalar field perturbations, using the Quasinormal Modes (QNMs) approach. In order to find the QNMs corresponding to a black hole geometry, we consider perturbations described by a massive scalar field nonminimally coupled to gravity. We find that the QNMs frequencies turn out to be pure imaginary leading to purely damped modes, in the range 0<zeta<1/4 of nonminimal coupling constant (zeta), and the QNMs acquires a real part if zeta>1/4 that is in agreement with the literature of dilatonic black holes. Our result exhibits the unstable behavior of the considered geometry against scalar perturbations. We study the instability for different values of nonminimal coupling constant. We extend our results to the 4+1 dimensional dilatonic black hole, where the metric is the product of a two-dimensional asymptotically flat geometry and a three-sphere with constant radius, which are completely decoupled from each other. The exact solution for the QNMs was obtained in the five-dimensional case.

Tunnelling from Gödel black holes

Ryan Kerner and R. B. Mann

Published 12 April 2007 (9 pages)

084022 Full Text: PDF (406 kB) | Buy Article

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We consider the spacetime structure of Kerr-Gödel black holes, analyzing their parameter space in detail. We apply the tunnelling method to compute their temperature and compare the results to previous calculations obtained via other methods. We claim that it is not possible to have the closed timelike curve (CTC) horizon in between the two black hole horizons and include a discussion of issues that occur when the radius of the CTC horizon is smaller than the radius of both black hole horizons.

Tachyon field in loop quantum cosmology: Inflation and evolution picture

Hua-Hui Xiong and Jian-Yang Zhu

Published 13 April 2007 (8 pages)

084023 Full Text: PDF (1374 kB) | Buy Article

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Loop quantum cosmology (LQC) predicts a nonsingular evolution of the universne through a bounce in the high energy region. We show that this is always true in tachyon matter LQC. Differing from the classical Friedman-Robertson-Walker (FRW) cosmology, the super inflation can appear in the tachyon matter LQC; furthermore, the inflation can be extended to the region where classical inflation stops. Using the numerical method, we give an evolution picture of the tachyon field with an exponential potential in the context of LQC. It indicates that the quantum dynamical solutions have the same attractive behavior as the classical solutions do. The whole evolution of the tachyon field is that in the distant past, the tachyon field—being in the contracting cosmology—accelerates to climb up the potential hill with a negative velocity; then at the boundary the tachyon field is bounced into an expanding universe with positive velocity rolling down to the bottom of the potential. In the slow roll limit, we compare the quantum inflation with the classical case in both an analytic and a numerical way.

Quasinormal mode analysis in BEC acoustic black holes

C. Barceló, A. Cano, L. J. Garay, and G. Jannes

Published 13 April 2007 (7 pages)

084024 Full Text: PDF (366 kB) | Buy Article

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We perform a quasinormal mode analysis of black-hole configurations in Bose-Einstein condensates (BECs). In this analysis we use the full Bogoliubov dispersion relation, not just the hydrodynamic or geometric approximation. We restrict our attention to one-dimensional flows in BECs with steplike discontinuities. For this case we show that in the hydrodynamic approximation quasinormal modes do not exist. The full dispersion relation, however, allows the existence of quasinormal modes. Remarkably, the spectrum of these modes is not discrete but continuous.

“Mass without mass” from thin shells in Gauss-Bonnet gravity

Elias Gravanis and Steven Willison

Published 13 April 2007 (12 pages)

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Five tensor equations are obtained for a thin shell in Gauss-Bonnet gravity. There is the well-known junction condition for the singular part of the stress tensor intrinsic to the shell, which we also prove to be well defined. There are also equations relating the geometry of the shell (jump and average of the extrinsic curvature as well as the intrinsic curvature) to the nonsingular components of the bulk stress tensor on the sides of the thin shell. The equations are applied to spherically symmetric thin shells in the vacuum. The shells are part of the vacuum; they carry no energy tensor. We classify these solutions of “thin shells of nothingness” in the pure Gauss-Bonnet theory. There are three types of solutions, with one, zero, or two asymptotic regions, respectively. The third kind of solutions are wormholes. Although vacuum solutions, they have the appearance of mass in the asymptotic regions. It is striking that in this theory, exotic matter is not needed in order for wormholes to exist—they can exist even with no matter.

Self-renormalization of the classical quasilocal energy

Andrew P. Lundgren, Bjoern S. Schmekel, and James W. York, Jr.

Published 16 April 2007 (7 pages)

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Pointlike objects cause many of the divergences that afflict physical theories. For instance, the gravitational binding energy of a point particle in Newtonian mechanics is infinite. In general relativity, the analog of a point particle is a black hole and the notion of binding energy must be replaced by quasilocal energy (QLE). The QLE derived by York, and elaborated by Brown and York, is finite outside the horizon but it was not considered how to evaluate it inside the horizon. We present a prescription for finding the QLE inside a horizon, and show that it is finite at the singularity for a variety of types of black holes. The energy is typically concentrated just inside the horizon, not at the central singularity.

Uniqueness of the Fock quantization of the Gowdy T³ model

Jerónimo Cortez, Guillermo A. Mena Marugán, and José M. Velhinho

Published 16 April 2007 (14 pages)

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After its reduction by a gauge-fixing procedure, the family of linearly polarized Gowdy T³ cosmologies admits a scalar field description whose evolution is governed by a Klein-Gordon type equation in a flat background in 1+1 dimensions with the spatial topology of S¹, though in the presence of a time-dependent potential. The model is still subject to a homogeneous constraint, which generates S¹-translations. Recently, a Fock quantization of this scalar field was introduced and shown to be unique under the requirements of unitarity of the dynamics and invariance under the gauge group of S¹-translations. In this work, we extend and complete this uniqueness result by considering other possible scalar field descriptions, resulting from reasonable field reparametrizations of the induced metric of the reduced model. In the reduced phase space, these alternate descriptions can be obtained by means of a time-dependent scaling of the field, the inverse scaling of its canonical momentum, and the possible addition of a time-dependent, linear contribution of the field to this momentum. Demanding again unitarity of the field dynamics and invariance under the gauge group, we prove that the alternate canonical pairs of fieldlike variables admit a Fock representation if and only if the scaling of the field is constant in time. In this case, there exists essentially a unique Fock representation, provided by the quantization constructed by Corichi, Cortez, and Mena Marugán. In particular, our analysis shows that the scalar field description proposed by Pierri does not admit a Fock quantization with the above unitarity and invariance properties.

Brane cosmic string compactification in Brans-Dicke theory

M. C. B. Abdalla, M. E. X. Guimarães, and J. M. Hoff da Silva

Published 16 April 2007 (4 pages)

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We investigate an alternative compactification of extra dimensions using local cosmic string in the Brans-Dicke gravity framework. In the context of dynamical systems it is possible to show that there exist a stable field configuration for the Einstein-Brans-Dicke equations. We explore the analogies between this particular model and the Randall-Sundrum scenario.

General variational principle for spherically symmetric perturbations in diffeomorphism covariant theories

Michael D. Seifert and Robert M. Wald

Published 17 April 2007 (12 pages)

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We present a general method for the analysis of the stability of static, spherically symmetric solutions to spherically symmetric perturbations in an arbitrary diffeomorphism covariant Lagrangian field theory. Our method involves fixing the gauge and solving the linearized gravitational field equations to eliminate the metric perturbation variables in terms of the matter variables. In a wide class of cases—which include f(R) gravity, the Einstein-æther theory of Jacobson and Mattingly, and Bekenstein's TeVeS theory—the remaining perturbation equations for the matter fields are second order in time. We show how the symplectic current arising from the original Lagrangian gives rise to a symmetric bilinear form on the variables of the reduced theory. If this bilinear form is positive definite, it provides an inner product that puts the equations of motion of the reduced theory into a self-adjoint form. A variational principle can then be written down immediately, from which stability can be tested readily. We illustrate our method in the case of Einstein's equation with perfect fluid matter, thereby rederiving, in a systematic manner, Chandrasekhar's variational principle for radial oscillations of spherically symmetric stars. In a subsequent paper, we will apply our analysis to f(R) gravity, the Einstein-æther theory, and Bekenstein's TeVeS theory.

Extremal black holes in D=4 Gauss-Bonnet gravity

Chiang-Mei Chen, Dmitri V. Gal'tsov, and Dmitry G. Orlov

Published 17 April 2007 (12 pages)

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We show that four-dimensional Einstein-Maxwell-dilaton-Gauss-Bonnet gravity admits asymptotically flat black hole solutions with a degenerate event horizon of the Reissner-Nordström type AdS2×S². Such black holes exist for the dilaton coupling constant within the interval 0<=a²<acr². Black holes must be endowed with an electric charge and (possibly) with magnetic charge (dyons) but they cannot be purely magnetic. Purely electric solutions are constructed numerically and the critical dilaton coupling is determined acr~=0.488 219 703. For each value of the dilaton coupling a within this interval and for a fixed value of the Gauss-Bonnet coupling alpha we have a family of black holes parametrized by their electric charge. The relation between the mass, the electric charge, and the dilaton charge at both ends of the allowed interval of a is reminiscent of the Bogomol'nyi-Prasad-Sommerfield condition for dilaton black holes in the Einstein-Maxwell-dilaton theory. The entropy of the dilaton-Gauss-Bonnet extremal black holes is twice the Bekenstein-Hawking entropy.

Modified teleparallel gravity: Inflation without an inflaton

Rafael Ferraro and Franco Fiorini

Published 18 April 2007 (5 pages)

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The Born-Infeld strategy to smooth theories having divergent solutions is applied to the teleparallel equivalent of general relativity. Differing from other theories of modified gravity, modified teleparallelism leads to second order equations, since the teleparallel Lagrangian only contains first derivatives of the vierbein. We show that the Born-Infeld-modified teleparallelism solves the particle horizon problem in a spatially flat Friedmann-Robertson-Walker (FRW) universe by providing an initial exponential expansion without resorting to an inflaton field.

New energy definition for higher-curvature gravities

S. Deser and Bayram Tekin

Published 19 April 2007 (3 pages)

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We propose a novel but natural definition of conserved quantities for gravity models of quadratic and higher order in curvature. Based on the spatial asymptotics of curvature rather than of metric, it avoids the more egregious problems—such as zero-energy “theorems” and failure in flat backgrounds—in this fourth-derivative realm. In D>4, the present expression indeed correctly discriminates between second-derivative Gauss-Bonnet and generic, fourth-derivative actions.

Multiple-event probability in general-relativistic quantum mechanics

Frank Hellmann, Mauricio Mondragon, Alejandro Perez, and Carlo Rovelli

Published 19 April 2007 (13 pages)

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We discuss the definition of quantum probability in the context of “timeless” general-relativistic quantum mechanics. In particular, we study the probability of sequences of events, or multievent probability. In conventional quantum mechanics this can be obtained by means of the “wave function collapse” algorithm. We first point out certain difficulties of some natural definitions of multievent probability, including the conditional probability widely considered in the literature. We then observe that multievent probability can be reduced to single-event probability, by taking into account the quantum nature of the measuring apparatus. In fact, by exploiting the von-Neumann freedom of moving the quantum/classical boundary, one can always trade a sequence of noncommuting quantum measurements at different times, with an ensemble of simultaneous commuting measurements on the joint system+apparatus system. This observation permits a formulation of quantum theory based only on single-event probability, where the results of the wave function collapse algorithm can nevertheless be recovered. The discussion also bears on the nature of the quantum collapse.

Extending the WMAP bound on the size of the Universe

Joey Shapiro Key, Neil J. Cornish, David N. Spergel, and Glenn D. Starkman

Published 19 April 2007 (10 pages)

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The topology of the Universe can leave an imprint on the cosmic microwave background (CMB) radiation. Clues to the shape of our Universe can be found by searching the CMB for matching circles of temperature patterns. A full sky search of the CMB, mapped extremely accurately by NASA's WMAP satellite, returned no detection of such matching circles and placed a lower bound on the size of the Universe at 24 Gpc. This lower bound can be extended by optimally filtering the WMAP power spectrum. More stringent bounds can be placed on specific candidate topologies by using a combination statistic. We use optimal filtering and the combination statistic to rule out the suggestion that we live in a Poincaré dodecahedral space.

Spin Hall effect of photons in a static gravitational field

Pierre Gosselin, Alain Bérard, and Hervé Mohrbach

Published 19 April 2007 (6 pages)

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Starting from a Hamiltonian description of the photon within the set of Bargmann-Wigner equations we derive new semiclassical equations of motion for the photon propagating in a static gravitational field. These equations which are obtained in the representation diagonalizing the Hamiltonian at the order [h-bar], present the first order corrections to the geometrical optics. The photon Hamiltonian shows a new kind of helicity-torsion coupling. However, even for a torsionless space-time, photons do not follow the usual null geodesic as a consequence of an anomalous velocity term. This term is responsible for the gravitational birefringence phenomenon: photons with distinct helicity follow different geodesics in a static gravitational field.

Scalar-tensor black holes coupled to Born-Infeld nonlinear electrodynamics

Ivan Zh. Stefanov, Stoytcho S. Yazadjiev, and Michail D. Todorov

Published 19 April 2007 (6 pages)

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The nonexistence of asymptotically flat, neutral black holes and asymptotically flat, charged black holes in the Maxwell electrodynamics, with a nontrivial scalar field has been proved for a large class of scalar-tensor theories. The no-scalar-hair theorems, however, do not apply in the case of nonlinear electrodynamics. In the present work numerical solutions describing charged black holes coupled to Born-Infeld type nonlinear electrodynamics in scalar-tensor theories of gravity with massless scalar field are found. The causal structure and properties of the solutions are studied, and a comparison between these solutions and the corresponding solutions in the general relativity is made. The presence of the scalar field leads to a much more simple causal structure. The present class of black holes has a single, nondegenerate horizon, i.e., its causal structure resembles that of the Schwarzschild black hole.

1/R multidimensional gravity with form-fields: Stabilization of extra dimensions, cosmic acceleration, and domain walls

Tamerlan Saidov and Alexander Zhuk

Published 25 April 2007 (11 pages)

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We study multidimensional gravitational models with scalar curvature nonlinearity of the type 1/R and with form-fields (fluxes) as a matter source. It is assumed that the higher dimensional space-time undergoes Freund-Rubin-like spontaneous compactification to a warped product manifold. It is shown that for certain parameter regions the model allows for a freezing stabilization of the internal space near the positive minimum of the effective potential which plays the role of the positive cosmological constant. This cosmological constant provides the observable late-time accelerating expansion of the Universe if the parameters of the model are fine tuned. Additionally, the effective potential has the saddle point. It results in domain walls in the Universe. We show that these domain walls do not undergo inflation.

Gravitational waves from nonlinear couplings of radial and polar nonradial modes in relativistic stars

Andrea Passamonti, Nikolaos Stergioulas, and Alessandro Nagar

Published 26 April 2007 (11 pages)

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The postbounce oscillations of newly-born relativistic stars are expected to lead to gravitational-wave emission through the excitation of nonradial oscillation modes. At the same time, the star is oscillating in its radial modes, with a central density variation that can reach several percent. Nonlinear couplings between radial oscillations and polar nonradial modes lead to the appearance of combination frequencies (sums and differences of the linear mode frequencies). We study such combination frequencies using a gauge-invariant perturbative formalism, which includes bilinear coupling terms between different oscillation modes. For typical values of the energy stored in each mode we find that gravitational waves emitted at combination frequencies could become detectable in galactic core-collapse supernovae with advanced interferometric or wideband resonant detectors.

Towards a wave-extraction method for numerical relativity. V. Estimating the gravitational-wave content of spatial hypersurfaces

Lior M. Burko

Published 26 April 2007 (11 pages)

084039 Full Text: PDF (167 kB) | Buy Article

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Physical Review D

April 2007

Volume 75, Number 8 , Articles (08xxxx)

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