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An effective model of the spacetime foam: An approximate model of the spacetime foam is offered in which each quantum
handle (wormhole) is a 5D wormhole-like solution. A spinor field is introduced
for an effective description of this foam. The topological handles of the
spacetime foam can be attached either to one space or connect two different
spaces. In the first case we have a wormhole with the quantum throat and such
object can demonstrate a model of preventing the formation the naked
singularity with relation $e > m$. In the second case the spacetime foam looks
as a dielectric with quantum handles as dipoles. It is supposed that
supergravity theories with a nonminimal interaction between spinor and
electromagnetic fields can be considered as an effective model approximately
describing the spacetime foam. | gr-qc |
Black hole mass and angular momentum in topologically massive gravity: We extend the Abbott-Deser-Tekin approach to the computation of the Killing
charge for a solution of topologically massive gravity (TMG) linearized around
an arbitrary background. This is then applied to evaluate the mass and angular
momentum of black hole solutions of TMG with non-constant curvature
asymptotics. The resulting values, together with the appropriate black hole
entropy, fit nicely into the first law of black hole thermodynamics. | gr-qc |
Suppression of long-wavelength CMB spectrum from the Hartle-Hawking wave
function in Starobinsky-type inflation model: The lack of correlations on the large scale cosmic microwave background (CMB)
anisotropy provides a potential window to probe beyond the standard
inflationary scenario. In this paper, we investigate the primordial power
spectrum based on the Hartle-Hawking (HH) no-boundary proposal for a
homogeneous, isotropic, and spatially-closed universe that leads to a
Starobinsky-type inflation after the classicalization. While we found that
there is no suppression at large scales in the standard R + R^2 theory, we also
found that it is possible to sufficiently suppress the large-scale power
spectrum if a pre-inflation stage is introduced to the Starobinsky-type model.
We calculate the C^TT_l correlation function and show that our proposal gives a
better fit to the Planck CMB data. l This suggests that our universe might have
begun with a compact HH state with a small positive curvature. | gr-qc |
Gravitational frequency shift of light in equatorial plane of a radially
moving Schwarzschild black hole: The kinematical effect induced by the transversal motion of a gravitational
lens on the frequency shift of light has been investigated in detail, while the
effect of the radial motion is thought to be much smaller than the transversal
one and thus has usually been neglected. In this work, we find that the radial
velocity effect on the frequency shift has the same order of magnitude as that
of the transversal velocity effect, when the light emitter (or the receiver) is
close to the gravitational lens with the distance between them being an impact
parameter scale. The significant velocity effect is usually transient due to
the motion of the gravitational lens relative to the light emitter or the
receiver. | gr-qc |
Negative mass bubbles in de Sitter space-time: We study the possibility of the existence of negative mass bubbles within a
de Sitter space-time background with matter content corresponding to a perfect
fluid. It is shown that there exist configurations of the perfect fluid, that
everywhere satisfy the dominant energy condition, the Einstein equations and
the equations of hydrostatic equilibrium, however asymptotically approach the
exact solution of Schwarzschid-de Sitter space-time with a negative mass. | gr-qc |
Universal ratios of critical physical quantities of charged AdS black
holes: We investigate the ratios of critical physical quantities related to the
$T-S$ criticality of charged AdS black holes. It is shown that the ratio
$\frac{T_cS_c}{Q_c}$ is universal while $\frac{T_cr_c}{Q_c}$ is not. This
finding is quite interesting considering the former observation that both the
$T-S$ graph and $T-r_+$ graph exhibit reverse van der Waals behavior. It is
also worth noting that the value of $\frac{T_cS_c}{Q_c}$ differs from that of
$\frac{P_cv_c}{T_c}$ for $P-V$ criticality. Moreover, we discuss universal
ratios for the $P-V$ criticality and $Q-\Phi$ criticality. We successfully
interpret the former finding that the ratio $\frac{\Phi_cQ_c}{T_c}$ is not
universal and construct two universal ratios for the $Q-\Phi$ criticality
instead. To the best of our knowledge, we are the first to introduce the
dimensional analysis technique to study the ratios of critical physical
quantities. It is expected that this technique can be generalized to probe the
universal ratios for $Y-X$ criticality in future research. | gr-qc |
Cosmological Implications of a Possible Class of Particles Able to
Travel Faster than Light (abridged version): Superluminal particles are not excluded by particle physics. The apparent
Lorentz invariance of the laws of physics does not imply that space-time is
indeed minkowskian. Matter made of solutions of Lorentz-invariant equations
would feel a relativistic space-time even if the actual space-time had a quite
different geometry (f.i. a galilean space-time). If Lorentz invariance is only
a property of equations describing a sector of matter at a given scale, an
absolute frame (the "vacuum rest frame") may exist without contradicting the
minkowskian structure felt by ordinary particles. Then c , the speed of light,
will not necessarily be the only critical speed in vacuum and superluminal
sectors of matter may equally exist feeling space-times with critical speeds
larger than c . We present a discussion of possible cosmological implications
of such a scenario, assuming that the superluminal sectors couple weakly to
ordinary matter. The universality of the equivalence between inertial and
gravitational mass will be lost. The Big Bang scenario will undergo important
modifications, and the evolution of the Universe may be strongly influenced by
superluminal particles. | gr-qc |
Comparing Post-Newtonian and Numerical-Relativity Precession Dynamics: Binary black-hole systems are expected to be important sources of
gravitational waves for upcoming gravitational-wave detectors. If the spins are
not colinear with each other or with the orbital angular momentum, these
systems exhibit complicated precession dynamics that are imprinted on the
gravitational waveform. We develop a new procedure to match the precession
dynamics computed by post-Newtonian (PN) theory to those of numerical binary
black-hole simulations in full general relativity. For numerical relativity NR)
simulations lasting approximately two precession cycles, we find that the PN
and NR predictions for the directions of the orbital angular momentum and the
spins agree to better than $\sim 1^{\circ}$ with NR during the inspiral,
increasing to $5^{\circ}$ near merger. Nutation of the orbital plane on the
orbital time-scale agrees well between NR and PN, whereas nutation of the spin
direction shows qualitatively different behavior in PN and NR. We also examine
how the PN equations for precession and orbital-phase evolution converge with
PN order, and we quantify the impact of various choices for handling partially
known PN terms. | gr-qc |
Universal Inflationary Attractors Implications on Static Neutron Stars: We study static neutron stars in the context of a class of non-minimally
coupled inflationary potentials, the universal attractors. Universal attractors
are known to generate a viable inflationary era, and they fall into the same
category of inflationary phenomenology as the $R^2$ model and other well-known
cosmological attractors. We present the essential features of universal
attractors in both the Einstein and Jordan frame, and we extract the
Tolman-Oppenheimer-Volkoff equations in the Einstein frame using the usual
notation of theoretical astrophysics. We use a python 3 based double shooting
numerical code for our numerical analysis and we construct the $M-R$ graphs for
the universal attractor potential, using piecewise polytropic equation of state
the small density part of which is the WFF1 or the APR or the SLy equation of
state. As we show, all the studied cases predict larger maximum masses for the
neutron stars, and all the results are compatible with the GW170817 constraints
imposed on the radii of the neutron stars. | gr-qc |
Gravity Effects on Hawking Radiation from Charged Black Strings in
Rastall Theory: The Rastall theory of gravity is the generalized form of the Einstein theory
which describes the conservation law of energy and momentum tensor. In our
work, we compute the charged black strings solution in the background of
Rastall theory by applying the Newman-Janis approach. After computing the
charged black strings solution in the background of Rastall theory, we study
the thermodynamical property (i.e., Hawking temperature) for the charged black
strings. Furthermore, we investigate the graphical representation of Hawking
temperature via event horizon to check the stability conditions of charged
black strings under the influence of Rastall theory. Moreover, we examine the
modified Hawking temperature for charged black strings in Rastall theory by
taking into account the quantum gravity effects. We also discuss the physical
state of charged black strings under the effects of quantum gravity and spin
parameter (appears due to Rastall theory in charged black strings solution). | gr-qc |
Line geometry and electromagnetism I: basic structures: Some key notions of line geometry are recalled, along with their application
to mechanics. It is then shown that most of the basic structures that one
introduces in the pre-metric formulation of electromagnetism can be interpreted
directly in terms of corresponding concepts in line geometry. The results are
summarized in a table. | gr-qc |
A note on wormholes in slightly modified gravitational theories: Wormholes that meet the flare-out condition violate the weak energy condition
in classical general relativity. The purpose of this note is to show that even
a slight modification of the gravitational theory could, under certain
conditions, avoid this violation for matter threading the wormhole. The first
part discusses some general criteria based on the field equations, while the
second part assumes a specific equation of state describing normal matter,
together with a particular type of shape function. The analysis is confined to
wormholes with zero tidal forces. | gr-qc |
The Newtonian limit of metric gravity theories with quadratic
Lagrangians: The Newtonian limit of fourth-order gravity is worked out discussing its
viability with respect to the standard results of General Relativity. We
investigate the limit in the metric approach which, with respect to the
Palatini formulation, has been much less studied in the recent literature, due
to the higher-order of the field equations. In addition, we refrain from
exploiting the formal equivalence of higher-order theories considering the
analogy with specific scalar-tensor theories, i.e. we work in the so-called
Jordan frame in order to avoid possible misleading interpretations of the
results. Explicit solutions are provided for several different types of
Lagrangians containing powers of the Ricci scalar as well as combinations of
the other curvature invariants. In particular, we develop the Green function
method for fourth-order theories in order to find out solutions. Finally, the
consistency of the results with respect to General Relativity is discussed. | gr-qc |
Geodesic analysis and black hole shadows on a general non--extremal
rotating black hole in five--dimensional gauged supergravity: In this work, motivated by the fact that higher-dimensional theories predict
the existence of black holes which differ from their four-dimensional
counterpart, we analyse the geodesics and black hole shadow cast by a general
non-extremal five-dimensional black hole. The system under consideration
corresponds to the Chong-Cveti\v{c}-L\"u-Pope (Phys. Rev.Lett.{\bf 95}, 161301
(2005)), which has the Myers--Perry black hole as a limit. | gr-qc |
Second Order Perturbations of Flat Dust FLRW Universes with a
Cosmological Constant: We summarize recent results concerning the evolution of second order
perturbations in flat dust irrotational FLRW models with $\Lambda\ne 0$. We
show that asymptotically these perturbations tend to constants in time, in
agreement with the cosmic no-hair conjecture. We solve numerically the second
order scalar perturbation equation, and very briefly discuss its all time
behaviour and some possible implications for the structure formation. | gr-qc |
General-relativistic Model of Magnetically Driven Jet: The general scheme for the construction of the general-relativistic model of
the magnetically driven jet is suggested. The method is based on the usage of
the 3+1 MHD formalism. It is shown that the critical points of the flow and the
explicit radial behavior of the physical variables may be derived through the
jet ``profile function." | gr-qc |
Electromagnetic fields around black holes and Meissner effect: The work on black holes immersed in external stationary magnetic fields is
reviewed in both test-field approximation and within exact solutions. In
particular we pay attention to the effect of the expulsion of the flux of
external fields across charged and rotating black holes which are approaching
extremal states. Recently this effect has been shown to occur for black hole
solutions in string theory and Kaluza-Klein theory. | gr-qc |
Perturbative quasinormal mode frequencies: We often encounter a situation that black hole solutions can be regarded as
continuous deformations of simpler ones, or modify general relativity by
continuous parameters. We develop a general framework to compute high-order
perturbative corrections to quasinormal mode frequencies in such deformed
problems. Our method has many applications, and allows to compute numerical
values of the high-order corrections very accurately. For several examples, we
perform this computation explicitly, and discuss analytic properties of the
quasinormal mode frequencies for deformation parameters. | gr-qc |
Phantom Cosmology without Big Rip Singularity: We construct phantom energy models with the equation-of-state parameter $w$
such that $w<-1$, but finite-time future singularity does not occur. Such
models can be divided into two classes: (i) energy density increases with time
("phantom energy" without "Big Rip" singularity) and (ii) energy density tends
to constant value with time ("cosmological constant" with asymptotically de
Sitter evolution). The disintegration of bound structure is confirmed in Little
Rip cosmology. Surprisingly, we find that such disintegration (on example of
Sun-Earth system) may occur even in asymptotically de Sitter phantom universe
consistent with observational data. We also demonstrate that non-singular
phantom models admit wormhole solutions as well as possibility of big trip via
wormholes. | gr-qc |
Aspects of Black Holes in Gravitational Theories with Broken Lorentz and
Diffeomorphism Symmetries: Since Stephen Hawking discovered that black holes emit thermal radiation,
black holes have become the theoretical laboratories for testing our ideas on
quantum gravity. This dissertation is devoted to the study of singularities,
the formation of black holes by gravitational collapse and the global structure
of spacetime. All our investigations are in the context of a recently proposed
approach to quantum gravity, which breaks Lorentz and diffeomorphism symmetries
at very high energies. | gr-qc |
On Einstein - Weyl unified model of dark energy and dark matter: Here we give a more detailed account of the part of the conference report
that was devoted to reinterpreting the Einstein `unified models of gravity and
electromagnetism' (1923) as the unified theory of dark energy (cosmological
constant) and dark matter (neutral massive vector particle having only
gravitational interactions). After summarizing Einstein's work and related
earlier work of Weyl and Eddington, we present an approach to finding
spherically symmetric solutions of the simplest variant of the Einstein models
that was earlier mentioned in Weyl's work as an example of his generalization
of general relativity. The spherically symmetric static solutions and
homogeneous cosmological models are considered in some detail. As the theory is
not integrable we study approximate solutions. In the static case, we show that
there may exist two horizons and derive solutions near the horizons. In
cosmology, we propose to study the corresponding expansions of possible
solutions near the origin and derive these expansions in a simplified model
neglecting anisotropy. The structure of the solutions seems to hint at a
possibility of an inflation mechanism that does not require adding scalar
fields. | gr-qc |
A Proposed Search for the Detection of Gravitational Waves from
Eccentric Binary Black Holes: Most of compact binary systems are expected to circularize before the
frequency of emitted gravitational waves (GWs) enters the sensitivity band of
the ground based interferometric detectors. However, several mechanisms have
been proposed for the formation of binary systems, which retain eccentricity
throughout their lifetimes. Since no matched-filtering algorithm has been
developed to extract continuous GW signals from compact binaries on orbits with
low to moderate values of eccentricity, and available algorithms to detect
binaries on quasi-circular orbits are sub-optimal to recover these events, in
this paper we propose a search method for detection of gravitational waves
produced from the coalescences of eccentric binary black holes (eBBH). We study
the search sensitivity and the false alarm rates on a segment of data from the
second joint science run of LIGO and Virgo detectors, and discuss the
implications of the eccentric binary search for the advanced GW detectors. | gr-qc |
Quantum Reference Frames at the Boundary of Spacetime: An analysis is given of the local phase space of gravity coupled to matter to
second order in perturbation theory. Working in local regions with boundaries
at finite distance, we identify matter, Coulomb, and additional boundary modes.
The boundary modes take the role of reference frames for both diffeomorphisms
and internal Lorentz rotations. Passing to the quantum level, we identify the
constraints that link the bulk and boundary modes. The constraints take the
form of a multi-fingered Schr\"odinger equation, which determines the
relational evolution of the quantum states in the bulk with respect to the
quantum reference fields at the boundary. | gr-qc |
Geometry of Black Holes and Multi-Black-Holes in 2+1 dimensions: Lecture given at ``Raychaudhuri session," ICGC-95 conference, Pune (India),
December 1995.
Contents:
I. Introduction
II. (2+1)-Dimensional Initial Values in Stereographic Projection
A. The single, non-rotating black hole
B. Non-rotating multi-black-holes
C. ``Black Hole" Universe Initial Values
III. Time Development: Enter the Raychaudhuri Equation
IV. Time Development in Stereographic Projection
A. The BTZ black hole spacetime
B. Multi-Black-Hole Spacetimes
V. Black Holes with Angular Momentum
VI. Analogous 3+1-Dimensional Black Holes
VII. Conclusions | gr-qc |
Regular Black Hole in General Relativity Coupled to Nonlinear
Electrodynamics: The first regular exact black hole solution in General Relativity is
presented. The source is a nonlinear electrodynamic field satisfying the weak
energy condition, which in the limit of weak field becomes the Maxwell field.
The solution corresponds to a charged black hole with |q| \leq 2 s_c m \approx
0.6 m, having the metric, the curvature invariants, and the electric field
regular everywhere. | gr-qc |
Semiclassical Approximations to Cosmological Perturbations: We apply several methods related to the WKB approximation to study
cosmological perturbations during inflation, obtaining the full power spectra
of scalar and tensor perturbations to first and to second order in the
slow-roll parameters. We compare our results with those derived by means of
other methods, in particular the Green's function method, and find agreement
for the slow-roll structure. Scalar wave propagation on the Schwarzschild
background is also considered. | gr-qc |
On Some Applications of the Sagnac Effect: Considering exact spacetimes representing rotating black holes and naked
singularities, we study the possibility that the Sagnac effect detects $i$)
higher dimensions, $ii$) rotation of black holes in higher dimension, and also,
$iii$) distinguishes black holes from naked singularities. The results indicate
that the Sagnac time delay gets affected by the presence of extra-dimension or
its associated angular momentum. This time delay is also different in the
spacetime of a naked singularity compared to that of a black hole. Hence, the
Sagnac effect may be used as an experiment for better understanding of
spacetimes with higher dimensions or those that admit naked singularities. | gr-qc |
Projected Constraints on Lorentz-Violating Gravity with Gravitational
Waves: Gravitational waves are excellent tools to probe the foundations of General
Relativity in the strongly dynamical and non-linear regime. One such foundation
is Lorentz symmetry, which can be broken in the gravitational sector by the
existence of a preferred time direction, and thus, a preferred frame at each
spacetime point. This leads to a modification in the orbital decay rate of
binary systems, and also in the generation and chirping of their associated
gravitational waves. We here study whether waves emitted in the late,
quasi-circular inspiral of non-spinning, neutron star binaries can place
competitive constraints on two proxies of gravitational Lorentz-violation:
Einstein-\AE{}ther theory and khronometric gravity. We model the waves in the
small-coupling (or decoupling) limit and in the post-Newtonian approximation,
by perturbatively solving the field equations in small deformations from
General Relativity and in the small-velocity/weak-gravity approximation. We
assume a gravitational wave consistent with General Relativity has been
detected with second- and third-generation, ground-based detectors, and with
the proposed space-based mission, DECIGO, with and without coincident
electromagnetic counterparts. Without a counterpart, a detection consistent
with General Relativity of neutron star binaries can only place competitive
constraints on gravitational Lorentz violation when using future,
third-generation or space-based instruments. On the other hand, a single
counterpart is enough to place constraints that are 10 orders of magnitude more
stringent than current binary pulsar bounds, even when using second-generation
detectors. This is because Lorentz violation forces the group velocity of
gravitational waves to be different from that of light, and this difference can
be very accurately constrained with coincident observations. | gr-qc |
Quantum Cosmology in some Scalar-tensor Theories: The Wheeler-DeWitt equation is solved for some scalar-tensor theories of
gravitation in the case of homogeneous and isotropic cosmological models.We
present general solutions corresponding to cosmological term:
(i)\lambda(\phi)=0$ and $(ii) \lambda(\phi)=q\phi$. | gr-qc |
A covariant formalism of spin precession with respect to a reference
congruence: We derive an effectively three-dimensional relativistic spin precession
formalism. The formalism is applicable to any spacetime where an arbitrary
timelike reference congruence of worldlines is specified. We employ what we
call a stopped spin vector which is the spin vector that we would get if we
momentarily make a pure boost of the spin vector to stop it relative to the
congruence. Starting from the Fermi transport equation for the standard spin
vector we derive a corresponding transport equation for the stopped spin
vector. Employing a spacetime transport equation for a vector along a
worldline, corresponding to spatial parallel transport with respect to the
congruence, we can write down a precession formula for a gyroscope relative to
the local spatial geometry defined by the congruence. This general approach has
already been pursued by Jantzen et. al. (see e.g. Jantzen, Carini and Bini,
Ann. Phys. 215 (1997) 1), but the algebraic form of our respective expressions
differ. We are also applying the formalism to a novel type of spatial parallel
transport introduced in Jonsson (Class. Quantum Grav. 23 (2006) 1), as well as
verifying the validity of the intuitive approach of a forthcoming paper
(Jonsson, Am. Journ. Phys. 75 (2007) 463) where gyroscope precession is
explained entirely as a double Thomas type of effect. We also present the
resulting formalism in explicit three-dimensional form (using the boldface
vector notation), and give examples of applications. | gr-qc |
Quasilocal rotating conformal Killing horizons: The formulation of quasi-local conformal Killling horizons(CKH) is extended
to include rotation. This necessitates that the horizon be foliated by
2-spheres which may be distorted. Matter degrees of freedom which fall through
the horizon is taken to be a real scalar field. We show that these rotating
CKHs also admit a first law in differential form. | gr-qc |
A New Test of the Einstein Equivalence Principle and the Isotropy of
Space: Recent research has established that nonsymmetric gravitation theories like
Moffat's NGT predict that a gravitational field singles out an orthogonal pair
of polarization states of light that propagate with different phase velocities.
We show that a much wider class of nonmetric theories encompassed by the $\chi
g$ formalism predict such violations of the Einstein equivalence principle.
This gravity-induced birefringence of space implies that propagation through a
gravitational field can alter the polarization of light. We use data from
polarization measurements of extragalactic sources to constrain birefringence
induced by the field of the Galaxy. Our new constraint is $10^8$ times sharper
than previous ones. | gr-qc |
Hamiltonians and canonical coordinates for spinning particles in curved
space-time: The spin-curvature coupling as captured by the so-called
Mathisson-Papapetrou-Dixon (MPD) equations is the leading order effect of the
finite size of a rapidly rotating compact astrophysical object moving in a
curved background. It is also a next-to-leading order effect in the phase of
gravitational waves emitted by extreme-mass-ratio inspirals (EMRIs), which are
expected to become observable by the LISA space mission. Additionally,
exploring the Hamiltonian formalism for spinning bodies is important for the
construction of the so-called Effective-One-Body waveform models that should
eventually cover all mass ratios.
The MPD equations require supplementary conditions determining the frame in
which the moments of the body are computed. We review various choices of these
supplementary spin conditions and their properties. Then, we give Hamiltonians
either in proper-time or coordinate-time parametrization for the
Tulczyjew-Dixon, Mathisson-Pirani, and Kyrian-Semer\'ak conditions. Finally, we
also give canonical phase-space coordinates parametrizing the spin tensor. We
demonstrate the usefulness of the canonical coordinates for symplectic
integration by constructing Poincar\'e surfaces of section for spinning bodies
moving in the equatorial plane in Schwarzschild space-time. We observe the
motion to be essentially regular for EMRI-ranges of the spin, but for larger
values the Poincar\'e surfaces of section exhibit the typical structure of a
weakly chaotic system. A possible future application of the numerical
integration method is the inclusion of spin effects in EMRIs at the precision
requirements of LISA. | gr-qc |
Reconstruction method in the kinetic gravity braiding theory with
shift-symmetric: We present a reconstruction method for flat Friedman-Robertson-Walker (FRW)
spacetime in a subclass of Horndeski theory -- specifically shiftsymmetric, the
kinetic gravity braiding (KGB) theory with a non-vanishing conserved current.
Choosing the form of the Hubble parameter and kinetic density $X$, we restore
the functions $G_2$, $G_3$ of the KGB model. In order to determine whether the
model is free of ghosts and Laplacian instabilities and thus cosmologically
viable, two conditions related to scalar perturbations are checked. Initially,
the Lagrangian does not include the term that describes, for example, the
perfect fluid with the EoS parameter $w\neq -1$. This fluid can provide a
dynamic solution $H(t)$, $X(t)$. In the presented method, dynamic solutions are
provided by a nonzero scalar charge associated with the shift symmetry
$\phi\rightarrow\phi+\phi_0$. Reconstruction examples are given for models: an
perfect fluid, a unified description dark energy-dark matter, a
post-inflationary transition to the radiation-dominated phase. | gr-qc |
The Lambert $W$ function: A newcomer in the Cosmology class?: We propose a novel equation of state (EoS) which explains the evolutionary
history of a flat Friedmann-Lemaitre-Robertson-Walker (FLRW) Universe. The
uniqueness of this EoS lies in the fact that it incorporates the Lambert $W$
function in a special fashion. It is explicitly demonstrated that with
observationally relevant values of the unknown parameters
$\vartheta_\mathrm{1}$ and $\vartheta_\mathrm{2}$, all the evolutionary phases
of the Universe can be reproduced. Moreover, it also shows that the initial
singularity is unavoidable and asserts that the late-time acceleration of the
Universe would continue forever. | gr-qc |
Evaluation of neutrinos mass based on ENU model: Based on principles of the Expansive Nondecelerative Universe model that
enables to quantify and localize the gravitational energy density, and stemming
from the see-saw mechanism, the mass of electron, muon and tau neutrinos are
determined in an independent way. | gr-qc |
An exact quantification of backreaction in relativistic cosmology: An important open question in cosmology is the degree to which the
Friedmann-Lemaitre-Robertson-Walker (FLRW) solutions of Einstein's equations
are able to model the large-scale behaviour of the locally inhomogeneous
observable universe. We investigate this problem by considering a range of
exact n-body solutions of Einstein's constraint equations. These solutions
contain discrete masses, and so allow arbitrarily large density contrasts to be
modelled. We restrict our study to regularly arranged distributions of masses
in topological 3-spheres. This has the benefit of allowing straightforward
comparisons to be made with FLRW solutions, as both spacetimes admit a discrete
group of symmetries. It also provides a time-symmetric hypersurface at the
moment of maximum expansion that allows the constraint equations to be solved
exactly. We find that when all the mass in the universe is condensed into a
small number of objects (<10) then the amount of backreaction in dust models
can be large, with O(1) deviations from the predictions of the corresponding
FLRW solutions. When the number of masses is large (>100), however, then our
measures of backreaction become small (<1%). This result does not rely on any
averaging procedures, which are notoriously hard to define uniquely in general
relativity, and so provides (to the best of our knowledge) the first exact and
unambiguous demonstration of backreaction in general relativistic cosmological
modelling. Discrete models such as these can therefore be used as laboratories
to test ideas about backreaction that could be applied in more complicated and
realistic settings. | gr-qc |
Quantum Brownian motion for a particle in analog expanding cosmologies
in the presence of disclination: In this paper we study the quantum brownian motion of a scalar point particle
in the analog Friedman-Robertson-Walker spacetime in the presence of a
disclination, in a condensed matter system. The analog spacetime is obtained as
an effective description of a Bose-Einstein condensate in terms of quantum
excitations of sound waves, named phonons. The dynamics of the phonons is
described by a massless real scalar field whose modes are also subjected to a
quasi-periodic condition. In this sense, we find exact solutions for the real
scalar field in this scenario and calculate the two-point function which makes
possible to analyze the mean squared velocity dispersion of the particle in all
directions. We, thus, analyze some interesting particular cases and show some
graphs where it is possible to see the consistency of our results. | gr-qc |
Searching for an oscillating massive scalar field as a dark matter
candidate using atomic hyperfine frequency comparisons: We use six years of accurate hyperfine frequency comparison data of the dual
rubidium and caesium cold atom fountain FO2 at LNE-SYRTE to search for a
massive scalar dark matter candidate. Such a scalar field can induce harmonic
variations of the fine structure constant, of the mass of fermions and of the
quantum chromodynamic mass scale, which will directly impact the
rubidium/caesium hyperfine transition frequency ratio. We find no signal
consistent with a scalar dark matter candidate but provide improved constraints
on the coupling of the putative scalar field to standard matter. Our limits are
complementary to previous results that were only sensitive to the fine
structure constant, and improve them by more than an order of magnitude when
only a coupling to electromagnetism is assumed. | gr-qc |
Chaotic Inflation and Reheating in Generalized Scalar-Tensor Gravity: In the present work, we study slow-roll inflation in scalar-tensor gravity
theories in the presence of both the non-minimal coupling between the scalar
field and curvature, and the Galileon self-interaction of the scalar field.
Furthermore, we give predictions for the duration of reheating as well as for
the reheating temperature after inflation. After working out the expressions
for the power spectra of scalar and tensor perturbations in the case of a
general non-minimal coupling function that depends solely on the scalar field
and a general scalar potential, we focus on the special cases of the power-law
coupling function and chaotic quadratic inflation. Thus, under the slow-roll
approximation we confront the predictions of the model with the current PLANCK
constraints on the spectral index $n_s$ and the tensor-to-scalar ratio $r$
using the $n_{s}-r$ plane. We found that the combination of the non-minimal
coupling and Galileon self-interaction effects allows us to obtain better
results for $r$ than in the case in which each effect is considered separately.
Particularly, we obtained that the predictions of the model are in agreement
with the current observational bounds on $n_{s}$ and $r$ within the $95 \%$ C.L
region and also slightly inside the $68 \%$ C.L region. Also, we investigate
the oscillatory regime after the end of inflation by solving the full
background equations, and then we determine the upper bound for the Galileon
and non-minimal coupling parameters under the condition that the scalar field
oscillates coherently during reheating. Finally, after approximating reheating
by a constant equation of state, we derive the relations between the reheating
duration, the temperature at the end of reheating, its equation of state, and
the number of $e$-folds of inflation and then we relate them all to the
inflationary observables. | gr-qc |
Comments on "Casimir Effect in the Kerr spacetime with Quintessence": This comment is devoted to the recalculation of the Casimir energy of a
massless scalar field in the Kerr black hole surrounded by quintessence derived
in [B. Toshmatov, Z. Stuchl\'{i}k and B. Ahmedov, Eur. Phys. J. Plus {\bf 132},
98 (2017)] and its comparison with the results recently obtained in [V. B.
Bezerra, M. S. Cunha, L. F. F. Freitas and C. R. Muniz, Mod. Phys. Lett. A {\bf
32}, 1750005 (2017)] in the spacetime [S. G. Ghosh, Eur. Phys. J. C {\bf 76},
222 (2016)]. We have shown that in the more realistic spacetime which does not
have the failures illustrated here, the Casimir energy is significantly bigger
than that derived in [V. B. Bezerra, M. S. Cunha, L. F. F. Freitas and C. R.
Muniz, Mod. Phys. Lett. A {\bf 32}, 1750005 (2017)], and the difference becomes
crucial especially in the regions of near horizons of the spacetime. | gr-qc |
On scattering of CMB radiation on wormholes: kinetic SZ-effect: The problem of scattering of CMB radiation on wormholes is considered. It is
shown that a static gas of wormholes does not perturb the spectrum of CMB. In
the first order by $v/c$ the presence of peculiar velocities gives rise to the
dipole contribution in $\Delta T/T$, which corresponds to the well-known
kinetic Sunyaev-Zel'dovich effect. In next orders there appears a more
complicated dependence of the perturbed CMB spectrum on peculiar velocities. We
also discuss some peculiar features of the scattering on a single wormhole. | gr-qc |
Chaotic Scattering and Capture of Strings by Black Hole: We consider scattering and capture of circular cosmic strings by a
Schwarzschild black hole. Although being a priori a very simple axially
symmetric two-body problem, it shows all the features of chaotic scattering. In
particular, it contains a fractal set of unstable periodic solutions; a
so-called strange repellor. We study the different types of trajectories and
obtain the fractal dimension of the basin-boundary separating the space of
initial conditions according to the different asymptotic outcomes. We also
consider the fractal dimension as a function of energy, and discuss the
transition from order to chaos. | gr-qc |
Canonical Chern-Simons Gravity: We study the canonical description of the axisymmetric vacuum in 2+1
dimensional gravity, treating Einstein's gravity as a Chern Simons gauge theory
on a manifold with the restriction that the dreibein is invertible. Our
treatment is in the spirit of Kucha\v r's description of the Schwarzschild
black hole in 3+1 dimensions, where the mass and angular momentum are expressed
in terms of the canonical variables and a series of canonical transformations
are performed that turn the curvature coordinates and their conjugate momenta
into new canonical variables. In their final form, the constraints are seen to
require that the momenta conjugate to the Killing time and curvature radius
vanish and what remains are the mass, the angular momentum and their conjugate
momenta, which we derive. The Wheeler-DeWitt equation is trivial and describes
time independent systems with wave functions described only by the total mass
and total angular momentum. | gr-qc |
Comment on "What does the Letelier-Gal'tsov metric describe?": We show that the Letelier-Gal'tsov (LG) metric describing multiple crossed
strings in relative motion does solve the Einstein equations, in spite of the
discontinuity uncovered recently by Krasnikov [gr-qc/0502090] provided the
strings are straight and moving with constant velocities. | gr-qc |
A regularization of the hamiltonian constraint compatible with the
spinfoam dynamics: We introduce a new regularization for Thiemann's Hamiltonian constraint. The
resulting constraint can generate the 1-4 Pachner moves and is therefore more
compatible with the dynamics defined by the spinfoam formalism. We calculate
its matrix elements and observe the appearence of the 15j Wigner symbol in
these. | gr-qc |
Conformally curved binary black hole initial data including tidal
deformations and outgoing radiation: (Abridged) By asymptotically matching a post-Newtonian (PN) metric to two
tidally perturbed Schwarzschild metrics, we generate approximate initial data
(in the form of a 4-metric) for a nonspinning black hole binary in a circular
orbit. We carry out this matching through O(v^4) in the binary's orbital
velocity v, so the resulting data are conformally curved. Far from the holes,
we use the appropriate PN metric that accounts for retardation, which we
construct using the highest-order PN expressions available to compute the
binary's past history. The data set's uncontrolled remainders are thus O(v^5)
throughout the timeslice; we also generate an extension to the data set that
has uncontrolled remainders of O(v^6) in the purely PN portion of the timeslice
(i.e., not too close to the holes). The resulting data are smooth, since we
join all the metrics together by smoothly interpolating between them. We
perform this interpolation using transition functions constructed to avoid
introducing excessive additional constraint violations. Due to their inclusion
of tidal deformations and outgoing radiation, these data should substantially
reduce the initial spurious ("junk") radiation observed in current simulations
that use conformally flat initial data. Such reductions in the nonphysical
components of the initial data will be necessary for simulations to achieve the
accuracy required to supply Advanced LIGO and LISA with the templates necessary
for parameter estimation. | gr-qc |
Accelerating universe with time variation of $G$ and $Λ$: We study a gravitational model in which scale transformations play the key
role in obtaining dynamical $G$ and $\Lambda$. We take a scale non-invariant
gravitational action with a cosmological constant and a gravitational coupling
constant. Then, by a scale transformation, through a dilaton field, we obtain a
new action containing cosmological and gravitational coupling terms which are
dynamically dependent on the dilaton field with Higgs type potential. The
vacuum expectation value of this dilaton field, through spontaneous symmetry
breaking on the basis of anthropic principle, determines the time variations of
$G$ and $\Lambda$. The relevance of these time variations to the current
acceleration of the universe, coincidence problem, Mach's cosmological
coincidence and those problems of standard cosmology addressed by inflationary
models, are discussed. The current acceleration of the universe is shown to be
a result of phase transition from radiation toward matter dominated eras. No
real coincidence problem between matter and vacuum energy densities exists in
this model and this apparent coincidence together with Mach's cosmological
coincidence are shown to be simple consequences of a new kind of scale factor
dependence of the energy momentum density as $\rho \sim a^{-4}$. This model
also provides the possibility for a super fast expansion of the scale factor at
very early universe by introducing exotic type matter like cosmic strings. | gr-qc |
Links between gravity and dynamics of quantum liquids: We consider the Landau-Khalatnikov two-fluid hydrodynamics of superfluid
liquid as an effective theory, which provides a self-consistent analog of
Einstein equations for gravity and matter. | gr-qc |
Primordial Cosmology in Mimetic Born-Infeld Gravity: The Eddington-inspired-Born-Infeld (EiBI) model is reformulated within the
mimetic approach. In the presence of a mimetic field, the model contains
non-trivial vacuum solutions which could be free of spacetime singularity
because of the Born-Infeld nature of the theory. We study a realistic
primordial vacuum universe and prove the existence of regular solutions, such
as primordial inflationary solutions of de Sitter type or bouncing solutions.
Besides, the linear instabilities present in the EiBI model are found to be
avoidable for some interesting bouncing solutions in which the physical metric
as well as the auxiliary metric are regular at the background level. | gr-qc |
Self-energy anomaly of an electric pointlike dipole in three-dimensional
static spacetimes: We calculate the self-energy anomaly of a pointlike electric dipole located
in a static $(2+1)$-dimensional curved spacetime. The energy functional for
this problem is invariant under an infinite-dimensional (gauge) group of
transformations parameterized by one scalar function of two variables. We
demonstrate that the problem of the calculation of the self-energy anomaly for
a pointlike dipole can be reduced to the calculation of quantum fluctuations of
an effective two-dimensional Euclidean quantum field theory. We reduced the
problem in question to the calculation of the conformal anomaly of an effective
scalar field in two dimensions and obtained an explicit expression for the
self-energy anomaly of an electric dipole in an asymptotically flat, regular
$(2+1)$-dimensional spacetime which may have electrically neutral
black-hole-like metrics with regular Killing horizon. | gr-qc |
Mapping the asymptotic inspiral of precessing binary black holes to
their merger remnants: Multiple approaches are required to study the evolution of black-hole
binaries. While the post-Newtonian approximation is sufficient to describe the
early inspiral (even from infinitely large orbital separation), only numerical
relativity can capture the full complexity of the dynamics near merger. We
combine multi-timescale post-Newtonian integrations with numerical-relativity
surrogate models, thus mapping the entire history of the binary from its
asymptotic configuration at past-time infinity to the post-merger remnant. This
approach naturally allows us to assess the impact of the precessional and
orbital phase on the properties - mass, spin, and kick - of the merger remnant.
These phases introduce a fundamental uncertainty when connecting the two
extrema of the binary evolution. | gr-qc |
A New Kind of Uniformly Accelerated Reference Frames: A new kind of uniformly accelerated reference frames with a line-element
different from the M{\o}ller and Rindler ones is presented, in which every
observer at $x, y, z=$consts. has the same constant acceleration. The laws of
mechanics are checked in the new kind of frames. Its thermal property is
studied. The comparison with the M{\o}ller and Rindler uniform accelerated
reference frames is also made. | gr-qc |
All metrics have curvature tensors characterised by its invariants as a
limit: the ε-property: We prove a generalisation of the $\epsilon$-property, namely that for any
dimension and signature, a metric which is not characterised by its polynomial
scalar curvature invariants, there is a frame such that the components of the
curvature tensors can be arbitrary close to a certain "background". This
"background" is defined by its curvature tensors: it is characterised by its
curvature tensors and has the same polynomial curvature invariants as the
original metric. | gr-qc |
Solving the Schrodinger Poisson System using the coordinate Adaptive
Moving Mesh method: In this paper, we implement the Adaptive Moving Mesh method (AMM) to the
solution of initial value problems involving the Schr\"odinger equation, and
more specifically the Schr\"odinger-Poisson system of equations. This method is
based on the solution of the problem on a discrete domain, whose resolution is
coordinate and time-dependent, and allows to dynamically assign numerical
resolution in terms of desired refinement criteria. We apply the method to
solve various test problems involving stationary solutions of the SP system,
and toy scenarios related to the disruption of subhalo s made of ultralight
bosonic dark matter traveling on top of host galaxies. | gr-qc |
Reheating-volume measure for random-walk inflation: The recently proposed "reheating-volume" (RV) measure promises to solve the
long-standing problem of extracting probabilistic predictions from cosmological
"multiverse" scenarios involving eternal inflation. I give a detailed
description of the new measure and its applications to generic models of
eternal inflation of random-walk type. For those models I derive a general
formula for RV-regulated probability distributions that is suitable for
numerical computations. I show that the results of the RV cutoff in random-walk
type models are always gauge-invariant and independent of the initial
conditions at the beginning of inflation. In a toy model where equal-time
cutoffs lead to the "youngness paradox," the RV cutoff yields unbiased results
that are distinct from previously proposed measures. | gr-qc |
Effective-one-body waveforms calibrated to numerical relativity
simulations: coalescence of non-spinning, equal-mass black holes: We calibrate the effective-one-body (EOB) model to an accurate numerical
simulation of an equal-mass, non-spinning binary black-hole coalescence
produced by the Caltech-Cornell collaboration. Aligning the EOB and numerical
waveforms at low frequency over a time interval of ~1000M, and taking into
account the uncertainties in the numerical simulation, we investigate the
significance and degeneracy of the EOB adjustable parameters during inspiral,
plunge and merger, and determine the minimum number of EOB adjustable
parameters that achieves phase and amplitude agreements on the order of the
numerical error. We find that phase and fractional amplitude differences
between the numerical and EOB values of the dominant gravitational wave mode
h_{22} can be reduced to 0.02 radians and 2%, respectively, until a time 26 M
before merger, and to 0.1 radians and 10%, at a time 16M after merger (during
ringdown), respectively. Using LIGO, Enhanced LIGO and Advanced LIGO noise
curves, we find that the overlap between the EOB and the numerical h_{22},
maximized only over the initial phase and time of arrival, is larger than 0.999
for equal-mass binary black holes with total mass 30-150 Msun. In addition to
the leading gravitational mode (2,2), we compare the dominant subleading modes
(4,4) and (3,2) and find phase and amplitude differences on the order of the
numerical error. We also determine the mass-ratio dependence of one of the EOB
adjustable parameters by fitting to numerical {\it inspiral} waveforms for
black-hole binaries with mass ratios 2:1 and 3:1. These results improve and
extend recent successful attempts aimed at providing gravitational-wave data
analysts the best analytical EOB model capable of interpolating accurate
numerical simulations. | gr-qc |
Entropies and The First Laws of Black Hole Thermodynamics in
Einstein-aether-Maxwell Theory: Using the solution phase space method, we investigate the thermodynamics of
black holes in Einstein-aether-Maxwell theory, for which the traditional Wald
method (covariant phase space method) fails. We show the first laws of
thermodynamics and definitive entropy expressions at both Killing and universal
horizons for some examples of exact black hole solutions, including
3-dimensional static charged quasi-BTZ black hole, two 4-dimensional static
charged black holes and 3-dimensional rotating solution. At Killing horizons
the entropies are exactly one quarter of the horizon area, but at universal
horizons of 3-dimensional black holes, the entropies have a corrected term in
addition to the one proportional to the horizon area. | gr-qc |
Accelerating expansion of the universe in modified symmetric
teleparallel gravity: The fundamental nature and origin of dark energy are one of the premier
mysteries of theoretical physics. In General Relativity Theory, the
cosmological constant $\Lambda$ is the simplest explanation for dark energy. On
the other hand, the cosmological constant $\Lambda$ suffers from a delicate
issue so-called fine-tuning problem. This motivates one to modify the spacetime
geometry of Einstein's GR. The $f(Q)$ gravity is a recently proposed modified
theory of gravity in which the non-metricity scalar $Q$ drives the
gravitational interaction. In this article, we consider a linear $f(Q)$ model,
specifically $f(Q)=\alpha Q + \beta$, where $\alpha$ and $\beta$ are free
parameters. Then we estimate the best fit values of model parameters that would
be in agreement with the recent observational data sets. We use 57 points of
the updated $H(z)$ data sets, 6 points of the BAO data sets, and 1048 points
from the Pantheon supernovae samples. We apply the Bayesian analysis and
likelihood function along with the Markov Chain Monte Carlo (MCMC) method.
Further, we analyse the physical behaviour of cosmological parameters such as
density, deceleration, and the EoS parameters corresponding to the constraint
values of the model parameters. The evolution of deceleration parameter
predicts a transition from decelerated to accelerated phases of the universe.
Further, the evolution of equation of state parameter depicts quintessence type
behaviour of the dark energy fluid part. We found that our $f(Q)$ cosmological
model can effectively describe the late time cosmic acceleration without
invoking any dark energy component in the matter part. | gr-qc |
New Generalizations of Cosmography Inspired by the Pade Approximant: The current accelerated expansion of the universe has been one of the most
important fields in physics and astronomy since 1998. Many cosmological models
have been proposed in the literature to explain this mysterious phenomenon.
Since the nature and cause of the cosmic acceleration are still unknown,
model-independent approaches to study the evolution of the universe are
welcome. One of the powerful model-independent approaches is the so-called
cosmography. It only relies on the cosmological principle, without postulating
any underlying theoretical model. However, there are several shortcomings in
the usual cosmography. For instance, it is plagued with the problem of
divergence (or an unacceptably large error), and it fails to predict the future
evolution of the universe. In the present work, we try to overcome or at least
alleviate these problems, and we propose two new generalizations of cosmography
inspired by the Pad\'e approximant. One is to directly parameterize the
luminosity distance based on the Pad\'e approximant, while the other is to
generalize cosmography with respect to a so-called $y_\beta$-shift
$y_\beta=z/(1+\beta z)$, which is also inspired by the Pad\'e approximant.
Then, we confront them with the observational data with the help of the Markov
chain Monte Carlo (MCMC) code emcee, and find that they work fairly well. | gr-qc |
Data formats for numerical relativity waves: This document proposes data formats to exchange numerical relativity results,
in particular gravitational waveforms. The primary goal is to further the
interaction between gravitational-wave source modeling groups and the
gravitational-wave data-analysis community. We present a simple and extendable
format which is applicable to various kinds of gravitational wave sources
including binaries of compact objects and systems undergoing gravitational
collapse, but is nevertheless sufficiently general to be useful for other
purposes. | gr-qc |
Quantum corrections to the stress-energy tensor in thermodynamic
equilibrium with acceleration: We show that the stress-energy tensor has additional terms with respect to
the ideal form in states of global thermodynamic equilibrium in flat spacetime
with non-vanishing acceleration and vorticity. These corrections are of quantum
origin and their leading terms are second order in the gradients of the
thermodynamic fields. Their relevant coefficients can be expressed in terms of
correlators of the stress-energy tensor operator and the generators of the
Lorentz group. With respect to previous assessments, we find that there are
more second order coefficients and that all thermodynamic functions including
energy density receive acceleration and vorticity dependent corrections.
Notably, also the relation between \rho and p, that is the equation of state,
is affected by acceleration and vorticity. We have calculated the corrections
for a free real scalar field -- both massive and massless -- and we have found
that they increase, particularly for a massive field, at very high acceleration
and vorticity and very low temperature. Finally, these non-ideal terms depend
on the explicit form of the stress-energy operator, implying that different
stress-energy tensor of the scalar field -- canonical or improved -- are
thermodynamically inequivalent. | gr-qc |
Scalar Cosmological Perturbations from Quantum Gravitational
Entanglement: A major challenge at the interface of quantum gravity and cosmology is to
explain how the large-scale structure of the Universe emerges from physics at
the Planck scale. In this letter, we take an important step in this direction
by extracting the dynamics of scalar isotropic cosmological perturbations from
full quantum gravity, as described by the causally complete Barrett-Crane group
field theory model. From the perspective of the underlying quantum gravity
theory, cosmological perturbations are represented as nearest-neighbor two-body
entanglement of group field theory quanta. Their effective dynamics is obtained
via mean-field methods and described relationally with respect to a physical
Lorentz frame causally coupled to the quantum geometry. We quantitatively study
these effective dynamical equations and show that at low energies they are
perfectly consistent with those of General Relativity, while for
trans-Planckian scales quantum effects become important. These results
therefore not only provide crucial insights into the potentially purely quantum
gravitational nature of cosmological perturbations, but also offer rich
phenomenological implications for the physics of the early Universe. | gr-qc |
Terrestrial Gravity Fluctuations: The article reviews the current state of the field, and also presents new
analyses especially with respect to the impact of seismic scattering on gravity
perturbations, active gravity noise cancellation, and time-domain models of
gravity perturbations from atmospheric and seismic point sources. Our
understanding of terrestrial gravity fluctuations will have great impact on the
future development of GW detectors and high-precision gravimetry in general,
and many open questions need to be answered still as emphasized in this
article. | gr-qc |
Exact solutions of Friedmann equation: The cosmological Friedmann equation for the universe filled with a scalar
field is reduced to a system of two equations of the first order, one of which
is an equation with separable variables. For the second equation the exact
solutions are given in closed form for potentials as constants and exponents.
For the same equation exact solutions for quadratic potential are written in
the form of a series in the spiral and attractor areas. Also exact solutions
for very arbitrary potentials are given in the neighborhood of endpoint and
infinity. The existence of all these classical solutions is proven. | gr-qc |
Energy flux and waveforms by coalescing spinless binary system in
effective one-body theory: We present a study on the energy radiation rate and waveforms of the
gravitational wave generated by coalescing spinless binary systems up to the
third post-Minkowskian approximation in the effective one-body theory. To
derive an analytical expansion of the null tetrad components of the
gravitational perturbed Weyl tensor $\varPsi_{4}$ in the effective spacetime,
we utilize the method proposed by Sasaki $et$ $al.$ During this investigation,
we discover more general integral formulas that provide a theoretical framework
for computing the results in any order. Subsequently, we successfully compute
the energy radiation rate and waveforms of the gravitational wave, which
include the results of the Schwarzschild case and the correction terms
resulting from the dimensionless parameters $a_{2}$ and $a_{3}$ in the
effective metric. | gr-qc |
Mass varying neutrinos, symmetry breaking, and cosmic acceleration: We introduce a new proposal for the onset of cosmic acceleration based on
mass-varying neutrinos. When massive neutrinos become nonrelativistic, the
$Z_2$ symmetry breaks, and the quintessence potential becomes positive from its
initially zero value. This positive potential behaves like a cosmological
constant at the present era and drives the Universe's acceleration during the
slow roll evolution of the quintessence. In contrast to $\Lambda$CDM model, the
dark energy in our model is dynamical, and the acceleration is not persistent.
Contrary to some of the previous models of dark energy with mass-varying
neutrinos, we do not use the adiabaticity condition which leads to instability. | gr-qc |
Quantum Cylindrical Waves and Sigma Models: We analyze cylindrical gravitational waves in vacuo with general polarization
and develop a viewpoint complementary to that presented recently by Niedermaier
showing that the auxiliary sigma model associated with this family of waves is
not renormalizable in the standard perturbative sense. | gr-qc |
A geometric construction of the Riemann scalar curvature in Regge
calculus: The Riemann scalar curvature plays a central role in Einstein's geometric
theory of gravity. We describe a new geometric construction of this scalar
curvature invariant at an event (vertex) in a discrete spacetime geometry. This
allows one to constructively measure the scalar curvature using only clocks and
photons. Given recent interest in discrete pre-geometric models of quantum
gravity, we believe is it ever so important to reconstruct the curvature scalar
with respect to a finite number of communicating observers. This derivation
makes use of a new fundamental lattice cell built from elements inherited from
both the original simplicial (Delaunay) spacetime and its circumcentric dual
(Voronoi) lattice. The orthogonality properties between these two lattices
yield an expression for the vertex-based scalar curvature which is strikingly
similar to the corresponding hinge-based expression in Regge calculus (deficit
angle per unit Voronoi dual area). In particular, we show that the scalar
curvature is simply a vertex-based weighted average of deficits per weighted
average of dual areas. | gr-qc |
Thin and thick bubble walls I: vacuum phase transitions: This is the first in a series of papers where we study the dynamics of a
bubble wall beyond usual approximations, such as the assumptions of spherical
bubbles and infinitely thin walls. In this paper, we consider a vacuum phase
transition. Thus, we describe a bubble as a configuration of a scalar field
whose equation of motion depends only on the effective potential. The thin-wall
approximation allows obtaining both an effective equation of motion for the
wall position and a simplified equation for the field profile inside the wall.
Several different assumptions are involved in this approximation. We discuss
the conditions for the validity of each of them. In particular, the minima of
the effective potential must have approximately the same energy, and we discuss
the correct implementation of this approximation. We consider different
improvements to the basic thin-wall approximation, such as an iterative method
for finding the wall profile and a perturbative calculation in powers of the
wall width. We calculate the leading-order corrections. Besides, we derive an
equation of motion for the wall without any assumptions about its shape. We
present a suitable method to describe arbitrarily deformed walls from the
spherical shape. We consider concrete examples and compare our approximations
with numerical solutions. In subsequent papers, we shall consider higher-order
finite-width corrections, and we shall take into account the presence of the
fluid. | gr-qc |
Center-of-Mass Equations of Motion and Conserved Integrals of Compact
Binary Systems at the Fourth Post-Newtonian Order: The dynamics of compact binary systems at the fourth post-Newtonian (4PN)
approximation of general relativity has been recently completed in a
self-consistent way. In this paper, we compute the ten Poincar\'e constants of
the motion and present the equations of motion in the frame of the center of
mass (CM), together with the corresponding CM Lagrangian, conserved energy and
conserved angular momentum. Next, we investigate the reduction of the CM
dynamics to the case of quasi-circular orbits. The non local (in time) tail
effect at the 4PN order is consistently included, as well as the relevant
radiation-reaction dissipative contributions to the energy and angular
momentum. | gr-qc |
Chameleon Cosmology Model Describing the Phantom Divide Line Crossing: An exact solution describing the evolution of the type Bang-to-Rip with the
phantom divide line crossing is constructed in the Chameleon cosmology model,
based on two independent functions of the scalar field. | gr-qc |
Eisenhart's theorem and the causal simplicity of Eisenhart's spacetime: We give a causal version of Eisenhart's geodesic characterization of
classical mechanics. We emphasize the geometric, coordinate independent
properties needed to express Eisenhart's theorem in light of modern studies on
the Bargmann structures (lightlike dimensional reduction, pp-waves). The
construction of the space metric, Coriolis 1-form and scalar potential through
which the theorem is formulated is shown in detail, and in particular it is
proved a one-to-one correspondence between Newtonian frames and Abelian
connections on suitable lightlike principal bundles. The relation of
Eisenhart's theorem in the lightlike case with a Fermat type principle is
pointed out. The operation of lightlike lift is introduced and the existence of
minimizers for the classical action is related to the causal simplicity of
Eisenhart's spacetime. | gr-qc |
Asymptotically AdS black hole with a conformally-coupled scalar field in
the first-order formalism of gravity: We present a novel asymptotically anti-de Sitter black hole solution with
conformally-coupled scalar fields in the first-order formalism of gravity in
four dimensions. To do so, we consider a one-parameter extension of conformal
transformations by exploiting the fact that the vielbein and spin connection
are regarded as independent fields. We solve the field equations analytically
and obtain a static black hole solution with nontrivial torsion sourced by the
conformal coupling between the scalar field and geometry. The presence of
torsion renders the scalar field everywhere regular, while the curvature and
torsion singularities coalesce into the origin. We show that this configuration
is continuously connected to previously reported solutions in the limit of
vanishing torsion and analyze its main properties, focusing on the consequences
of the torsional singularity. | gr-qc |
Equation of state of dark energy in f(R) gravity: f(R) gravity is one of the simplest generalizations of general relativity,
which may explain the accelerated cosmic expansion without introducing a
cosmological constant. Transformed into the Einstein frame, a new scalar degree
of freedom appears and it couples with matter fields. In order for f(R)
theories to pass the local tests of general relativity, it has been known that
the chameleon mechanism with a so-called thin-shell solution must operate. If
the thin-shell constraint is applied to a cosmological situation, it has been
claimed that the equation-of-state parameter of dark energy w must be extremely
close to -1. We argue this is due to the incorrect use of the Poisson equation
which is valid only in the static case. By solving the correct Klein-Gordon
equation perturbatively, we show that a thin-shell solution exists even if w
deviates appreciably from -1. | gr-qc |
The Einstein-Maxwell-Particle System in the York Canonical Basis of ADM
Tetrad Gravity: III) The Post-Minkowskian N-Body Problem, its Post-Newtonian
Limit in Non-Harmonic 3-Orthogonal Gauges and Dark Matter as an Inertial
Effect: We conclude the study of the Post-Minkowskian linearization of ADM tetrad
gravity in the York canonical basis for asymptotically Minkowskian space-times
in the family of non-harmonic 3-orthogonal gauges parametrized by the York time
${}^3K(\tau, \vec \sigma)$ (the inertial gauge variable, not existing in Newton
gravity, describing the general relativistic remnant of the freedom in clock
synchronization in the definition of the instantaneous 3-spaces). As matter we
consider only N scalar point particles with a Grassmann regularization of the
self-energies and with a ultraviolet cutoff making possible the PM
linearization and the evaluation of the PM solution for the gravitational
field. We study in detail all the properties of these PM space-times
emphasizing their dependence on the gauge variable ${}^3{\cal K}_{(1)} =
{1\over {\triangle}}\, {}^3K_{(1)}$ (the non-local York time): Riemann and Weyl
tensors, 3-spaces, time-like and null geodesics, red-shift and luminosity
distance. Then we study the Post-Newtonian (PN) expansion of the PM equations
of motion of the particles. We find that in the two-body case at the 0.5PN
order there is a damping (or anti-damping) term depending only on ${}^3{\cal
K}_{(1)}$. This open the possibility to explain dark matter in Einstein theory
as a relativistic inertial effect: the determination of ${}^3{\cal K}_{(1)}$
from the masses and rotation curves of galaxies would give information on how
to find a PM extension of the existing PN Celestial frame (ICRS) used as
observational convention in the 4-dimensional description of stars and
galaxies. Dark matter would describe the difference between the inertial and
gravitational masses seen in the non-Euclidean 3-spaces, without a violation of
their equality in the 4-dimensional space-time as required by the equivalence
principle. | gr-qc |
Banks of templates for directed and all-sky narrow-band searches of
continuous gravitational waves from spinning neutron stars with several
spindowns: We construct efficient banks of templates suitable for searches of continuous
gravitational waves from isolated spinning neutron stars. We assume that the
search algorithm is based on the time-domain maximum-likelihood
$\mathcal{F}$-statistic and we consider narrow-band searches with several
spindown parameters included. Our template banks are suitable for both all-sky
and directed searches and they enable the usage of the fast Fourier transform
in the computation of the $\mathcal{F}$-statistic as well as, in the case of
all-sky searches, efficient resampling of data to barycentric time. | gr-qc |
BSSN in Spherical Symmetry: The BSSN (Baumgarte-Shapiro-Shibata-Nakamura) formulation of the Einstein
evolution equations is written in spherical symmetry. These equations can be
used to address a number of technical and conceptual issues in numerical
relativity in the context of a single Schwarzschild black hole. One of the
benefits of spherical symmetry is that the numerical grid points can be tracked
on a Kruskal--Szekeres diagram. Boundary conditions suitable for puncture
evolution of a Schwarzschild black hole are presented. Several results are
shown for puncture evolution using a fourth--order finite difference
implementation of the equations. | gr-qc |
Inflation from a chaotic potential with a step: In this work, we study the effects on the relevant observational parameters
of an inflationary universe from a chaotic potential with a step. We
numerically evolve the perturbation equations within both cold inflation and
warm inflation. On the one hand, in a cold inflation scenario we analyse the
scalar power spectrum $P_{\mathcal{R}}$ in terms of the number of e-folds
$N_{e}$, and in terms of the ratio $k/k_{0}$, where $k_{0}$ is our pivot scale.
We show how $P_{\mathcal{R}}$ oscillates around $0.2< k/k_{0} < 20$.
Additionally, we present the evolution of two relevant parameters: the scalar
spectral index $n_\mathrm{s}$ and the tensor-to-scalar ratio $r$. In fact, more
than one region of $(n_\mathrm{s},r)$ lies within the observable window (Planck
2018). On the other hand, in the warm inflationary case, we also examine the
evolution of $P_{\mathcal{R}}$ in terms of $N_{e}$ and $k/k_{0}$. Perturbations
are amplified in WI; in fact, $P_{\mathcal{R}}$ can be much larger than the CMB
value $P_{\mathcal{R}}> 2.22\times 10^{-9}$. This time, the spectral index
$n_\mathrm{s}$ is clearly blue-tilted, at smaller scales, and the
tensor-to-scalar ratio $r$ becomes too low. However, $n_\mathrm{s}$ can change
from blue-tilted towards red-tilted, since $P_{\mathcal{R}}$ starts oscillating
around $k/k_{0}\sim 40$. Indeed, the result from the step potential skims the
Planck contours. Finally, one key aspect of this research was to contrast the
features of an inflationary potential between both paradigms, and, in fact,
they show similarities and differences. Due to a featured background and a
combined effect of entropy fluctuations (only in warm inflation), in both
scenarios certain fluctuation scales are not longer ``freeze in'' on
super-horizon scales. | gr-qc |
Black holes and entropy in loop quantum gravity: An overview: Black holes in equilibrium and the counting of their entropy within Loop
Quantum Gravity are reviewed. In particular, we focus on the conceptual setting
of the formalism, briefly summarizing the main results of the classical
formalism and its quantization. We then focus on recent results for small,
Planck scale, black holes, where new structures have been shown to arise, in
particular an effective quantization of the entropy. We discuss recent results
that employ in a very effective manner results from number theory, providing a
complete solution to the counting of black hole entropy. We end with some
comments on other approaches that are motivated by loop quantum gravity. | gr-qc |
Hidden regimes during preheating: The Effective Field Theory (EFT) of Preheating with scalar fields, implies
three types of derivative couplings between the inflaton and the reheating
field. Two of these couplings lead to scales below which only one of the two
species appear as the low energy modes. In this paper, the variety of low
energy regimes in terms of the species they accommodate are explored by
studying the scales introduced by the derivative couplings and the dispersion
relations they lead to. It is noted that the EFT of two scalar fields can give
rise to non-trivial sound speed for both the inflation and reheating sector
even at scales where modes of both species propagate freely, suggesting the
presence of additional heavy fields. The regimes where one of the species
affects the dispersion relation of the other while not appearing as an
effective mode itself, are named as "Hidden Regimes" during preheating. | gr-qc |
Local Scale Invariant Kaluza-Klein Reduction: We perform the 4-dimensional Kaluza-Klein (KK) reduction of the 5-dimensional
locally scale invariant Weyl-Dirac gravity. While compactification unavoidably
introduces an explicit length scale into the theory, it does it in such a way
that the KK radius can be integrated out from the low energy regime, leaving
the KK vacuum to still enjoy local scale invariance at the classical level.
Imitating a $U(1)\times\tilde{U}(1)$ gauge theory, the emerging 4D theory is
characterized by a kinetic Maxwell-Weyl mixing whose diagonalization procedure
is carried out in detail. In particular, we identify the unique linear
combination which defines the 4D Weyl vector, and fully classify the 4D scalar
sector. The later consists of (using Weyl language) a co-scalar and two
in-scalars. The analysis is performed for a general KK $m$-ansatz, parametrized
by the power $m$ of the scalar field which factorizes the 4D metric. The
no-ghost requirement, for example, is met provided $-\frac{1}{2}\leq m \leq 0$.
An $m$-dependent dictionary is then established between the original 5D
Brans-Dicke parameter $\omega_5$ and the resulting 4D $\omega_4$. The critical
$\omega_5=-\frac{4}{3}$ is consistently mapped into critical $\omega_4 =
-\frac{3}{2}$. The KK reduced Maxwell-Weyl kinetic mixing cannot be scaled away
as it is mediated by a 4D in-scalar (residing within the 5D Weyl vector). The
mixing is explicitly demonstrated within the Einstein frame for the special
physically motivated choice of $m=-\frac{1}{3}$. For instance, a super critical
Brans-Dicke parameter induces a tiny positive contribution to the original (if
introduced via the 5-dimensional scalar potential) cosmological constant.
Finally, some no-scale quantum cosmological aspects are studied at the
universal mini-superspace level. | gr-qc |
Two-Sided Gravitational Mirror: Sealing off Curvature Singularities: A gravitational mirror is a non-singular finite redshift surface which
bounces all incident null geodesics. While a white mirror (outward bouncing)
resembles 't Hooft's brick wall, a black mirror (inward bouncing) offers a
novel mechanism for sealing off curvature singularities. The geometry
underlying a two-sided mirror is characterized by a single signature change, to
be contrasted with the signature flip which governs the black hole geometry. To
demonstrate the phenomenon analytically, we derive an exact, static, radially
symmetric, two-sided mirror solution, which asymptotes the massless BTZ black
hole background, and then probe the local structure of a massive mirror. | gr-qc |
Electrically charged gravastar configurations: The notion of a compact object immune to the horizon problem and comprising
an anisotropic inhomogeneous fluid with a specific radial pressure behavior,
i.e. the gravastar, is extended by introducing an electrically charged
component. Einstein-Maxwell field equations are solved in the asymptotically de
Sitter interior where a source of the electric field is coupled to the fluid
energy density. Two different solutions which satisfy the dominant energy
condition are given: one is the delta-shell model for which the analysis is
carried out within Israel's thin shell formalism, the other approach - the
continuous profile model - is solved numerically and the interior solutions
have been (smoothly) joined with the Reissner-Nordstrom exterior. The effect of
electric charge is considered, and the equation of state, the speed of sound
and the surface redshift are calculated for both models. | gr-qc |
Anisotropic solution for polytropic stars in 4D Einstein-Gauss-Bonnet
gravity: In the present work we have investigated a new anisotropic solution for
polytropic star in the framework of $4D$ Einstein-Gauss-Bonnet (EGB) gravity.
The possibility of determining the masses and radii of compact stars which puts
some limitations on equation of state (EoS) above the nuclear saturation
density. For this purpose, the $4D$ EGB field equations are solved by taking a
generalized polytropic equation of state (EoS) with Finch-Skea ansatz. The
generalized solution for anisotropic model has been tested for different values
of Gauss-Bonnet constant $\alpha$ which satisfies all the physical criteria
including causality with static stability via mass vs central mass density
($M-\rho_c$), Bondi and Abreu criterion. The adiabatic index shows a minor
influence of the GB coupling constant whereas the central and surface redshifts
in the EGB gravity always remain lower than the GR. We present the possibility
of fitting the mass and radius for some known compact star via $M-R$ curve
which satisfies the recent gravitational wave observations from GW 170817
event. | gr-qc |
Symmetry algebra in gauge theories of gravity: Diffeomorphisms and an internal symmetry (e.g., local Lorentz invariance) are
typically regarded as the symmetries of any geometrical gravity theory,
including general relativity. In the first-order formalism, diffeomorphisms can
be thought of as a derived symmetry from the so-called local translations,
which have improved properties. In this work, the algebra of an arbitrary
internal symmetry and the local translations is obtained for a generic gauge
theory of gravity, in any spacetime dimensions, and coupled to matter fields.
It is shown that this algebra closes off shell suggesting that these symmetries
form a larger gauge symmetry. In addition, a mechanism to find the symmetries
of theories that have nondynamical fields is proposed. It turns out that the
explicit form of the local translations depend on the internal symmetry and
that the algebra of local translations and the internal group still closes off
shell. As an example, the unimodular Einstein-Cartan theory in four spacetime
dimensions, which is only invariant under volume preserving diffeomorphisms, is
studied. | gr-qc |
Realizability of the Lorentzian (n,1)-Simplex: In a previous article [JHEP 1111 (2011) 072; arXiv:1108.4965] we have
developed a Lorentzian version of the Quantum Regge Calculus in which the
significant differences between simplices in Lorentzian signature and Euclidean
signature are crucial. In this article we extend a central result used in the
previous article, regarding the realizability of Lorentzian triangles, to
arbitrary dimension. This technical step will be crucial for developing the
Lorentzian model in the case of most physical interest: 3+1 dimensions.
We first state (and derive in an appendix) the realizability conditions on
the edge-lengths of a Lorentzian n-simplex in total dimension n=d+1, where d is
the number of space-like dimensions. We then show that in any dimension there
is a certain type of simplex which has all of its time-like edge lengths
completely unconstrained by any sort of triangle inequality. This result is the
d+1 dimensional analogue of the 1+1 dimensional case of the Lorentzian
triangle. | gr-qc |
N=1 de Sitter Supersymmetry Algebra: Recalling the universal covering group of de Sitter, the transformation
properties of the spinor fields $\psi(x)$ and ${\bar\psi}(x)$, in the ambient
space notation, are presented in this paper. The charge conjugation symmetry of
the de Sitter spinor field is then discussed in the above notation. Using this
spinor field and charge conjugation, de Sitter supersymmetry algebra in the
ambient space notation has been established. It is shown that a novel
dS-superalgebra can be attained by the use of spinor field and charge
conjugation in the ambient space notation. | gr-qc |
Gravitational-wave surrogate models powered by artificial neural
networks: The ANN-Sur for waveform generation: Inferring the properties of black holes and neutron stars is a key science
goal of gravitational-wave (GW) astronomy. To extract as much information as
possible from GW observations we must develop methods to reduce the cost of
Bayesian inference. In this paper, we use artificial neural networks (ANNs) and
the parallelisation power of graphics processing units (GPUs) to improve the
surrogate modelling method, which can produce accelerated versions of existing
models. As a first application of our method, ANN-Sur, we build a time-domain
surrogate model of the spin-aligned binary black hole (BBH) waveform model
SEOBNRv4. We achieve median mismatches of 2e-5 and mismatches no worse than
2e-3. For a typical BBH waveform generated from 12 Hz with a total mass of $60
M_\odot$ the original SEOBNRv4 model takes 1812 ms. Existing bespoke code
optimisations (SEOBNRv4opt) reduced this to 91.6 ms and the interpolation
based, frequency-domain surrogate SEOBNRv4ROM can generate this waveform in 6.9
ms. Our ANN-Sur model, when run on a CPU takes 2.7 ms and just 0.4 ms when run
on a GPU. ANN-Sur can also generate large batches of waveforms simultaneously.
We find that batches of up to 10^4 waveforms can be evaluated on a GPU in just
163 ms, corresponding to a time per waveform of 0.016 ms. This method is a
promising way to utilise the parallelisation power of GPUs to drastically
increase the computational efficiency of Bayesian parameter estimation. | gr-qc |
News from horizons in binary black hole mergers: In a binary black hole merger, it is known that the inspiral portion of the
waveform corresponds to two distinct horizons orbiting each other, and the
merger and ringdown signals correspond to the final horizon being formed and
settling down to equilibrium. However, we still lack a detailed understanding
of the relation between the horizon geometry in these three regimes and the
observed waveform. Here we show that the well known inspiral chirp waveform has
a clear counterpart on black hole horizons, namely, the shear of the outgoing
null rays at the horizon. We demonstrate that the shear behaves very much like
a compact binary coalescence waveform with increasing frequency and amplitude.
Furthermore, the parameters of the system estimated from the horizon agree with
those estimated from the waveform. This implies that even though black hole
horizons are causally disconnected from us, assuming general relativity to be
true, we can potentially infer some of their detailed properties from
gravitational wave observations. | gr-qc |
Comments on the entropic gravity proposal: Explicit tests are presented of the conjectured entropic origin of the
gravitational force. The gravitational force on a test particle in the vicinity
of the horizon of a large Schwarzschild black hole in arbitrary spacetime
dimensions is obtained as entropic force. The same conclusion can be reached
for the cases of a large electrically charged black hole and a large slowly
rotating Kerr black hole. The generalization along the same lines to a test
mass in the field of an arbitrary spherical star is also studied and found not
to be possible. Our results thus reinforce the argument that the entropic
gravity proposal cannot account for the gravitational force in generic
situations. | gr-qc |
Matching slowly rotating spacetimes split by dynamic thin shells: We investigate within the Darmois-Israel thin shell formalism the match of
neutral and asymptotically flat, slowly rotating spacetimes (up to the second
order in the rotation parameter) when their boundaries are dynamic. It has
several important applications in general relativistic systems, such as black
holes and neutron stars, which we exemplify. We mostly focus on stability
aspects of slowly rotating thin shells in equilibrium and surface degrees of
freedom on the hypersurfaces splitting the matched slowly rotating spacetimes,
e.g., surface energy density and surface tension. We show that the stability
upon perturbations in the spherically symmetric case automatically implies
stability in the slow rotation case. In addition, we show that when matching
slowly rotating Kerr spacetimes through thin shells in equilibrium, surface
degrees of freedom can decrease compared to their Schwarzschild counterparts,
meaning that energy conditions could be weakened. Frame-dragging aspects of the
match of slowly rotating spacetimes are also briefly discussed. | gr-qc |
Extended diffeomorphism algebras in (quantum) gravitational physics: We construct an explicit representation of the algebra of local
diffeomorphisms of a manifold with realistic dimensions. This is achieved in
the setting of a general approach to the (quantum) dynamics of a physical
system which is characterized by the fundamental role assigned to a basic
underlying symmetry. The developed mathematical formalism makes contact with
the relevant gravitational notions by means of the addition of some extra
structure. The specific manners in which this is accomplished, together with
their corresponding physical interpretation, lead to different gravitational
models. Distinct strategies are in fact briefly outlined, showing the
versatility of the present conceptual framework. | gr-qc |
A Model of Macroscopic Geometrical Uncertainty: A model quantum system is proposed to describe position states of a massive
body in flat space on large scales, excluding all standard quantum and
gravitational degrees of freedom. The model is based on standard quantum spin
commutators, with operators interpreted as positions instead of spin, and a
Planck-scale length $\ell_P$ in place of Planck's constant $\hbar$. The algebra
is used to derive a new quantum geometrical uncertainty in direction, with
variance given by $\langle \Delta \theta^2\rangle = \ell_P/L$ at separation
$L$, that dominates over standard quantum position uncertainty for bodies
greater than the Planck mass. The system is discrete and holographic, and
agrees with gravitational entropy if the commutator coefficient takes the exact
value $\ell_P= l_P/\sqrt{4\pi}$, where $l_P\equiv \sqrt{\hbar G/c^3}$ denotes
the standard Planck length. A physical interpretation is proposed that connects
the operators with properties of classical position in the macroscopic limit:
Approximate locality and causality emerge in macroscopic systems if position
states of multiple bodies are entangled by proximity. This interpretation
predicts coherent directional fluctuations with variance $\langle \Delta
\theta^2\rangle $ on timescale $\tau \approx L/c$ that lead to precisely
predictable correlations in signals between adjacent interferometers. It is
argued that such a signal could provide compelling evidence of Planck scale
quantum geometry, even in the absence of a complete dynamical or fundamental
theory. | gr-qc |
Thermodynamics and Phase transition from regular Bardeen black hole: In this paper, thermodynamics and phase transition are investigated for the
regular Bardeen black hole. Considering the metric of the Bardeen spacetime, we
derived the Unruh-Verlinde temperature. Using the first law of thermodynamics,
we derived the expression of the specific heat and plot its behavior. It
results that the magnetic monopole charge $\beta$ reduces the temperature and
induces a thermodynamics phase transition in the spacetime. Moreover, when
increasing $\beta$, the transition point moves to higher entropy. | gr-qc |
Thermodynamics of scalar-tensor gravity: Previously, the Einstein equation has been described as an equation of state,
general relativity as the equilibrium state of gravity, and $f({\cal R})$
gravity as a non-equilibrium one. We apply Eckart's first order thermodynamics
to the effective dissipative fluid describing scalar-tensor gravity.
Surprisingly, we obtain simple expressions for the effective heat flux,
"temperature of gravity", shear and bulk viscosity, and entropy density, plus a
generalized Fourier law in a consistent Eckart thermodynamical picture.
Well-defined notions of temperature and approach to equilibrium, missing in the
current thermodynamics of spacetime scenarios, naturally emerge. | gr-qc |
Is gravitational entropy quantized ?: In Einstein's gravity, the entropy of horizons is proportional to their area.
Several arguments given in the literature suggest that, in this context, both
area and entropy should be quantized with an equally spaced spectrum for large
quantum numbers. But in more general theories (like, for e.g, in the black hole
solutions of Gauss-Bonnet or Lanczos-Lovelock gravity) the horizon entropy is
\emph{not} proportional to area and the question arises as to which of the two
(if at all) will have this property. We give a general argument that in all
Lanczos-Lovelock theories of gravity, it is the \emph{entropy} that has equally
spaced spectrum. In the case of Gauss-Bonnet gravity, we use the asymptotic
form of quasi normal mode frequencies to explicitly demonstrate this result.
Hence, the concept of a quantum of area in Einstein Hilbert (EH) gravity needs
to be replaced by a concept of \emph{quantum of entropy} in a more general
context. | gr-qc |
New directions in Background Independent Quantum Gravity: We discuss the meaning of background independence in quantum theories of
gravity where geometry and gravity are emergent and illustrate the
possibilities using the framework of quantum causal histories. | gr-qc |
Extended gravity from noncommutativity: We review the first order theory of gravity (vierbein formulation) on
noncommutative spacetime studied in [1, 2]. The first order formalism allows to
couple the theory to fermions. This NC action is then reinterpreted (using the
Seiberg-Witten map) as a gravity theory on commutative spacetime that contains
terms with higher derivatives and higher powers of the curvature and depend on
the noncommutativity parameter \theta. When the noncommutativity is switched
off we recover the usual gravity action coupled to fermions. The first
nontrival corrections to the usual gravity action coupled to fermions are
presented in a manifest Lorentz invariant form. | gr-qc |
The Jacobi map for gravitational lensing: the role of the exponential
map: We present a formal derivation of the key equations governing gravitational
lensing in arbitrary space-times, starting from the basic properties of Jacobi
fields and their expressions in terms of the exponential map. A careful
analysis of Jacobi fields and Jacobi classes near the origin of a light beam
determines the nature of the singular behavior of the optical deformation
matrix. We also show that potential problems that could arise from this
singularity do not invalidate the conclusions of the original argument
presented by Seitz, Schneider & Ehlers (1994). | gr-qc |
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