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Improved ($\barμ$-Scheme) Effective Dynamics of Full Loop Quantum
Gravity: We propose a new derivation from the full Loop Quantum Gravity (LQG) to the
Loop Quantum Cosmology (LQC) improved $\bar{\mu}$-scheme effective dynamics,
based on the reduced phase space formulation of LQG and a proposal of effective
Hamiltonian/action in the full LQG. A key step of our program is an improved
regularization of the full LQG Hamiltonian on a cubic lattice. The improved
Hamiltonian uses a set of "dressed holonomies" $h_\Delta(\mathfrak{s})$ which
not only depend on the connection $A$ but also depend on the length of the
curve $\mathfrak{s}$. With the improved Hamiltonian, we propose a quantum
effective action and derive a new set effective equations of motion (EOMs) for
the full LQG. Then we show that these new EOMs imply the $\bar{\mu}$-scheme
effective dynamics for both the homogeneous-isotropic and Bianchi-I cosmology,
and predict bounce and Planckian critical density. As a byproduct, although the
model is defined on a cubic lattice, we find that the improved effective
Hamiltonian of cosmology is invariant under the lattice refinement. The
cosmological effective dynamics, predictions of bounce and critical density are
results at the continuum limit. | gr-qc |
Entropic issues in contemporary cosmology: Penrose [1] has emphasized how the initial big bang singularity requires a
special low entropy state. We address how recent brane cosmological schemes
address this problem and whether they offer any apparent resolution. Pushing
the start time back to $t=-\infty$ or utilizing maximally symmetric AdS spaces
simply exacerbates or transfers the problem.
Because the entropy of de Sitter space is $S\leq 1/\Lambda$, using the
present acceleration of the universe as a low energy $(\Lambda\sim 10^{-120}$)
inflationary stage, as in cyclic ekpyrotic models, produces a gravitational
heat death after one cycle. Only higher energy driven inflation, together with
a suitable, quantum gravity holography style, restriction on {\em ab initio}
degrees of freedom, gives a suitable low entropy initial state. We question the
suggestion that a high energy inflationary stage could be naturally reentered
by Poincare recurrence within a finite causal region of an accelerating
universe.
We further give a heuristic argument that so-called eternal inflation is not
consistent with the 2nd law of thermodynamics within a causal patch. | gr-qc |
Generic warp drives violate the null energy condition: Three very recent articles have claimed that it is possible to, at least in
theory, either set up positive energy warp drives satisfying the weak energy
condition (WEC), or at the very least, to minimize the WEC violations. These
claims are at best incomplete, since the arguments presented only demonstrate
the existence of one set of timelike observers, the co-moving Eulerian
observers, who see "nice" physics. While these observers might see a positive
energy density, the WEC requires all timelike observers to see positive energy
density. Therefore, one should revisit this issue. A more careful analysis
shows that the situation is actually much grimmer than advertised -- all
physically reasonable warp drives will violate the null energy condition, and
so also automatically violate the WEC, and both the strong and dominant energy
conditions. While warp drives are certainly interesting examples of speculative
physics, the violation of the energy conditions, at least within the framework
of standard general relativity, is unavoidable. Even in modified gravity,
physically reasonable warp drives will still violate the purely geometrical
null convergence condition and the timelike convergence condition which, in
turn, will place very strong constraints on any modified-gravity warp drive. | gr-qc |
Galactic dark matter as a bulk effect on the brane: The behavior of the angular velocity of a test particle moving in a stable
circular orbit in the vacuum on the brane is considered. In the brane world
scenario, the four dimensional effective Einstein equation acquire extra terms,
called dark radiation and dark pressure, respectively, which arise from the
embedding of the 3-brane in the bulk. A large number of independent
observations have shown that the rotational velocities of test particles
gravitating around galaxies tend, as a function of the distance from the
galactic center, toward constant values. By assuming a constant tangential
velocity, the general solution of the vacuum gravitational field equations on
the brane can be obtained in an exact analytic form. This allows us to obtain
the explicit form of the projections of the bulk Weyl tensor on the brane, and
the equation of state of the dark pressure as a function of the dark radiation.
The physical and geometrical quantities are expressed in terms of
observable/measurable parameters, like the tangential velocity, the baryonic
mass and the radius of the galaxy. We also analyze the dynamics of test
particles by using methods from the qualitative analysis of dynamical systems,
by assuming a simple linear equation of state for the dark pressure. The
obtained results provide a theoretical framework for the observational testing
at the extra-galactic scale of the predictions of the brane world models. | gr-qc |
Observational constraints on bimetric gravity: Ghost-free bimetric gravity is a theory of two interacting spin-2 fields, one
massless and one massive, in addition to the standard matter particles and
fields, thereby generalizing Einstein's theory of general relativity. To
parameterize the theory, we use five observables with specific physical
interpretations. We present, for the first time, observational constraints on
these parameters that: (i) apply to the full theory, (ii) are consistent with a
working screening mechanism (i.e., restoring general relativity locally), (iii)
exhibit a continuous, real-valued background cosmology (without the Higuchi
ghost). For the cosmological constraints, we use data sets from the cosmic
microwave background, baryon acoustic oscillations, and type Ia supernovae.
Bimetric cosmology provides a good fit to data even for large values of the
mixing angle between the massless and massive gravitons. Interestingly, the
best-fit model is a self-accelerating solution where the accelerated expansion
is due to the dynamical massive spin-2 field, without a cosmological constant.
Due to the screening mechanism, the models are consistent with local tests of
gravity such as solar system tests and gravitational lensing by galaxies. We
also comment on the possibility of alleviating the Hubble tension with this
theory. | gr-qc |
Induced gravitational waves as a probe of thermal history of the
universe: The scalar perturbation induced gravitational waves are a probe of the
primordial density perturbation spectrum on small scales. In this paper, we
show that they can also probe the thermal history of the universe. We assume
the universe underwent a stage with a constant equation of state parameter $w$,
followed by the radiation-dominated stage of the conventional big bang
universe. We find that the infrared slope of the power spectrum of the induced
stochastic gravitational wave background for decelerating cosmologies is
related to the equation of state of the universe. Furthermore, the induced
gravitational wave spectrum has in general a broken power-law shape around the
scale of reheating. Interestingly, below the threshold $w=0$ of the equation of
state parameter, the broken power-law presents a peak for a Dirac delta peak in
the scalar spectrum. For a finite width peak, the threshold changes to
$w=-1/15$ depending on the value of the width. In some cases, such a broken
power-law gravitational wave spectrum may degenerate to the spectrum from other
sources like phase transitions or global cosmic strings. | gr-qc |
Self-interacting dipolar boson stars and their dynamics: We construct and dynamically evolve dipolar, self-interacting scalar boson
stars in a model with sextic (+ quartic) self-interactions. The domain of
existence of such dipolar $Q$-stars has a similar structure to that of the
fundamental monopolar stars of the same model. For the latter it is structured
in a Newtonian plus a relativistic branch, wherein perturbatively stable
solutions exist, connected by a middle unstable branch. Our evolutions support
similar dynamical properties of the dipolar $Q$-stars that: 1) in the Newtonian
and relativistic branches are dynamically robust over time scales longer than
those for which dipolar stars without self-interactions are seen to decay; 2)
in the middle branch migrate to either the Newtonian or the relativistic
branch; 3) beyond the relativistic branch decay to black holes. Overall, these
results strengthen the observation, seen in other contexts, that
self-interactions can mitigate dynamical instabilities of scalar boson star
models. | gr-qc |
Recreation of the temperature power spectrum for some inflationary
models: This paper presents the CMB angular power spectrum obtained using the CAMB
code for three different models of inflation: the Starobinsky inflationary
model, the generalized Starobinsky inflationary model, and the chaotic
inflationary model with a step. The results are compared with the most recent
data reported for the Planck mission. An analysis of the large ($l \lesssim
90$), intermediate ($90 \lesssim l \lesssim 900$), and small ($l \gtrsim 900 $)
angular scales is performed. We report the position of the peaks in the
intermediate region so as the cosmological parameters obtained in each of the
models: age of the universe, $\Omega_m$, $\Omega_b$, $\Omega_{\Lambda}$,
$\Omega_K$ and $n_S$. | gr-qc |
Quasi-Rip universe induced by the fluid with inhomogeneous equation of
state: We investigate a specific model for dark energy, which lead to the Quasi-Rip
cosmology. In the Quasi-Rip model, the equation of the state parameter $w$ is
less than -1 in the first stage, but then in the second stage is larger than
-1. The conditions for the appearance the Quasi-Rip in the terms of the
parameters equation of state are received. | gr-qc |
Tsunamis and Ripples: Effects of Scalar Waves on Screening in the Milky
Way: Modified gravity models which include an additional propagating degree of
freedom are typically studied in the quasi-static limit, where the propagation
is neglected, and the wave equation of the field is replaced with a
Poisson-type equation. Recently, it has been proposed that, in the context of
models with symmetron- or chameleon-type screening, scalar waves from
astrophysical or cosmological events could have a significant effect on the
screening of the Solar System, and hence invalidate these models. Here, we
quantitatively investigate the impact of scalar waves by solving the full field
equation linearised in the wave amplitude. In the symmetron case, we find that
the quantitative effect of waves is generally negligible, even for the largest
amplitudes of waves that are physically expected. In order to spoil the
screening in the Solar System, a significant amount of wave energy would have
to be focused on the Solar System by arranging the sources in a spherical shell
centred on Earth. In the chameleon case, we are able to rule out any
significant effects of propagating waves on Solar System tests. | gr-qc |
A nested sampling code for targeted searches for continuous
gravitational waves from pulsars: This document describes a code to perform parameter estimation and model
selection in targeted searches for continuous gravitational waves from known
pulsars using data from ground-based gravitational wave detectors. We describe
the general workings of the code and characterise it on simulated data
containing both noise and simulated signals. We also show how it performs
compared to a previous MCMC and grid-based approach to signal parameter
estimation. Details how to run the code in a variety of cases are provided in
Appendix A. | gr-qc |
Interacting vector fields in Relativity without Relativity: Barbour, Foster and \'{O} Murchadha have recently developed a new framework,
called here {\it{the 3-space approach}}, for the formulation of classical
bosonic dynamics. Neither time nor a locally Minkowskian structure of spacetime
are presupposed. Both arise as emergent features of the world from
geodesic-type dynamics on a space of 3-dimensional metric--matter
configurations. In fact gravity, the universal light cone and Abelian gauge
theory minimally coupled to gravity all arise naturally through a single common
mechanism. It yields relativity -- and more -- without presupposing relativity.
This paper completes the recovery of the presently known bosonic sector within
the 3-space approach. We show, for a rather general ansatz, that 3-vector
fields can interact among themselves only as Yang--Mills fields minimally
coupled to gravity. | gr-qc |
Shadow thermodynamics of the Hayward-AdS black hole: In this paper, the phase structure of the Hayward-AdS black hole (BH) is
studied using shadow formalism. It has been found that the shadow radius is a
monotonic function of the horizon radius and can therefore play an equivalent
role to the horizon radius in characterizing the thermodynamics of Hayward-AdS
BH. The thermodynamic phase transition (PT) of the Hayward-AdS BH is
investigated with the shadow radius. It is shown that as the magnetic charge
increases, the shadow radius becomes larger, while the coexistence temperature
becomes lower. The thermal profile of the Hayward-AdS BH is established by
combining the temperature diagram and the shadow cast diagram, which shows that
for a fixed magnetic charge, the temperature of the Hayward-AdS BH increases
with the pressure while the region of the thermal profile decreases with the
pressure. In particular, the temperature of the Hayward-AdS BH follows an
N-type change trend when it is smaller than the critical temperature. This
imply that the BH shadow may be used to investigate the thermodynamics of the
Hayward-AdS BH. | gr-qc |
Binary black hole merger dynamics and waveforms: We study dynamics and radiation generation in the last few orbits and merger
of a binary black hole system, applying recently developed techniques for
simulations of moving black holes. Our analysis of the gravitational radiation
waveforms and dynamical black hole trajectories produces a consistent picture
for a set of simulations with black holes beginning on circular-orbit
trajectories at a variety of initial separations. We find profound agreement at
the level of one percent among the simulations for the last orbit, merger and
ringdown. We are confident that this part of our waveform result accurately
represents the predictions from Einstein's General Relativity for the final
burst of gravitational radiation resulting from the merger of an astrophysical
system of equal-mass non-spinning black holes. The simulations result in a
final black hole with spin parameter a/m=0.69. We also find good agreement at a
level of roughly 10 percent for the radiation generated in the preceding few
orbits. | gr-qc |
Resonant Graviton-Photon Conversion with Stochastic Magnetic Field in
the Expanding Universe: We investigate graviton-photon oscillations sourced by cosmological magnetic
fields from Gertsenshtein effect. We adopt a robust perturbative approach and
we find that the conversion probability from graviton to photon can be
resonantly enhanced in monochromatic, multi-chromatic and scale invariant
spectrum models of stochastic magnetic field fluctuations. In addition, the
expansion of the Universe acts as a decoherence factor, which demands a natural
discretization scheme along the line of sight. Including also decoherence from
cosmic acceleration, we find that conversion probabilities for stochastic
magnetic fields are completely different than results predicted from existing
magnetic domain-like models in a wide range of magnetic strengths and coherence
lengths. Resonances can be tested by radio telescopes as a probe of high
frequency gravitational wave sources and primordial magnetogenesis mechanisms. | gr-qc |
Primordial power spectra for scalar perturbations in loop quantum
cosmology: We provide the power spectrum of small scalar perturbations propagating in an
inflationary scenario within loop quantum cosmology. We consider the hybrid
quantization approach applied to a Friedmann--Robertson--Walker spacetime with
flat spatial sections coupled to a massive scalar field. We study the quantum
dynamics of scalar perturbations on an effective background within this hybrid
approach. We consider in our study adiabatic states of different orders. For
them, we find that the hybrid quantization is in good agreement with the
predictions of the dressed metric approach. We also propose an initial vacuum
state for the perturbations, and compute the primordial and the anisotropy
power spectrum in order to qualitatively compare with the current observations
of Planck mission. We find that our vacuum state is in good agreement with
them, showing a suppression of the power spectrum for large scale anisotropies.
We compare with other choices already studied in the literature. | gr-qc |
How to reduce the suspension thermal noise in LIGO without improving the
Q's of the pendulum and violin modes: The suspension noise in interferometric gravitational wave detectors is
caused by losses at the top and the bottom attachments of each suspension
fiber. We use the Fluctuation-Dissipation theorem to argue that by careful
positioning of the laser beam spot on the mirror face it is possible to reduce
the contribution of the bottom attachment point to the suspension noise by
several orders of magnitude. For example, for the initial and enhanced LIGO
design parameters (i.e. mirror masses and sizes, and suspension fibers' lengths
and diameters) we predict a reduction of $\sim 100$ in the "bottom" spectral
density throughout the band $35-100\hbox{Hz}$ of serious thermal noise. We then
propose a readout scheme which suppresses the suspension noise contribution of
the top attachment point. The idea is to monitor an averaged horizontal
displacement of the fiber of length $ l$; this allows one to record the
contribution of the top attachment point to the suspension noise, and later
subtract it it from the interferometer readout. For enhanced LIGO this would
allow a suppression factor about 100 in spectral density of suspension thermal
noise. | gr-qc |
Deflationary Universe Scenario: We show that it is possible to realize an inflationary scenario even without
conversion of the false vacuum energy to radiation. Such cosmological models
have a deflationary stage in which $Ha^2$ is decreasing and radiation produced
by particle creation in an expanding Universe becomes dominant. The preceding
inflationary stage ends since the inflaton potential becomes steep. False
vacuum energy is finally (partly) converted to the inflaton kinetic energy ,
the potential energy rapidly decreases and the Universe comes to the
deflationary stage with a scale factor $a(t) \propto t^{1/3}$. Basic features
and observational consequences of this scenario are indicated. | gr-qc |
A family of solutions to the inverse problem in gravitation: building a
theory around a metric: A method is presented to construct a particular, non-minimally coupled
scalar-tensor theory such that a given metric is an exact vacuum solution in
that theory. In contrast to the standard approach in studies of gravitational
dynamics, where one begins with an action and then solves the equations of
motion, this approach allows for an explicit theory to be built around some
pre-specified geometry. Starting from a parameterized black hole spacetime with
generic, non-Kerr hairs, it is shown how an overarching family of theories can
be designed to fit the metric exactly. | gr-qc |
Analytic conformal compactification of Schwarzschild spacetime: Among the coordinates used to construct a conformal compactification of the
Schwarzschild spacetime, none of them simultaneously extend smoothly both
through an event horizon and beyond null infinity.To construct such
coordinates, instead of starting with the Kruskal-Szekeres coordinates we
assume direct analytic transformation between Schwarzschild and compactified
coordinates and determine their behavior on the event horizon and at null
infinity. We then propose an example of such coordinates and illustrate the way
they cover the conformally extended Schwarzschild spacetime as well as their
suitability for numerical applications. | gr-qc |
Mixed neutron-star-plus-wormhole systems: Linear stability analysis: We consider configurations consisting of a neutron star with a wormhole at
the core. The wormhole is held open by a ghost scalar field with a quartic
coupling. The neutron matter is described by a perfect fluid with a polytropic
equation of state. We obtain static regular solutions for these systems. A
stability analysis, however, shows that they are unstable with respect to
linear perturbations. | gr-qc |
Spherical scalar waves and gravity - red shift and backscattering: This article investigates the interaction of a spherically symmetric massless
scalar field with a strong gravitational field. It focuses on the propagation
of waves in regions outside any horizons. The two factors acting on the waves
can be identified as a redshift and a backscattering. The influence of
backscattering on the intensity of the outgoing radiation is studied and
rigorous quantitative upper bounds obtained. These show that the total flux may
be decreased if the sources are placed in a region adjoining an apparent
horizon. Backscattering can be neglected in the case $2m_0 /R<< 1$, that is
when the emitter is located at a distance from a black hole much larger than
the Schwarzschild radius. This backscattering may have noticeable astrophysical
consequences. | gr-qc |
Relative Unitary Implementability of Perturbed Quantum Field Dynamics on
de-Sitter Space: In this article, we study the quantum dynamics of a Klein-Gordon field on
de-Sitter space. We prove time evolution is not unitarily implementable. We
also consider a Klein-Gordon field perturbed by a local potential V. In this
case we prove that the deviation from the V=0 dynamics is unitarily
implementable. | gr-qc |
Black hole-naked singularity dualism and the repulsion of two Kerr black
holes due to spin-spin interaction: We report about the possibility for interacting Kerr sources to exist in two
different states - black holes or naked singularities - both states
characterized by the same masses and angular momenta. Another surprising
discovery reported by us is that in spite of the absence of balance between two
Kerr black holes, the latter nevertheless can repel each other, which provides
a good opportunity for experimental detection of the spin-spin repulsive force
through the observation of astrophysical black-hole binaries. | gr-qc |
Premetric teleparallel theory of gravity and its local and linear
constitutive law: We continue to investigate the premetric teleparallel theory of gravity (TG)
with the coframe (tetrad) as gravitational potential. We start from the field
equations and a local and linear constitutive law. We create a Tonti diagram of
TG in order to disclose the structure of TG. Subsequently we irreducibly
decompose the 6th order constitutive tensor under the linear group. Moreover,
we construct the most general constitutive tensors from the metric and the
totally antisymmetric Levi-Civita symbol, and we demonstrate that they
encompass nontrivial axion and skewon type pieces. Using these tools, we derive
for TG in the geometric-optics approximation propagating massless spin 0, 1,
and 2 waves, including the special case of Einstein's general relativity. | gr-qc |
Evolution of Nonlinear Perturbations Inside Einstein-Yang-Mills Black
Holes: We present our results on numerical study of evolution of nonlinear
perturbations inside spherically symmetric black holes in the SU(2)
Einstein-Yang-Mills (EYM) theory. Recent developments demonstrate a new type of
the behavior of the metric for EYM black hole interiors; the generic metric
exhibits an infinitely oscillating approach to the singularity, which is a
spacelike but not of the mixmaster type. The evolution of various types of
spherically symmetric perturbations, propagating from the internal vicinity of
the external horizon towards the singularity is investigated in a
self-consistent way using an adaptive numerical algorithm. The obtained results
give a strong numerical evidence in favor of nonlinear stability of the generic
EYM black hole interiors. Alternatively, the EYM black hole interiors of
S(chwarzschild)-type, which form only a zero measure subset in the space of all
internal solutions are found to be unstable and transform to the generic type
as perturbations are developed. | gr-qc |
New black holes in the brane-world?: It is known that the Einstein field equations in five dimensions admit more
general spherically symmetric black holes on the brane than four-dimensional
general relativity. We propose two families of analytic solutions (with
g_tt\not=-1/g_rr), parameterized by the ADM mass and the PPN parameter beta,
which reduce to Schwarzschild for beta=1. Agreement with observations requires
|\beta-1| |\eta|<<1. The sign of eta plays a key role in the global causal
structure, separating metrics which behave like Schwarzschild (eta<0) from
those similar to Reissner-Nordstroem (eta>0). In the latter case, we find a
family of black hole space-times completely regular. | gr-qc |
Constraint on the fifth force through perihelion precession of planets: The equivalence principle is important in fundamental physics. The fifth
force, as a describing formalism of the equivalence principle, may indicate the
property of an unknown theory. Dark matter is one of the most mysterious
objects in the current natural science. It is interesting to constrain the
fifth force of dark matter. We propose a new method to use perihelion
precession of planets to constrain the long-range fifth force of dark matter.
Due to the high accuracy of perihelion precession observation, and the large
difference of matter composition between the Sun and planets, we get one of the
strongest constraints on the fifth force of dark matter. In the near future,
the BepiColombo mission will be capable to improve the test by another factor
of ten. | gr-qc |
Cosmology and stellar equilibrium using Newtonian hydrodynamics with
general relativistic pressure: We revisit the analysis made by Hwang and Noh [JCAP 1310 (2013)] aiming the
construction of a Newtonian set of equations incorporating pressure effects
typical of the General Relativity theory. We explicitly derive the Hwang-Noh
equations, comparing them with similar computations found in the literature.
Then, we investigate $i)$ the cosmological expansion, $ii)$ linear cosmological
perturbations theory and $iii)$ stellar equilibrium by using the new set of
equations and comparing the results with those coming from the usual Newtonian
theory, from the Neo-Newtonian theory and from the General Relativity theory.
We show that the predictions for the background evolution of the Universe are
deeply changed with respect to the General Relativity theory: the acceleration
of the Universe is achieved with positive pressure. On the other hand, the
behaviour of small cosmological perturbations reproduces the one found in the
relativistic context, even if only at small scales. We argue that this last
result may open new possibilities for numerical simulations for structure
formation in the Universe. Finally, the properties of neutron stars are
qualitatively reproduced by Hwang-Noh equations, but the upper mass limit is at
least one order of magnitude higher than the one obtained in General
Relativity. | gr-qc |
Time Asymmetry of Cosmic Background Evolution in Loop Quantum Cosmology: We discuss the asymmetry of cosmic background evolution in time with respect
to the quantum bounce in the Loop Quantum Cosmology (LQC), employing the value
of scalar field at the bounce $\phi_{\rm B}$. We use the Chaotic and the $R^2$
potentials to demonstrate that a possible deflation before the bounce may
counteract the inflation that is needed for resolving the cosmological
conundrums, so a certain level of time asymmetry is required for the models in
LQC. This $\phi_{\rm B}$ is model dependent and closely related to the amounts
of deflation and inflation, so we may use observations to confine $\phi_{\rm
B}$ and thus the model parameters. With further studies this formalism should
be useful in providing an observational testbed for the LQC models. | gr-qc |
On the Komar Energy and the Generalized Smarr Formula for a Charged
Black Hole of Noncommutative Geometry: We calculate the Komar energy $E$ for a charged black hole inspired by
noncommutative geometry and identify the total mass ($M_{0}$) by considering
the asymptotic limit. We also found the generalized Smarr formula, which shows
a deformation from the well known relation $M_{0}-\frac{Q_{0}^{2}}{r}=2ST$
depending on the noncommutative scale length $\ell$ . | gr-qc |
Neutrino spin oscillations in conformally gravity coupling models and
quintessence surrounding a black hole: In this paper, we study the spin transitions of neutrinos caused by the
interaction with a gravitational field. We consider a model with a scalar field
(describing screening effects) conformally coupled to matter and neutrinos. The
presence of screening effects suppresses the neutrino spin-flip probability as
compared with General Relativity predictions. Such a result could be used,
combined with neutrino astronomy, for testing modified theories of gravity and,
in turn, screening effects invoked to bypass the solar system and Lab tests.
Such an analysis has been also extended to the case of the quintessence field
surrounding a black hole. Here we investigate the flavor and spin transitions,
showing that also in such a case exists a suppression of the effect compared to
General Relativity prediction. | gr-qc |
Quantum Probe of Hořava-Lifshitz Gravity: Particle probe analysis of the Kehagias - Sfetsos black hole spacetime of Ho%
\v{r}ava-Lifshitz gravity is extended to wave probe analysis within the
framework of quantum mechanics. The timelike naked singularity that develops
when $\omega M^{2}<1/2$, is probed with quantum fields obeying Klein-Gordon and
Chandrasekhar-Dirac equations. Quantum field probe of the naked singularity has
revealed that both the spatial part of the wave and the Hamiltonian operators
of Klein-Gordon and Chandrasekhar-Dirac equations are essentially self-adjoint
and thus, the naked singularity in the Kehagias - Sfetsos spacetime become
quantum mechanically non - singular. | gr-qc |
Embedding Causal Sets into Minkowski Spacetime: We present a new method for embedding a causal set into an interval of
Minkowski spacetime. The method uses spacetime volumes for causally related
elements to define causal set analogs of Minkowski inner products. These are
used to construct matrices of inner products which are then factored using the
singular value decomposition to give coordinates in Minkowski spacetime.
Results are presented showing good quality embeddings into Minkowski spacetime
for dimensions d=2,3,4. The method applies in any dimension and does not
require spacelike distances to be used as an input. It offers a new way to
define spatial orientation and spacelike distances in a causal set. | gr-qc |
Potentials between D-Branes in a Supersymmetric Model of Space-Time Foam: We study a supersymmetric model of space-time foam with two stacks each of
eight D8-branes with equal string tensions, separated by a single bulk
dimension containing D0-brane particles that represent quantum fluctuations.
The ground-state configuration with static D-branes has zero vacuum energy,
but, when they move, the interactions among the D-branes and D-particles due to
the exchanges of strings result in a non-trivial, positive vacuum energy. We
calculate its explicit form in the limits of small velocities and large or
small separations between the D-branes and/or the D-particles. This non-trivial
vacuum energy appears as a central charge deficit in the non-critical stringy
$\sigma$ model describing perturbative string excitations on a moving D-brane.
These calculations enable us to characterise the ground state of the
D-brane/D-particle system, and provide a framework for discussing brany
inflation and the possibility of residual Dark Energy in the present-day
Universe. | gr-qc |
Les potentiels non gravitationnels et la structure de l'espace-temps: The subject of this dissertation consists in analyzing a recent proposition,
advanced by C.C.Barros, in which the non gravitational interactions can affect
the space-time metric as in gravity. In fact, in the context of the
Schwarzschild solution, the hydrogen atom is described in a completely new way
: instead of following the usual approach to describe the electron under the
Coulomb potential by using the minimal coupling, the "proton-electron"
interaction is rather incorporated in the metric. In this context, we reproduce
in this dissertation the same equations as predicted in Dirac theory for the
weak potential approximation. Contrary to the statement made by Barros,
claiming that he brought an insignificant correction to the electron levels, at
the end of our analysis, we assert that this new approach has the merit of
reproducing the relativistic spectrum as known in the Dirac theory. These
spectacular results incite us to wonder about the role of the Principle of
Equivalence in the foundations of the general theory of relativity. | gr-qc |
Regular charged black holes, energy conditions and quasinormal modes: We discuss energy conditions and quasinormal modes for scalar perturbations
of regular charged black holes within the framework of General Relativity
coupled to non-linear electrodynamics. The frequencies are computed numerically
adopting the WKB method, while in the eikonal limit an analytic expression for
the spectra is obtained. The impact of the electric charge, the angular degree,
and the overtone number on the spectra is investigated in detail. We find that
all frequencies are characterized by a negative imaginary part, and that each
type of energy conditions imply a different quasinormal spectrum. | gr-qc |
Remarks on Superenergy Tensors: We define super-energy tensors for arbitrary physical fields, including the
gravitational, electromagnetic and massless scalar fields. We also define
super-super-energy tensors, and so on. All these tensors satisfy the so-called
"Dominant Superenergy Property" among other interesting and good properties.
The possibility of interchange of superenergy between gravitational and other
fields is considered. | gr-qc |
On the Entropy of Minimally Coupled and Non-minimally Coupled Gravities: We investigate whether the gravitational thermodynamic properties of the
scalar-tensor theory of gravity are affected by the conformal transformation or
not. As an explicit example, we consider an electrically charged static
spherical black hole in the 4-dimensional low energy effective theory of
bosonic string. | gr-qc |
Quantum Stress Tensor Fluctuations and their Physical Effects: We summarize several aspects of recent work on quantum stress tensor
fluctuations and their role in driving fluctuations of the gravitational field.
The role of correlations and anticorrelations is emphasized. We begin with a
review of the properties of the stress tensor correlation function. We next
consider some illuminating examples of non-gravitational effects of stress
tensors fluctuations, specifically fluctuations of the Casimir force and
radiation pressure fluctuations. We next discuss passive fluctuations of
spacetime geometry and some of their operational signatures. These include
luminosity fluctuations, line broadening, and angular blurring of a source
viewed through a fluctuating gravitational field. Finally, we discuss the
possible role of quantum stress tensor fluctuations in the early universe,
especially in inflation. The fluctuations of the expansion of a congruence of
comoving geodesics grows during the inflationary era, due to non-cancellation
of anticorrelations that would have occurred in flat spacetime. This results in
subsequent non-Gaussian density perturbations and allows one to infer an upper
bound on the duration of inflation. This bound is consistent with adequate
inflation to solve the horizon and flatness problems. | gr-qc |
Entanglement/Brick-wall entropies correspondence: There have been many attempts to understand the statistical origin of
black-hole entropy. Among them, entanglement entropy and the brick wall model
are strong candidates. In this paper we show a relation between entanglement
entropy and the brick wall model: the brick wall model seeks the maximal value
of the entanglement entropy. In other words, the entanglement approach reduces
to the brick wall model when we seek the maximal entanglement entropy . | gr-qc |
Electromagnetic memory in arbitrary curved space-times: The gravitational memory effect and its electromagnetic (EM) analog are
potential probes in the strong gravity regime. In the literature, this effect
is derived for static observers at asymptotic infinity. While this is a
physically consistent approach, it restricts the space-time geometries for
which one can obtain the EM memory effect. To circumvent this, we evaluate the
EM memory effect for comoving observers (defined by the 4-velocity $u_{\mu}$)
in arbitrary curved space-times. Using the covariant approach, we split
Maxwell's equations into two parts -- projected parallel to the 4-velocity
$u_{\mu}$ and into the 3-space orthogonal to $u_{\mu}$. Further splitting the
equations into $1+1+2$-form, we obtain \emph{master equation} for the EM memory
in an arbitrary curved space-time. We provide a geometrical understanding of
the contributions to the memory effect. We then obtain EM memory for specific
space-time geometries and discuss the salient features. | gr-qc |
Analytical treatment of SUSY Quasi-normal modes in a non-rotating
Schwarzschild black hole: We use the Fock-Ivanenko formalism to obtain the Dirac equation which
describes the interaction of a massless 1/2-spin neutral fermion with a
gravitational field around a Schwarzschild black hole (BH). We obtain
approximated analytical solutions for the eigenvalues of the energy
(quasi-normal frequencies) and their corresponding eigenstates (quasi-normal
states). The interesting result is that all the excited states [and their
supersymmetric (SUSY) partners] have a purely imaginary frequency, which can be
expressed in terms of the Hawking temperature. Furthermore, as one expects for
SUSY Hamiltonians, the isolated bottom state has a real null energy eigenvalue. | gr-qc |
Gravity Theories via Algebra Gauging: This work presents instructive, yet comprehensive derivation of quantized
gravity theories in relativistic, classical, and semi-classical spacetime
structure based on the Poincar\'e, Galilean, and Bargmann algebra,
respectively. The technique of algebra gauging to construct the spacetime
dynamics - inspired by the approach of notable previous works - is introduced
to complement the standard vielbein formulation. The key characteristics and
anomalies of Galilean gravity will then be analyzed: the degenerate metric
structure, the additional degree of freedom in metric connection and the
additional necessary conditions of Galilean invariance among others. General
metric connection solution in Galilean spacetime differs fundamentally from
that of general relativity; this will be thoroughly investigated and an
explicit formula for such solution - equivalent to the parameterization by
Hartong and Obers (2015) - shall be derived. Multiple derivations of the
Bargmann algebra will be provided, together with both physical and algebraic
motivation for the extended Bargmann frame bundle. Finally, the physical impact
of constraining temporal torsion in classical spacetime will be discussed with
emphasis on the geometrical interpretation of time foliations. | gr-qc |
Multidimensional perfect fluid cosmology with stable compactified
internal dimensions: Multidimensional cosmological models in the presence of a bare cosmological
constant and a perfect fluid are investigated under dimensional reduction to
4-dimensional effective models. Stable compactification of the internal spaces
is achieved for a special class of perfect fluids. The external space behaves
in accordance with the standard Friedmann model. Necessary restrictions on the
parameters of the models are found to ensure dynamical behavior of the external
(our) universe in agreement with observations. | gr-qc |
Quantum geometry and quantum dynamics at the Planck scale: Canonical quantum gravity provides insights into the quantum dynamics as well
as quantum geometry of space-time by its implications for constraints. Loop
quantum gravity in particular requires specific corrections due to its
quantization procedure, which also results in a discrete picture of space. The
corresponding changes compared to the classical behavior can most easily be
analyzed in isotropic models, but perturbations around them are more involved.
For one type of corrections, consistent equations have been found which shed
light on the underlying space-time structure at the Planck scale: not just
quantum dynamics but also the concept of space-time manifolds changes in
quantum gravity. Effective line elements provide indications for possible
relationships to other frameworks, such as non-commutative geometry. | gr-qc |
The generalized Heun equation in QFT in curved space-times: In this article we give a brief outline of the applications of the
generalized Heun equation (GHE) in the context of Quantum Field Theory in
curved space-times. In particular, we relate the separated radial part of a
massive Dirac equation in the Kerr-Newman metric and the static perturbations
for the non-extremal Reissner-Nordstr\"{o}m solution to a GHE. | gr-qc |
Impact of Bayesian prior on the characterization of binary black hole
coalescences: In a regime where data are only mildly informative, prior choices can play a
significant role in Bayesian statistical inference, potentially affecting the
inferred physics. We show this is indeed the case for some of the parameters
inferred from current gravitational-wave measurements of binary black hole
coalescences. We reanalyze the first detections performed by the twin LIGO
interferometers using alternative (and astrophysically motivated) prior
assumptions. We find different prior distributions can introduce deviations in
the resulting posteriors that impact the physical interpretation of these
systems. For instance, (i) limits on the $90\%$ credible interval on the
effective black hole spin $\chi_{\rm eff}$ are subject to variations of $\sim
10\%$ if a prior with black hole spins mostly aligned to the binary's angular
momentum is considered instead of the standard choice of isotropic spin
directions, and (ii) under priors motivated by the initial stellar mass
function, we infer tighter constraints on the black hole masses, and in
particular, we find no support for any of the inferred masses within the
putative mass gap $M \lesssim 5 M_\odot$. | gr-qc |
Godel Universes in String Theory: We show that homogeneous G\"odel spacetimes need not contain closed timelike
curves in low-energy-effective string theories. We find exact solutions for the
G\"odel metric in string theory for the full $O(\alpha ^{\prime})$ action
including both dilaton and axion fields. The results are valid for bosonic,
heterotic and super-strings. To first order in the inverse string tension
$\alpha ^{\prime}$, these solutions display a simple relation between the
angular velocity of the G\"odel universe, $\Omega ,$ and the inverse string
tension of the form $\alpha ^{\prime}=1/\Omega ^2$ in the absence of the axion
field. The generalization of this relationship is also found when the axion
field is present. | gr-qc |
Three Dimensional Black Hole in the Low Energy Heterotic String Theory: We study the BTZ black holes (2+1 dimensional space-time) in the low energy
heterotic string theory (BTZ-Sen BH). This concept requires us to include a
non-trivial dilaton $ \phi $ and a 3-form $ H_{\mu\nu\rho} $ field. By using
the Hassan-Sen transformation and BTZ black hole as a seed solution, we obtain
the solution in the string frame. Some properties of the black hole solutions
are discussed. | gr-qc |
Light Propagation in the vicinity of the ModMax black hole: ModMax is a nonlinear electrodynamics theory with the same symmetries as
Maxwell electrodynamics. Static spherically symmetric solutions have been
derived by coupling ModMax electrodynamics with the Einstein equations, which
can represent a black hole. In this paper, we analyze light propagation in the
vicinity of the ModMax black hole. We determine birefringence, light
trajectories, deflection, redshifts, as well as the shadow of the black hole
using the effective or optical metric to determine the optical paths of light;
comparison is done with the corresponding effects in the neighborhood of the
Reissner-Nordstrom black hole, that is the solution to the Einstein-Maxwell
equations. | gr-qc |
No New Symmetries of the Vacuum Einstein Equations: In this note we examine some recently proposed solutions of the linearized
vacuum Einstein equations. We show that such solutions are {\it not} symmetries
of the Einstein equations, because of a crucial integrability condition. | gr-qc |
Constraining the mass of the graviton with the planetary ephemeris INPOP: We use the planetary ephemeris INPOP17b to constrain the mass of the graviton
in the Newtonian limit. We also give an interpretation of this result for a
specific case of fifth force framework. We find that the residuals for the
Cassini spacecraft significantly (90\% C.L.) degrade for Compton wavelengths of
the graviton smaller than $1.83\times 10^{13}$ km, corresponding to a graviton
mass bigger than $6.76\times 10^{-23} eV/c^2$. This limit is comparable in
magnitude to the one obtained by the LIGO-Virgo collaboration in the radiative
regime. We also use this specific example to illustrate why constraints on
alternative theories of gravity obtained from postfit residuals are generically
overestimated. | gr-qc |
Turning big bang into big bounce: II. Quantum dynamics: We analyze the big bounce transition of the quantum FRW model in the setting
of the nonstandard loop quantum cosmology (LQC). Elementary observables are
used to quantize composite observables. The spectrum of the energy density
operator is bounded and continuous. The spectrum of the volume operator is
bounded from below and discrete. It has equally distant levels defining a
quantum of the volume. The discreteness may imply a foamy structure of
spacetime at semiclassical level which may be detected in astro-cosmo
observations. The nonstandard LQC method has a free parameter that should be
fixed in some way to specify the big bounce transition. | gr-qc |
Loss of Quantum Coherence and Positivity of Energy Density in
Semiclassical Quantum Gravity: In the semiclassical quantum gravity derived from the Wheeler-DeWitt
equation, the energy density of a matter field loses quantum coherence due to
the induced gauge potential from the parametric interaction with gravity in a
non-static spacetime. It is further shown that the energy density takes only
positive values and makes superposition principle hold true. By studying a
minimal massive scalar field in a FRW spacetime background, we illustrate the
positivity of energy density and obtain the classical Hamiltonian of a complex
field from the energy density in coherent states. | gr-qc |
Marginally Outer Trapped Tori in Black Hole Spacetimes: During a binary black hole merger, multiple intermediary marginally outer
trapped tubes connect the initial pair of apparent horizons with the final
(single) apparent horizon. The marginally outer trapped surfaces (MOTSs) that
foliate these tubes can have complicated geometries as well as non-spherical
topologies. In particular, toroidal MOTSs form inside both of the original
black holes during the early stages of a head-on merger that starts from
time-symmetric initial data [1]. We show that toroidal MOTSs also form in the
maximal analytic extension of the Schwarzschild spacetime as Kruskal time
advances from the $T=0$ moment of time symmetry. As for the merger simulations,
they cross the Einstein-Rosen bridge and are tightly sandwiched between the
apparent horizons in the two asymptotic regions at early times. This strongly
suggests that their formation is a consequence of the initial conditions rather
than merger physics. Finally, we consider MOTSs of spherical topology in the
Kruskal-Szekeres slicing and study their properties. All of these are contained
within the apparent horizon but some do not enclose the wormhole. | gr-qc |
Interplay between curvature and Planck-scale effects in astrophysics and
cosmology: Several recent studies have considered the implications for astrophysics and
cosmology of some possible nonclassical properties of spacetime at the Planck
scale. The new effects, such as a Planck-scale-modified energy-momentum
(dispersion) relation, are often inferred from the analysis of some quantum
versions of Minkowski spacetime, and therefore the relevant estimates depend
heavily on the assumption that there could not be significant interplay between
Planck-scale and curvature effects. We here scrutinize this assumption, using
as guidance a quantum version of de Sitter spacetime with known Inonu-Wigner
contraction to a quantum Minkowski spacetime. And we show that, contrary to
common (but unsupported) beliefs, the interplay between Planck-scale and
curvature effects can be significant. Within our illustrative example, in the
Minkowski limit the quantum-geometry deformation parameter is indeed given by
the Planck scale, while in the de Sitter picture the parameter of quantization
of geometry depends both on the Planck scale and the curvature scalar. For the
much-studied case of Planck-scale effects that intervene in the observation of
gamma-ray bursts we can estimate the implications of "quantum spacetime
curvature" within robust simplifying assumptions. For cosmology at the present
stage of the development of the relevant mathematics one cannot go beyond
semiheuristic reasoning, and we here propose a candidate approximate
description of a quantum FRW geometry, obtained by patching together pieces
(with different spacetime curvature) of our quantum de Sitter. This
semiheuristic picture, in spite of its limitations, provides rather robust
evidence that in the early Universe the interplay between Planck-scale and
curvature effects could have been particularly significant. | gr-qc |
Observation of critical phenomena and self-similarity in the
gravitational collapse of radiation fluid: We observe critical phenomena in spherical collapse of radiation fluid. A
sequence of spacetimes $\cal{S}[\eta]$ is numerically computed, containing
models ($\eta\ll 1$) that adiabatically disperse and models ($\eta\gg 1$) that
form a black hole. Near the critical point ($\eta_c$), evolutions develop a
self-similar region within which collapse is balanced by a strong,
inward-moving rarefaction wave that holds $m(r)/r$ constant as a function of a
self-similar coordinate $\xi$. The self-similar solution is known and we show
near-critical evolutions asymptotically approaching it. A critical exponent
$\beta \simeq 0.36$ is found for supercritical ($\eta>\eta_c$) models. | gr-qc |
Cosmological Hyperfluids, Torsion and Non-metricity: We develop a novel model for Cosmological Hyperfluids, that is fluids with
intrinsic hypermomentum that induce spacetime torsion and non-metricity.
Imposing the Cosmological Principle to Metric-Affine Spaces, we present the
most general covariant form of the hypermomentum tensor in an FLRW Universe
along with its conservation laws and therefore construct a novel hyperfluid
model for Cosmological purposes. Extending the previous model of the
unconstrained hyperfluid in a Cosmological setting we establish the
conservation laws for energy-momentum and hypermomentum and therefore provide
the complete Cosmological setup to study non-Riemannian effects in Cosmology.
With the help of this we find the forms of torsion and non-metricity that were
earlier reported in the literature and also obtain the most general form of the
Friedmann equations with torsion and non-metricity. We also discuss some
applications of our model, make contact with the known results in the
literature and point to future directions. | gr-qc |
Characterization of (asymptotically) Kerr-de Sitter-like spacetimes at
null infinity: We investigate solutions $(\mathcal{M}, g)$ to Einstein's vacuum field
equations with positive cosmological constant $\Lambda$ which admit a smooth
past null infinity $\mathcal{J}^-$ \`a la Penrose and a Killing vector field
whose associated Mars-Simon tensor (MST) vanishes. The main purpose of this
work is to provide a characterization of these spacetimes in terms of their
Cauchy data on $\mathcal{J}^-$.
Along the way, we also study spacetimes for which the MST does not vanish. In
that case there is an ambiguity in its definition which is captured by a scalar
function $Q$. We analyze properties of the MST for different choices of $Q$. In
doing so, we are led to a definition of "asymptotically Kerr-de Sitter-like
spacetimes", which we also characterize in terms of their asymptotic data on
$\mathcal{J}^-$. | gr-qc |
Retrograde Polish Doughnuts around Boson Stars: We investigate Polish doughnuts with a uniform constant specific angular
momentum distribution in the space-times of rotating boson stars. In such
space-times thick tori can exhibit unique features not present in Kerr
space-times. For instance, in the context of retrograde tori, they may possess
two centers connected or not by a cusp. Rotating boson stars also feature a
static ring, neither present in Kerr space-times. This static ring consists of
static orbits, where particles are at rest with respect to a zero angular
momentum observer at infinity. Here we show that the presence of a static ring
allows for an associated static surface of a retrograde thick torus, where
inside the static surface the fluid moves in prograde direction. We classify
the retrograde Polish doughnuts and present several specific examples. | gr-qc |
Synchronization Gauges and the Principles of Special Relativity: The axiomatic bases of Special Relativity Theory (SRT) are thoroughly
re-examined from an operational point of view, with particular emphasis on the
status of Einstein synchronization in the light of the possibility of arbitrary
synchronization procedures in inertial reference frames. Once correctly and
explicitly phrased, the principles of SRT allow for a wide range of `theories'
that differ from the standard SRT only for the difference in the chosen
synchronization procedures, but are wholly equivalent to SRT in predicting
empirical facts. This results in the introduction, in the full background of
SRT, of a suitable synchronization gauge. A complete hierarchy of
synchronization gauges is introduced and elucidated, ranging from the useful
Selleri synchronization gauge (which should lead, according to Selleri, to a
multiplicity of theories alternative to SRT) to the more general Mansouri-Sexl
synchronization gauge and, finally, to the even more general
Anderson-Vetharaniam-Stedman's synchronization gauge. It is showed that all
these gauges do not challenge the SRT, as claimed by Selleri, but simply lead
to a number of formalisms which leave the geometrical structure of Minkowski
spacetime unchanged. Several aspects of fundamental and applied interest
related to the conventional aspect of the synchronization choice are discussed,
encompassing the issue of the one-way velocity of light on inertial and
rotating reference frames, the GPS's working, and the recasting of Maxwell
equations in generic synchronizations. Finally, it is showed how the gauge
freedom introduced in SRT can be exploited in order to give a clear explanation
of the Sagnac effect for counter-propagating matter beams. | gr-qc |
Time-domain inspiral templates for spinning compact binaries in
quasi-circular orbits described by their orbital angular momenta: We present a prescription to compute the time-domain gravitational wave (GW)
polarization states associated with spinning compact binaries inspiraling along
quasi-circular orbits. We invoke the orbital angular momentum $\vek L$ rather
than its Newtonian counterpart $\vek L_{\rm N}$ to describe the binary orbits
while the two spin vectors are freely specified in an inertial frame associated
with the initial direction of the total angular momentum. We show that the use
of $\vek L$ to describe the orbits leads to additional 1.5PN order amplitude
contributions to the two GW polarization states compared to the $\vek L_{\rm
N}$-based approach and discuss few implications of our approach. Further, we
provide a plausible prescription for GW phasing based on few theoretical
considerations and which may be treated as the natural circular limit to GW
phasing for spinning compact binaries in inspiraling eccentric orbits
[Gopakumar A and Sch{\"a}fer G 2011 {\em Phys. Rev. D} {\bf 84} 124007]. | gr-qc |
Role of pressure anisotropy on relativistic compact stars: We investigate a compact spherically symmetric relativistic body with
anisotropic particle pressure profiles. The distribution possesses
characteristics relevant to modeling compact stars within the framework of
general relativity. For this purpose, we consider a spatial metric potential of
Korkina and Orlyanskii [Ukr. Phys. J. 36, 885 (1991)] type in order to solve
the Einstein field equations. An additional prescription we make is that the
pressure anisotropy parameter takes the functional form proposed by Lake [Phys.
Rev. D 67, 104015 (2003)]. Specifying these two geometric quantities allows for
further analysis to be carried out in determining unknown constants and
obtaining a limit of the mass-radius diagram, which adequately describes
compact strange star candidates like Her X-1 and SMC X-1. Using the anisotropic
Tolman-Oppenheimer-Volkoff equations, we explore the hydrostatic equilibrium
and the stability of such compact objects. Then, we investigate other physical
features of this models, such as the energy conditions, speeds of sound and
compactness of the star in detail and show that our results satisfy all the
required elementary conditions for a physically acceptable stellar model. The
results obtained are useful in analyzing the stability of other anisotropic
compact objects like white dwarfs, neutron stars, and gravastars. | gr-qc |
Semiclassical limit problems with concurrent use of several clocks in
quantum cosmology: We revisit a recent proposal for a definition of time in quantum cosmology,
to investigate the effects of having more than one possible type of clock "at
the same time". We use as test tube an extension of Einstein gravity with a
massless scalar field in which the gravitational coupling $G_N$ is only a
constant on-shell, mimicking the procedure for $\Lambda$ in unimodular gravity.
Hence we have two "simultaneous" clocks in the theory: a scalar field clock,
and the conjugate of $G_N$. We find that attempts to use two coherent clocks
concurrently are disastrous for recovering the classical limit. The Heisenberg
relations, instead of being saturated, are always realized abundantly above
their bound, with strong quantum effects expected at least in parts of the
trajectory. Semi-classical states always result from situations where we
effectively impose a single clock, either by making the other clock a failed
clock (i.e. by choosing a state where its conjugate constant is infinitely
sharp), or by choosing a basis of constants where all clocks but one are
redundant, i.e. motion or change in phase space does not occur with the passing
of their "times". We show how this conclusion generalizes to fluids with any
equation of state. It also applies to systems where "sub-clocks" of the same
type could be used, for example in mixtures of different fluids with the same
equation of state. | gr-qc |
MATTERS OF GRAVITY, a newsletter for the gravity community, Number 3: Table of contents
Editorial
Correspondents
Gravity News:
Open Letter to gravitational physicists, Beverly Berger
A Missouri relativist in King Gustav's Court, Clifford Will
Gary Horowitz wins the Xanthopoulos award, Abhay Ashtekar
Research briefs:
Gamma-ray bursts and their possible cosmological implications, Peter Meszaros
Current activity and results in laboratory gravity, Riley Newman
Update on representations of quantum gravity, Donald Marolf
Ligo project report: December 1993, Rochus E. Vogt
Dark matter or new gravity?, Richard Hammond
Conference reports:
Gravitational waves from coalescing compact binaries, Curt Cutler
Mach's principle: from Newton's bucket to quantum gravity, Dieter Brill
Cornelius Lanczos international centenary conference, David Brown
Third Midwest relativity conference, David Garfinkle | gr-qc |
Nonperturbative analysis of the evolution of cosmological perturbations
through a nonsingular bounce: In bouncing cosmology, the primordial fluctuations are generated in a cosmic
contraction phase before the bounce into the current expansion phase. For a
nonsingular bounce, curvature and anisotropy grow rapidly during the bouncing
phase, raising questions about the reliability of perturbative analysis. In
this paper, we study the evolution of adiabatic perturbations in a nonsingular
bounce by nonperturbative methods including numerical simulations of the
nonsingular bounce and the covariant formalism for calculating nonlinear
perturbations. We show that the bounce is disrupted in regions of the universe
with significant inhomogeneity and anisotropy over the background energy
density, but is achieved in regions that are relatively homogeneous and
isotropic. Sufficiently small perturbations, consistent with observational
constraints, can pass through the nonsingular bounce with negligible alteration
from nonlinearity. We follow scale invariant perturbations generated in a
matter-like contraction phase through the bounce. Their amplitude in the
expansion phase is determined by the growing mode in the contraction phase, and
the scale invariance is well preserved across the bounce. | gr-qc |
Testing gravitational waveform models using angular momentum: The anticipated enhancements in detector sensitivity and the corresponding
increase in the number of gravitational wave detections will make it possible
to estimate parameters of compact binaries with greater accuracy assuming
general relativity(GR), and also to carry out sharper tests of GR itself.
Crucial to these procedures are accurate gravitational waveform models. The
systematic errors of the models must stay below statistical errors to prevent
biases in parameter estimation and to carry out meaningful tests of GR.
Comparisons of the models against numerical relativity (NR) waveforms provide
an excellent measure of systematic errors. A complementary approach is to use
balance laws provided by Einstein's equations to measure faithfulness of a
candidate waveform against exact GR. Each balance law focuses on a physical
observable and measures the accuracy of the candidate waveform vis a vis that
observable. Therefore, this analysis can provide new physical insights into
sources of errors. In this paper we focus on the angular momentum balance law,
using post-Newtonian theory to calculate the initial angular momentum,
surrogate fits to obtain the remnant spin and waveforms from models to
calculate the flux. The consistency check provided by the angular momentum
balance law brings out the marked improvement in the passage from
\texttt{IMRPhenomPv2} to \texttt{IMRPhenomXPHM} and from \texttt{SEOBNRv3} to
\texttt{SEOBNRv4PHM} and shows that the most recent versions agree quite well
with exact GR. For precessing systems, on the other hand, we find that there is
room for further improvement, especially for the Phenom models. | gr-qc |
Regular decoupling sector and exterior solutions in the context of MGD: We implement the Gravitational Decoupling through the Minimal Geometric
Deformation method and explore its effect on exterior solutions by imposing a
regularity condition in the Tolman--Oppenheimer--Volkoff equation of the
decoupling sector. We obtain that the decoupling function can be expressed
formally in terms of an integral involving the $g_{tt}$ component of the metric
of the seed solution. As a particular example, we implement the method by using
the Schwarzschild exterior as a seed and we obtain that the asymptotic behavior
of the extended geometry corresponds to a manifold with constant curvature. | gr-qc |
Weak Cosmic Censorship Conjecture in Kerr-(Anti-)de Sitter Black Hole
with Scalar Field: We investigate the weak cosmic censorship conjecture in Kerr-(anti-)de Sitter
black holes under the scattering of a scalar field. We test the conjecture in
terms of whether the black hole can exceed the extremal condition with respect
to its change caused by the energy and angular momentum fluxes of the scalar
field. Without imposing the laws of thermodynamics, we prove that the
conjecture is valid in all the initial states of the black hole (non-extremal,
near-extremal, and extremal black holes). The validity in the case of the
near-extremal black hole is different from the results of similar tests
conducted by adding a particle because the fluxes represent the energy and
angular momentum transferred to the black hole during the time interval not
included in the tests involving the particle. Using the time interval, we show
that the angular velocity of the black hole with the scalar field of a constant
state takes a long time for saturation to the frequency of the scalar field. | gr-qc |
Gravitationally Collapsing Shells in (2+1) Dimensions: We study gravitationally collapsing models of pressureless dust, fluids with
pressure, and the generalized Chaplygin gas (GCG) shell in (2+1)-dimensional
spacetimes. Various collapse scenarios are investigated under a variety of the
background configurations such as anti-de Sitter(AdS) black hole, de Sitter
(dS) space, flat and AdS space with a conical deficit. As with the case of a
disk of dust, we find that the collapse of a dust shell coincides with the
Oppenheimer-Snyder type collapse to a black hole provided the initial density
is sufficiently large. We also find -- for all types of shell -- that collapse
to a naked singularity is possible under a broad variety of initial conditions.
For shells with pressure this singularity can occur for a finite radius of the
shell. We also find that GCG shells exhibit diverse collapse scenarios, which
can be easily demonstrated by an effective potential analysis. | gr-qc |
Gravi-Weak Unification and the Black-Hole-Hedgehog's Solution with
Magnetic Field Contribution: In the present paper, we investigated the gravitational black-hole-hedgehog's
solution with magnetic field contribution in the framework of the f(R)--gravity
described by the Gravi-Weak unification model. Assuming the Multiple Point
Principle (MPP), we considered the existence of the two degenerate vacua of the
Universe: the first Electroweak (EW) vacuum with $v_1 \approx 246$ GeV ("true
vacuum"), and the second Planck scale ("false vacuum") with $v_2 \sim 10^{18}$
GeV. In these vacua, we investigated different topological defects. The main
aim of this paper is an investigation of the black-hole-hedgehog configurations
as defects of the "false vacuum". We have obtained the solution which
corresponds to a global monopole, that has been "swallowed" by the black-hole
with core mass $M_{BH}\approx 3.65\times 10^{18}\,\, {\rm{GeV}}$ and radius
$\delta \approx 6\cdot 10^{-21} {\rm{GeV}}^{-1}.$ We investigated the metric in
the vicinity of the black-hole-hedgehog and estimated its horizon radius:
$r_h\approx 1.14 \delta$. We have considered the phase transition from the
"false vacuum" to the "true vacuum" and confirmed the stability of the
EW--vacuum. | gr-qc |
Photon rockets and gravitational radiation: The absence of gravitational radiation in Kinnersley's ``photon rocket''
solution of Einstein's equations is clarified by studying the mathematically
well-defined problem of point-like photon rockets in Minkowski space (i.e.
massive particles emitting null fluid anisotro\-pically and accelerating
because of the recoil). We explicitly compute the (uniquely defined) {\it
linearized} retarded gravitational waves emitted by such objects, which are the
coherent superposition of the gravitational waves generated by the motion of
the massive point-like rocket and of those generated by the energy-momentum
distribution of the photon fluid. In the special case (corresponding to
Kinnersley's solution) where the anisotropy of the photon emission is purely
dipolar we find that the gravitational wave amplitude generated by the
energy-momentum of the photons exactly cancels the usual $1/r$ gravitational
wave amplitude generated by the accelerated motion of the rocket. More general
photon anisotropies would, however, generate genuine gravitational radiation at
infinity. Our explicit calculations show the compatibility between the
non-radiative character of Kinnersley's solution and the currently used
gravitational wave generation formalisms based on post-Minkowskian perturbation
theory. | gr-qc |
Two-body problem in Scalar-Tensor theories as a deformation of General
Relativity : an Effective-One-Body approach: In this paper we address the two-body problem in massless Scalar-Tensor (ST)
theories within an Effective-One-Body (EOB) framework. We focus on the first
building block of the EOB approach, that is, mapping the conservative part of
the two-body dynamics onto the geodesic motion of a test particle in an
effective external metric. To this end, we first deduce the second
post-Keplerian (2PK) Hamiltonian of the two-body problem from the known 2PK
Lagrangian. We then build, by means of a canonical transformation a
ST-deformation of the general relativistic EOB Hamiltonian which allows to
incorporate the Scalar-Tensor (2PK) corrections to the currently best available
General Relativity EOB results. This EOB-ST Hamiltonian defines a resummation
of the dynamics that may provide information on the strong-field regime, in
particular, the ISCO location and associated orbital frequency and can be
compared to other, e.g. tidal, corrections. | gr-qc |
Study of gravastars under $f(\mathbb{T})$ gravity: In the present paper we propose a stellar model under the $f(\mathbb{T})$
gravity following the conjecture of Mazur-Mottola~[Report number: LA-UR-01-5067
(2001); Proc. Natl. Acad. Sci. USA 101 (2004) 9545] known in literature as {\it
gravastar}, a viable alternative to the black hole. This gravastar has three
different regions, viz., (A) Interior core region, (B) Intermediate thin shell,
and (C) Exterior spherical region. It is assumed that in the interior region
the fluid pressure is equal to a negative matter-energy density providing a
constant repulsive force over the spherical thin shell. This shell at the
intermediate region is assumed to be formed by a fluid of ultrarelativistic
plasma and the pressure, which is directly proportional to the matter-energy
density according to Zeldovich's conjecture of stiff fluid~[Zeldovich, Mon.
Not. R. Astron. Soc. 160 (1972) 1], does nullify the repulsive force exerted by
the interior core region for a stable configuration. On the other hand, the
exterior spherical region can be described by the exterior Schwarzschild-de
Sitter solution. With all these specifications we have found out a set of exact
and singularity-free solutions of the gravastar which presents several
physically interesting as well as valid features within the framework of
alternative gravity. | gr-qc |
A solution of the Einstein-Maxwell equations describing conformally flat
spacetime outside a charged domain wall: We derive and discuss the physical interpretation of a conformally flat,
static solution of the Einstein-Maxwell equations. It is argued that it
describes a conformally flat, static spacetime outside a charged spherically
symmetric domain wall. The acceleration of gravity is directed away from the
wall in spite of the positive gravitational mass of the electric field outside
the wall, as given by the Tolman-Whittaker expression. The reason for the
repulsive gravitation is the strain of the wall which is calculated using the
Israel formalism for singular surfaces. | gr-qc |
Nonlinear and quantum effects in analogue gravity: The present thesis deals with some properties of classical and quantum scalar
fields in an inhomogeneous and/or time-dependent background, focusing on models
where the latter can be described as a curved space-time with an event horizon.
While naturally formulated in a gravitational context, such models extend to
many physical systems with an effective Lorentz invariance at low energy. We
shall see how this effective symmetry allows one to relate the behavior of
perturbations in these systems to black-hole physics, what are its limitations,
and in which sense results thus obtained are "analogous" to their general
relativistic counterparts. The first chapter serves as a general introduction.
A few notions from Einstein's theory of gravity are introduced and a derivation
of Hawking radiation is sketched. The correspondence with low-energy systems is
then explained through three important examples. The next four chapters each
details one of the works completed during this thesis, updated and slightly
reorganized to account for new developments which occurred after their
publication. The other articles I contributed to are summarized in the last
chapter, before the general conclusion. | gr-qc |
Mixed Elliptic and Hyperbolic Systems for the Einstein Equations: We analyse the mathematical underpinnings of a mixed hyperbolic-elliptic form
of the Einstein equations of motion in which the lapse function is determined
by specified mean curvature and the shift is arbitrary. We also examine a new
recently-published first order symmetric hyperbolic form of the equations of
motion. This paper is dedicated to Andre Lichnerowicz on the occasion of his
80th birthday and will appear in a volume edited by G. Ferrarese. | gr-qc |
The characteristic initial value problem for colliding plane waves: The
linear case: The physical situation of the collision and subsequent interaction of plane
gravitational waves in a Minkowski background gives rise to a well-posed
characteristic initial value problem in which initial data are specified on the
two null characteristics that define the wavefronts. In this paper, we analyse
how the Abel transform method can be used in practice to solve this problem for
the linear case in which the polarization of the two gravitational waves is
constant and aligned. We show how the method works for some known solutions,
where problems arise in other cases, and how the problem can always be solved
in terms of an infinite series if the spectral functions for the initial data
can be evaluated explicitly. | gr-qc |
Classification of image distortions in terms of Petrov types: An observer surrounded by sufficiently small spherical light sources at a
fixed distance will see a pattern of elliptical images distributed over the
sky, owing to the distortion effect (shearing effect) of the spacetime geometry
upon light bundles. In lowest non-trivial order with respect to the distance,
this pattern is completely determined by the conformal curvature tensor (Weyl
tensor) at the observation event. In this paper we derive formulas that allow
to calculate these distortion patterns in terms of the Newman-Penrose
formalism. Then we represent the distortion patterns graphically for all Petrov
types, and we discuss their dependence on the velocity of the observer. | gr-qc |
Integration of Einstein's Equations in the Weak Field Domain Using the
"Einstein" Gauge: We propose a new alternative gauge for the Einstein equations instead of the
de Donder gauge, which allows in the limit of weak fields a straightforward
integration of these equations. The Newtonian potential plays a new interesting
role in this framework. The calculations are demonstrated explicitely for 2
simple astrophysical models. | gr-qc |
Quantum corrections to the thermodynamics and phase transition of a
black hole surrounded by a cavity in the extended phase space: In the extended phase space, we investigate the rainbow gravity-corrected
thermodynamic phenomena and phase structure of the Schwarzschild black hole
surrounded by a spherical cavity. The results show that rainbow gravity has a
very significant effect on the thermodynamic phenomena and phase structure of
the black hole. It prevents the black hole from total evaporation and leads to
a remnant with a limited temperature but no mass. Additionally, we restore the
$P-V$ criticality and obtaine the critical quantities of the canonical
ensemble. When the temperature or pressure is smaller than the critical
quantities, the system undergoes two Hawking-Page-like phase transitions and
one first-order phase transition, which never occurs in the original case.
Remarkably, our findings demonstrate that the thermodynamic behavior and phase
transition of the rainbow SC black hole surrounded by a cavity in the extended
phase space are analogous to those of the Reissner-Nordstr\"{o}m anti-de Sitter
black hole. Therefore, rainbow gravity activates the effect of electric charge
and cutoff factor in the evolution of the black hole. | gr-qc |
Modified FRW cosmologies arising from states of the hybrid quantum Gowdy
model: We construct approximate solutions of the hybrid quantum Gowdy cosmology with
three-torus topology, linear polarization, and local rotational symmetry, in
the presence of a massless scalar field. More specifically, we determine some
families of states for which the complicated inhomogeneous and anisotropic
Hamiltonian constraint operator of the Gowdy model is approximated by a much
simpler one. Our quantum states follow the dynamics governed by this simpler
constraint, while being at the same time also approximate solutions of the full
Gowdy model. This is so thanks to the quantum correlations that the considered
states present between the isotropic and anisotropic sectors of the model.
Remarkably, this simpler constraint can be regarded as that of a flat
Friedmann-Robertson-Walker universe filled with different kinds of perfect
fluids and geometrically corrected by homogeneous and isotropic curvature-like
terms. Therefore, our quantum states, which are intrinsically inhomogeneous,
admit approximate homogeneous and isotropic effective descriptions similar to
those considered in modified theories of gravity. | gr-qc |
Lorentz-breaking Theory and Tunneling Radiation Correction to
Vaidya-Banner de Sitter Black Hole: In Vaidya-Bonner de Sitter Black hole space-time, the tunneling radiation
characteristics of fermions and bosons are corrected by taking Lorentz symmetry
breaking theory into account. The corresponding gamma matrices and ether-like
field vectors of the black hole are constructed, then the new modified form of
Dirac equation for the fermion with spin 1/2 and the new modified form of
Klein-Gordon equation for boson in the curved space-time of the black hole are
obtained. Through solving the two equations, new and corrected expressions of
surface gravity, Hawking temperature and tunneling rate of the black hole are
obtained, and the results obtained are also discussed. | gr-qc |
Energy and entropy in the Geometrical Trinity of gravity: All energy is gravitational energy. That is the consequence of the
equivalence principle, according to which gravity is the universal interaction.
The physical charges of this interaction have remained undisclosed, but the
Adventof the Geometrical Trinity opened a new approach to this foundational
problem. Here it is shown to provide a background-independent unification of
the previous, non-covariant approaches of Bergmann-Thomson, Cooperstock,
Einstein, von Freud, Landau-Lifshitz, Papapetrou and Weinberg. First, the
Noether currents are derived for a generic Palatini theory of gravity coupled
with generic matter fields, and then the canonical i.e. the unique charges are
robustly derived and analysed, particularly in the metric teleparallel and the
symmetric teleparallel versions of General Relativity. These results, and their
application to black holes and gravitational waves, are new. | gr-qc |
The relation between F(R) gravity and Einstein-conformally invariant
Maxwell source: In this paper, we consider the special case of $F(R)$ gravity, in which
$F(R)= R^{N}$ and obtain its topological black hole solutions in higher
dimensions. We show that, the same as higher dimensional charged black hole,
these solutions may be interpreted as black hole solutions with two event
horizons, extreme black hole and naked singularity provided the parameters of
the solutions are chosen suitably. But, the presented black hole is different
from the standard higher-dimensional Reissner-Nordstrom solutions. Next, we
focus on the conformally invariant Maxwell field coupled to Einstein gravity
and discuss about its black hole solutions. Comparing these two class of
solutions, shows that there is a correspondence between the
Einstein-conformally invariant Maxwell solutions and the solutions of $F(R)$
gravity without matter field in arbitrary dimensions. | gr-qc |
Brans-Dicke Wormhole Revisited -- II: In a recent paper [arXiv:0910.1109], a wormhole range in the Jordan frame,
-3/2<{\omega}<-4/3 for the vacuum Brans-Dicke Class I solution was derived. On
general grounds and under certain conditions, it is shown in a theorem that
static wormhole solutions in the scalar-tensor theory are not possible. We
agree with the conclusion within its framework but report that a
singularity-free wormhole can be obtained from Class I solution by performing
certain operations on it, a fact possibly not yet widely known. The transformed
solution is regular everywhere, produces a wormhole with two asymtotically flat
regions for a revised new range -2<{\omega}<-3/2, together with a wormhole
analogue (of Horowitz-Ross naked black hole) that we discovered earlier. This
new range lies in the ghost regime in the Einstein frame consistent with the
theorem. We further conclude that there is a fixed point at {\omega}=-3/2, the
values {\omega}>-3/2 correspond to singular wormholes, while values
{\omega}<-3/2 correspond to singularity-free wormholes. | gr-qc |
A simple toy model for a unified picture of dark energy, dark matter,
and inflation: A specific scale factor in Robertson-Walker metric with the prospect of
giving the overall cosmic history in a unified picture roughly is considered.
The corresponding energy-momentum tensor is identified as that of two scalar
fields where one plays the roles of both inflaton and dark matter while the
other accounts for dark energy. A preliminary phenomenological analysis gives
an order of magnitude agreement with observational data. The resulting picture
may be considered as a first step towards a single model for all epochs of
cosmic evolution. | gr-qc |
Compact Anisotropic Models in General Relativity by Gravitational
Decoupling: Durgapal's fifth isotropic solution describing spherically symmetric and
static matter distribution is extended to an anisotropic scenario. To do so we
employ the gravitational decoupling through the minimal geometric deformation
scheme. This approach allows to split Einstein's field equations in two simply
set of equations, one corresponding to the isotropic sector and other to the
anisotropic sector described by an extra gravitational source. The isotropic
sector is solved by the Dugarpal's model and the anisotropic sector is solved
once a suitable election on the minimal geometric deformation is imposes. The
obtained model is representing some strange stars candidates and fulfill all
the requirements in order to be a well behaved physical solution to the
Einstein's field equations. | gr-qc |
Electron in higher-dimensional weakly charged rotating black hole
spacetimes: We demonstrate separability of the Dirac equation in weakly charged rotating
black hole spacetimes in all dimensions. The electromagnetic field of the black
hole is described by a test field approximation, with vector potential
proportional to the primary Killing vector field. It is shown that the
demonstrated separability can be intrinsically characterized by the existence
of a complete set of mutually commuting first order symmetry operators
generated from the principal Killing-Yano tensor. The presented results
generalize the results on integrability of charged particle motion and
separability of charged scalar field studied in [1]. | gr-qc |
Field redefinitions, Weyl invariance, and nature of mavericks: In the theories of gravity with non-minimally coupled scalar fields there are
"mavericks" -- unexpected solutions with odd properties, e.g., black holes with
scalar hair in theories with scalar potential bounded from below. Probably the
most famous example is Bocharova-Bronnikov-Melnikov-Bekenstein (BBMB) black
hole solution in a theory with a scalar field conformally coupled to the
gravity and with vanishing potential. Its existence naively violates no-hair
conjecture without violating no-hair theorems because of the singular behavior
of the scalar field at the horizon. Despite being discovered more than 40 years
ago, nature of BBMB solution is still the subject of research and debate. We
argue here that the key in understanding nature of maverick solutions is the
proper choice of field redefinition schemes in which the solutions are regular.
It appears that in such "regular" schemes mavericks have different physical
interpretations, in particular they are not elementary but composite objects.
For example, BBMB solution is not an extremal black hole but a collection of a
wormhole and a naked singularity. In the process we show that Weyl-invariant
formulation of gravity is a perfect tool for such analyzes. | gr-qc |
Constant scalar curvature hypersurfaces in extended Schwarzschild
space-time: We present a class of spherically symmetric hypersurfaces in the
Kruskal extension of the Schwarzschild space-time. The hypersurfaces have
constant negative scalar curvature, so they are hyperboloidal in the regions of
space-time which are asymptotically flat. | gr-qc |
OPERATOR WEAK VALUES AND BLACK HOLE COMPLEMENTARITY: In conventional field theories, the emission of Hawking radiation in the
background of a collapsing star requires transplanckian energy fluctuations.
These fluctuations are encoded in the weak values of the energy-momentum
operator constructed from matrix elements between both -in and -out states. It
is argued that taming of these weak values by back-reaction may lead to
geometrical backgrounds which are also build from weak values of the
gravitational field operators. This leads to different causal histories of the
black hole as reconstructed by observers crossing the horizon at different
times but reduces, in accordance with the equivalence principle, to the
classical description of the collapse for the proper history of the star as
recorded by an observer comoving with it. For observers never crossing the
horizon, the evaporation would be interpreted within a topologically trivial
``achronon geometry" void of horizon and singularity: after the initial
ignition of the radiation from pair creation out of the vacuum of the
collapsing star of mass M, as in the conventional theory, the source of the
thermal radiation would shift gradually to the star itself in a time at least
of order $4M\ln 2M$. The burning of the star could be consistent with a quantum
unitary evolution along the lines suggested by 't Hooft. A provisional formal
expression of general black hole complementarity is proposed and its possible
relevance for testing features of a theory of quantum gravity is suggested. | gr-qc |
Deformed Weitzenböck Connections and Teleparallel Gravity: We study conditions on a generic connection written in terms of first-order
derivatives of the vielbein in order to obtain (possible) equivalent theories
to Einstein Gravity. We derive the equations of motion for these theories which
are based on the new connections. We recover the Teleparallel Gravity equations
of motion as a particular case. The analysis of this work might be useful to
Double Field Theory to find other connections determined in terms of the
physical fields. | gr-qc |
Backgrounds of squeezed relic photons and their spatial correlations: We discuss the production of multi-photons squeezed states induced by the
time variation of the (Abelian) gauge coupling constant in a string
cosmological context. Within a fully quantum mechanical approach we solve the
time evolution of the mean number of produced photons in terms of the squeezing
parameters and in terms of the gauge coupling. We compute the first (amplitude
interference) and second order (intensity interference) correlation functions
of the magnetic part of the photon background. The photons produced thanks to
the variation of the dilaton coupling are strongly bunched for the realistic
case where the growth of the dilaton coupling is required to explain the
presence of large scale magnetic fields and, possibly of a Faraday rotation of
the Cosmic Microwave Background. | gr-qc |
Hyperbolic slicings of spacetime: singularity avoidance and gauge shocks: I study the Bona-Masso family of hyperbolic slicing conditions, considering
in particular its properties when approaching two different types of
singularities: focusing singularities and gauge shocks. For focusing
singularities, I extend the original analysis of Bona et. al and show that both
marginal and strong singularity avoidance can be obtained for certain types of
behavior of the slicing condition as the lapse approaches zero. For the case of
gauge shocks, I re-derive a condition found previously that eliminates them.
Unfortunately, such a condition limits considerably the type of slicings
allowed. However, useful slicing conditions can still be found if one asks for
this condition to be satisfied only approximately. Such less restrictive
conditions include a particular member of the 1+log family, which in the past
has been found empirically to be extremely robust for both Brill wave and black
hole simulations. | gr-qc |
Nonlocal Gravity: Modification of Newtonian Gravitational Force in the
Solar System: Nonlocal gravity (NLG) is a classical nonlocal generalization of Einstein's
theory of gravitation developed in close analogy with the nonlocal
electrodynamics of media. It appears that the nonlocal aspect of the universal
gravitational interaction could simulate dark matter. Within the Newtonian
regime of NLG, we investigate the deviation of the gravitational force from the
Newtonian inverse square law as a consequence of the existence of the effective
dark matter. In particular, we work out the magnitude of this deviation in the
solar system out to 100 astronomical units. Moreover, we give an improved lower
limit for the short-range parameter of the reciprocal kernel of NLG. | gr-qc |
Existence of Spinorial States in Pure Loop Quantum Gravity: We demonstrate the existence of spinorial states in a theory of canonical
quantum gravity without matter. This should be regarded as evidence towards the
conjecture that bound states with particle properties appear in association
with spatial regions of non-trivial topology. In asymptotically trivial general
relativity the momentum constraint generates only a subgroup of the spatial
diffeomorphisms. The remaining diffeomorphisms give rise to the mapping class
group, which acts as a symmetry group on the phase space. This action induces a
unitary representation on the loop state space of the Ashtekar formalism.
Certain elements of the diffeomorphism group can be regarded as asymptotic
rotations of space relative to its surroundings. We construct states that
transform non-trivially under a $2\pi$-rotation: gravitational quantum states
with fractional spin. | gr-qc |
Anisotropic Cosmological Model with Variable G and Lambda: Anisotropic Bianchi-III cosmological model is investigated with variable
gravitational and cosmological constants in the framework of Einstein's general
relativity. The shear scalar is considered to be proportional to the expansion
scalar. The dynamics of the anisotropic universe with variable G and Lambda are
discussed. Without assuming any specific forms for Lambda and the metric
potentials, we have tried to extract the time variation of G and Lambda from
the anisotropic model. The extracted G and Lambda are in conformity with the
present day observation. Basing upon the observational limits, the behaviour
and range of the effective equation of state parameter are discussed. | gr-qc |
The case of Quantum Gravity with Spontaneous Collapse of the Wave
Function: When about half a century ago the concept of universal spontaneous collapse
of the wave function was conceived it was an attempt to alter standard
non-relativistic quantum physics. As such, it was largely ignored by
relativistic field theory and quantum gravity communities. A central motivation
of spontaneous collapse community has been to replace the standard
collapse-by-measurement that annoyed many. With few exceptions, it did not
annoy the field theory and quantum gravity communities. Concept of certain
general-relativity-related universal irreversibility in quantum field theory
had been initiated very long ago by Wheeler, Hawking and a few others
independently from the concept of spontaneous collapse. Lately the two concepts
started to converge and support each other. | gr-qc |
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