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Implications of a Time-Varying Fine Structure Constant: Much work has been done after the possibility of a fine structure constant
being time-varying. It has been taken as an indication of a time-varying speed
of light. Here we prove that this is not the case. We prove that the speed of
light may or may not vary with time, independently of the fine structure
constant being constant or not. Time variations of the speed of light, if
present, have to be derived by some other means and not from the fine structure
constant. No implications based on the possible variations of the fine
structure constant can be imposed on the speed of light. | gr-qc |
Transition to light-like trajectories in thin shell dynamics: It was recently shown that a massive thin shell that is sandwiched between a
flat interior and an exterior geometry given by the outgoing Vaidya metric
becomes null in a finite proper time. We investigate this transition for a
general spherically-symmetric metric outside the shell and find that it occurs
generically. Once the shell is null its persistence on a null trajectory can be
ensured by several mechanisms that we describe. Using the outgoing Vaidya
metric as an example we show that if a dust shell acquires surface pressure on
its transition to a null trajectory it can evade the Schwarzschild radius
through its collapse. Alternatively, the pressureless collapse may continue if
the exterior geometry acquires a more general form. | gr-qc |
Quantum gravity corrections to accretion onto a Schwarzschild black hole: Quantum gravity corrections to accretion onto a Schwarzschild black hole are
considered in the context of asymptotically safe scenario. The possible
positions of the critical points are discussed and the general conditions for
critical points are obtained. The explicit expressions for matter density
compression and temperature profile both below the critical radius and at the
event horizon are derived. For polytropic matter, we determine the corrected
temperature and the integrated flux resulting from quantum gravity effects at
the event horizon, which might be as a test of asymptotically safe scenario. | gr-qc |
Fermionic sectors for the Kodama state: Diffeomorphisms not connected to the identity can act nontrivially on the
quantum state space for gravity. However, in stark contrast to the case of
nonabelian Yang-Mills field theories, for which the quantum state space is
always in 1 dimensional representation of the large gauge transformations, the
quantum state space for gravity can have higher dimensional representations. In
particular, the Kodama state will have 2 dimensional representations, that is
sectors with spin 1/2, for many topologies that admit positive scalar
curvature. The existence of these spin 1/2 states are used to point out a
possible answer to certain criticisms raised recently in the literature. | gr-qc |
Soliton stars in the breather limit: This paper presents an asymptotic reduction of the Einstein-Klein-Gordon
system with real scalar field ("soliton star problem"). A periodic solution of
the reduced system, similar to the sine-Gordon breather, is obtained by a
variational method. This tallies with numerical computations. As a consequence,
a time-periodic redshift for sources close to the center of the star is
obtained. | gr-qc |
Gravity Sector of the SME: In this talk, the gravity sector of the effective field theory description of
local Lorentz violation is discussed, including minimal and nonminimal
curvature couplings. Also, recent experimental and observational analyses
including solar-system ephemeris and short-range gravity tests are reviewed. | gr-qc |
Cosmological future singularities in interacting dark energy models: The existence of interactions between dark matter and dark energy has been
widely studied, since they can fit well the observational data and may provide
new physics through such an interaction. In this work we analyze these models
and investigate their potential relation with future cosmological
singularities. We find that every future singularity found in the literature
can be mapped into a singularity of the interaction term, that we call
$Q$-singularity, where the energy flow between the dark components diverges.
Furthermore, this framework allows to identify a new type of future singularity
induced by the divergence of the first derivative of the dark energy equation
of state parameter. | gr-qc |
Signatures of Einstein-Maxwell dilaton-axion gravity from the observed
jet power and the radiative efficiency: The Einstein-Maxwell dilaton-axion (EMDA) gravity arises in the low energy
effective action of the heterotic string theory and provides a simple framework
to explore the signatures of the same. The dilaton and the axion fields
inherited in the action from string compactifications have interesting
consequences in inflationary cosmology and in explaining the present
accelerated expansion of the universe. It is therefore worthwhile to search for
the footprints of these fields in the available astrophysical observations.
Since Einstein gravity is expected to receive quantum corrections in the high
curvature domain, the near horizon regime of black holes seems to be the ideal
astrophysical laboratory to test these deviations from general relativity.
Exact, stationary and axisymmetric black hole solution in EMDA gravity
corresponds to the Kerr-Sen spacetime which carries dilaton charge, while the
angular momentum is sourced by the axion field. The ballistic jets and the peak
emission of the continuum spectrum from the accretion disk are believed to be
launched very close to the event horizon and hence should bear the imprints of
the background spacetime. We compute the jet power and the radiative efficiency
derived from the continuum spectrum in the Kerr-Sen background and compare them
with the corresponding observations of microquasars. Our analysis reveals that
Kerr black holes are more favored compared to Kerr-Sen black holes with dilaton
charges. | gr-qc |
Probabilistic bound on extreme fluctuations in isolated quantum systems: We ask to what extent an isolated quantum system can eventually "contract" to
be contained within a given Hilbert subspace. We do this by starting with an
initial random state, considering the probability that all the particles will
be measured in a fixed subspace, and maximizing this probability over all time.
This is relevant, for example, in a cosmological context, which may have access
to indefinite timescales. We find that when the subspace is much smaller than
the entire space, this maximal probability goes to $1/2$ for real initial wave
functions, and to $\pi^2/16$ when the initial wave function has been drawn from
a complex ensemble. For example when starting in a real generic state, the
chances of collapsing all particles into a small box will be less than but come
arbitrarily close to $50\%$. This contraction corresponds to an entropy
reduction by a factor of approximately two, thus bounding large downward
fluctuations in entropy from generic initial states. | gr-qc |
Shadows of rotating black holes in plasma environments with aberration
effects: The shadows of black holes encode significant information about the
properties of black holes and the spacetime surrounding them. So far, the
effects of dispersive media, such as plasma, and relativistic aberration on the
propagation of light around compact objects have been treated separately in the
literature. In this paper, we will employ the Konoplya, Stuchlik, and Zhidenko
family of stationary, axially symmetric, and asymptotically flat metrics to
describe the spacetime around rotating black holes. We will study how the
parameters of the black hole, the chromatic effects resulting from the presence
of a non-magnetized, pressureless plasma environment, and the effects of
relativistic aberration of a moving observer modify the morphology of the
shadow. | gr-qc |
Cosmological expansion governed by a scalar field from a 5D vacuum: We consider a single field governed expansion of the universe from a five
dimensional (5D) vacuum state. Under an appropiate change of variables the
universe can be viewed in a effective manner as expanding in 4D with an
effective equation of state which describes different epochs of its evolution.
In the example here worked the universe fistly describes an inflationary phase,
followed by a decelerated expansion. Thereafter, the universe is accelerated
and describes a quintessential expansion to finally, in the future, be vacuum
dominated. | gr-qc |
What actually happens when you approach a gravitational singularity?: Roger Penrose's 2020 Nobel Prize in Physics recognises that his
identification of the concepts of "gravitational singularity" and an
"incomplete, inextendible, null geodesic" is physically very important. The
existence of an incomplete, inextendible, null geodesic doesn't say much,
however, if anything, about curvature divergence, nor is it a helpful
definition for performing actual calculations. Physicists have long sought for
a coordinate independent method of defining where a singularity is located,
given an incomplete, inextendible, null geodesic, that also allows for standard
analytic techniques to be implemented. In this essay we present a solution to
this issue. It is now possible to give a concrete relationship between an
incomplete, inextendible, null geodesic and a gravitational singularity, and to
study any possible curvature divergence using standard techniques. | gr-qc |
Simple derivation of the Generalized Moller-Wu-Lee transformations.Born
rigid constant accelerated motion on a curved Lorentzian manifold: Simple derivation of the classical generalized Moller-Wu-Lee transformations
from general master equation is presented.We will argue that in fact we can
implement Born's notion of rigid motion in both flat spacetime and arbitrary
curved non-holonomic spacetimes containing classical and Colombeau's
distributional sources. | gr-qc |
Black Hole Information and Thermodynamics: This SpringerBrief is based on a masters course on black hole thermodynamics
and the black hole information problem taught by Dieter L\"ust during the
summer term 2017 at the Ludwig-Maximilians-Universit\"at in Munich; it was
written by Ward Vleeshouwers. It provides a short introduction to general
relativity, which describes gravity in terms of the curvature of space-time,
and examines the properties of black holes. These are central objects in
general relativity which arise when sufficient energy is compressed into a
finite volume, so that even light cannot escape its gravitational pull. We will
see that black holes exhibit a profound connection with thermodynamic systems.
Indeed, by quantizing a field theory on curved backgrounds, one can show that
black holes emit thermal (Hawking) radiation, so that the connection with
thermodynamics is more than a formal similarity. Hawking radiation gives rise
to an apparent conflict between general relativity and quantum mechanics known
as the black hole information problem. If a black hole formed from a pure
quantum state evaporates to form thermal radiation, which is in a mixed state,
then the unitarity postulate of quantum mechanics is violated. We will examine
the black hole information problem, which has plagued the physics community for
over four decades, and consider prominent examples of proposed solutions, in
particular, the string theoretical construction of the Tangherlini black hole,
and the infinite number of asymptotic symmetries given by BMS-transformations. | gr-qc |
Second post-Newtonian gravitational wave polarizations for compact
binaries in elliptical orbits: The second post-Newtonian (2PN) contribution to the `plus' and `cross'
gravitational wave polarizations associated with gravitational radiation from
non-spinning, compact binaries moving in elliptic orbits is computed. The
computation starts from our earlier results on 2PN generation, crucially
employs the 2PN accurate generalized quasi-Keplerian parametrization of
elliptic orbits by Damour, Sch\"afer and Wex and provides 2PN accurate
expressions modulo the tail terms for gravitational wave polarizations
incorporating effects of eccentricity and periastron precession. | gr-qc |
Bulk Entropy in Loop Quantum Gravity: In the framework of loop quantum gravity (LQG), having quantum black holes in
mind, we generalize the previous boundary state counting (gr-qc/0508085) to a
full bulk state counting. After a suitable gauge fixing we are able to compute
the bulk entropy of a bounded region (the "black hole") with fixed boundary.
This allows us to study the relationship between the entropy and the boundary
area in details and we identify a holographic regime of LQG where the leading
order of the entropy scales with the area. We show that in this regime we can
fine tune the factor between entropy and area without changing the Immirzi
parameter. | gr-qc |
On the evolution of a large class of inhomogeneous scalar field
cosmologies: The asymptotic behaviour of a family of inhomogeneous scalar field
cosmologies with exponential potential is studied. By introducing new variables
we can perform an almost complete analysis of the evolution of these
cosmologies. Unlike the homogeneous case (Bianchi type solutions), when k^2<2
the models do not isotropize due to the presence of the inhomogeneities | gr-qc |
Area evolution, bulk viscosity and entropy principles for dynamical
horizons: We derive from Einstein equation an evolution law for the area of a trapping
or dynamical horizon. The solutions to this differential equation show a causal
behavior. Moreover, in a viscous fluid analogy, the equation can be interpreted
as an energy balance law, yielding to a positive bulk viscosity. These two
features contrast with the event horizon case, where the non-causal evolution
of the area and the negative bulk viscosity require teleological boundary
conditions. This reflects the local character of trapping horizons as opposed
to event horizons. Interpreting the area as the entropy, we propose to use an
area/entropy evolution principle to select a unique dynamical horizon and time
slicing in the Cauchy evolution of an initial marginally trapped surface. | gr-qc |
On stability of a new model of wormhole: We investigate the stability of the wormhole of the
Mors-Thorne-Ellis-Bronnikov type. In our model the matter inside it consists of
a radial magnetic field and an ideal phantom-fluid. Properties of the matter
are described in section 1, 2 and 3 of this paper. We consider spherical
perturbations only and find examples of the stable wormholes against these
perturbations. | gr-qc |
Null Cones in Schwarzschild Geometry: Light cones of Schwarzschild geometry are studied in connection to the Null
Surface Formulation and gravitational lensing. The paper studies the light cone
cut function's singularity structure, gives exact gravitational lensing
equations, and shows that the "pseudo-Minkowski" coordinates are well defined
within the model considered. | gr-qc |
The virial theorem and the dynamics of clusters of galaxies in the brane
world models: A version of the virial theorem, which takes into account the effects of the
non-compact extra-dimensions, is derived in the framework of the brane world
models. In the braneworld scenario, the four dimensional effective Einstein
equation has some extra terms, called dark radiation and dark pressure,
respectively, which arise from the embedding of the 3-brane in the bulk. To
derive the generalized virial theorem we use a method based on the
collisionless Boltzmann equation. The dark radiation term generates an
equivalent mass term (the dark mass), which gives an effective contribution to
the gravitational energy. This term may account for the well-known virial
theorem mass discrepancy in actual clusters of galaxies. An approximate
solution of the vacuum field equations on the brane, corresponding to weak
gravitational fields, is also obtained, and the expressions for the dark
radiation and dark mass are derived. The qualitative behavior of the dark mass
is similar to that of the observed virial mass in clusters of galaxies. We
compare our model with the observational data for galaxy clusters, and we
express all the physical parameters of the model in terms of observable
quantities. In particular, we predict that the dark mass must extend far beyond
the presently considered virial radius. The behavior of the galaxy cluster
velocity dispersion in brane world models is also considered. Therefore the
study of the matter distribution and velocity dispersion at the extragalactic
scales could provide an efficient method for testing the multi-dimensional
physical models. | gr-qc |
General relativistic laser interferometric observables of the
GRACE-Follow-On mission: We develop a high-precision model for laser ranging interferometric (LRI)
observables of the GRACE Follow-On (GRACE-FO) mission. For this, we study the
propagation of an electromagnetic wave in the gravitational field in the
vicinity of an extended body, in the post-Newtonian approximation of the
general theory of relativity. We present a general relativistic model for the
phase of a plane wave that accounts for contributions of all the multipoles of
the gravitating body, its angular momentum, as well as the contribution of
tidal fields produced by external sources. We develop a new approach to model a
coherent signal transmission in the gravitational field of the solar system
that relies on a relativistic treatment of the phase. We use this approach to
describe high-precision interferometric measurements on GRACE-FO and formulate
the key LRI observables, namely the phase and phase rate of a coherent laser
link between the two spacecraft. We develop a relativistic model for the
LRI-enabled range between the two GRACE-FO spacecraft, accurate to less than 1
nm, and a high-precision model for the corresponding range rate, accurate to
better than 0.1 nm/s. We also formulate high-precision relativistic models for
the double one-way range (DOWR) and DOWR-enabled range rate observables
originally used on GRACE and now studied for interferometric measurements on
GRACE-FO. Our formulation justifies the basic assumptions behind the design of
the GRACE-FO mission and highlights the importance of achieving nearly circular
and nearly identical orbits for the GRACE-FO spacecraft. | gr-qc |
Magnetized accretion disks around Kerr black holes with scalar hair --
Nonconstant angular momentum disks: We present new equilibrium solutions of stationary models of magnetized thick
disks (or tori) around Kerr black holes with synchronised scalar hair. The
models reported here largely extend our previous results based on constant
radial distributions of the specific angular momentum along the equatorial
plane. We introduce a new way to prescribe the distribution of the disk's
angular momentum based on a combination of two previous proposals and compute
the angular momentum distribution outside the equatorial plane by resorting to
the construction of von Zeipel cylinders. We find that the effect of the scalar
hair on the black hole spacetime can yield significant differences in the disk
morphology and properties compared to what is found if the spacetime is purely
Kerr. Some of the tori built within the most extreme, background hairy black
hole spacetime of our sample exhibit the appearance of two maxima in the
gravitational energy density which impacts the radial profile distributions of
the disk's thermodynamical quantities. The models reported in this paper can be
used as initial data for numerical evolutions with GRMHD codes to study their
stability properties. Moreover, they can be employed as illuminating sources to
build shadows of Kerr black holes with scalar hair which might help further
constrain the no-hair hypothesis as new observational data is collected. | gr-qc |
Combinatorial Hopf algebra for the Ben Geloun-Rivasseau tensor field
theory: The Ben Geloun-Rivasseau quantum field theoretical model is the first tensor
model shown to be perturbatively renormalizable. We define here an appropriate
Hopf algebra describing the combinatorics of this new tensorial
renormalization. The structure we propose is significantly different from the
previously defined Connes-Kreimer combinatorial Hopf algebras due to the
involved combinatorial and topological properties of the tensorial Feynman
graphs. In particular, the 2- and 4-point function insertions must be defined
to be non-trivial only if the superficial divergence degree of the associated
Feynman integral is conserved. | gr-qc |
Variable Speed of Light Cosmology and Bimetric Gravity: An Alternative
to Standard Inflation: A scalar-tensor bimetric gravity model of early universe cosmology is
reviewed. The metric frame with a variable speed of light (VSL) and a constant
speed of gravitational waves is used to describe a Friedmann-Robertson-Walker
universe. The Friedmann equations are solved for a radiation dominated equation
of state and the power spectrum is predicted to be scale invariant with a
scalar mode spectral index $n_s=0.97$. The scalar modes are born in a ground
state superhorizon and the fluctuation modes are causally connected by the VSL
mechanism. The cosmological constant is equated to zero and there is no
significant dependence on the scalar field potential energy. A possible way of
distinguishing the metric gravity model from standard inflationary models is
discussed. | gr-qc |
Velocity memory effect without soft particles: We study the behavior of geodesics in the plane-fronted wave background of
the three-dimensional (3D) gravity with propagating torsion, which possesses
only massive degrees of freedom. We discover the velocity memory effect, in
contrast to the current belief that its existence is due to the presence of
soft particles. | gr-qc |
Equation of states in the curved spacetime of spherical degenerate stars: In the study of spherical degenerate stars such as neutron stars, general
relativistic effects are incorporated by using Tolman-Oppenheimer-Volkoff
equations to describe their interior spacetime. However, the equation of states
employed in such studies are invariably computed in flat spacetime. We show
that the equation of states computed in the curved spacetime of these stars
depend explicitly on the metric function. Further, we show that ignoring such
metric-dependent gravitational time dilation effect leads one to grossly
underestimate the mass limits of these compact stars. | gr-qc |
Vacua and Exact Solutions in Lower-$D$ Limits of EGB: We consider the action principles that are the lower dimensional limits of
the Einstein-Gauss-Bonnet gravity {\it via} the Kaluza-Klein route. We study
the vacua and obtain some exact solutions. We find that the reality condition
of the theories may select one vacuum over the other from the two vacua that
typically arise in Einstein-Gauss-Bonnet gravity. We obtain exact black hole
and cosmological solutions carrying scalar hair, including scalar hairy BTZ
black holes with both mass and angular momentum turned on. We also discuss the
holographic central charges in the asymptotic AdS backgrounds. | gr-qc |
Trumpet Initial Data for Boosted Black Holes: We describe a procedure for constructing initial data for boosted black holes
in the moving-punctures approach to numerical relativity that endows the
initial time slice from the outset with trumpet geometry within the black hole
interiors. We then demonstrate the procedure in numerical simulations using an
evolution code from the Einstein Toolkit that employs 1+log slicing. The
Lorentz boost of a single black hole can be precisely specified and multiple,
widely separated black holes can be treated approximately by superposition of
single hole data. There is room within the scheme for later improvement to
re-solve (iterate) the constraint equations in the multiple black hole case.
The approach is shown to yield an initial trumpet slice for one black hole that
is close to, and rapidly settles to, a stationary trumpet geometry. Initial
data in this new approach is shown to contain initial transient (or "junk")
radiation that is suppressed by as much as two orders of magnitude relative to
that in comparable Bowen-York initial data. | gr-qc |
Searching for double-peak and doubly broken gravitational-wave spectra
from Advanced LIGO-Virgo's first three observing runs: The current LIGO-Virgo observing run has been pushing the sensitivity limit
to touch the stochastic gravitational-wave backgrounds (SGWBs). However, no
significant detection has been reported to date for any single dominated source
of SGWBs with a single broken-power-law (BPL) spectrum. Nevertheless, it could
equally well escape from existing Bayesian searches from, for example, two
comparable dominated sources with two separate BPL spectra (double-peak case)
or a single source with its power-law behavior in the spectrum broken twice
(doubly broken case). In this paper, we put constraints on these two cases but
specifically for the model with cosmological first-order phase transitions from
Advanced LIGO-Virgo's first three observing runs. We found strong negative
evidence for the double-peak case and hence place 95\% C.L. upper limits
$\Omega_\mathrm{BPL,1}<5.8\times10^{-8}$ and
$\Omega_\mathrm{BPL,2}<4.4\times10^{-8}$ on the two BPL spectra amplitudes with
respect to the unresolved compact binary coalescence (CBC) amplitude
$\Omega_\mathrm{CBC}<5.6\times10^{-9}$. We further found weak negative evidence
for the doubly broken case and hence place 95\% C.L. upper limit
$\Omega_\mathrm{DB}<1.2\times10^{-7}$ on the overall amplitude of the doubly
broken spectrum with respect to $\Omega_\mathrm{CBC}<6.0\times10^{-9}$. In
particular, the results from the double-peak case have marginally ruled out the
strong super-cooling first-order phase transitions at LIGO-Virgo band. | gr-qc |
Preferred foliation effects in Quantum General Relativity: We investigate the infrared (IR) effects of Lorentz violating terms in the
gravitational sector using functional renormalization group methods similar to
Reuter and collaborators. The model we consider consists of pure quantum
gravity coupled to a preferred foliation, described effectively via a scalar
field with non-standard dynamics. We find that vanishing Lorentz violation is a
UV attractive fixed-point of this model in the local potential approximation.
Since larger truncations may lead to differing results, we study as a first
example effects of additional matter fields on the RG running of the Lorentz
violating term and provide a general argument why they are small. | gr-qc |
Anisotropic Solutions in Symmetric Teleparallel $f\left(
Q\right)$-theory: Kantowski-Sachs and Bianchi III LRS Cosmologies: We investigate the existence of anisotropic self-similar exact solutions in
symmetric teleparallel $f\left( Q\right)$-theory. For the background geometry
we consider the Kantowski-Sachs and the Locally Rotationally Symmetric Bianchi
type III geometries. These two anisotropic spacetimes are of special interest
because in the limit of isotropy they are related to the closed and open
Friedmann--Lema\^{\i}tre--Robertson--Walker cosmologies respectively. For each
spacetime there exist two distinct families of flat, symmetric connections,
which share the symmetries of the spacetime. We present the field equations,
and from them, we determine the functional form of the $f\left( Q\right)$
Lagrangian which yields self-similar solutions. We initially consider the
vacuum case and subsequently we introduce a matter source in terms of a perfect
fluid. Last but not least, we report some self-similar solutions corresponding
to static spherically symmetric spacetimes. | gr-qc |
Electrodynamics as a particular case of the most general relativistic
force field: A new approach to classical electrodynamics is presented, showing that it can
be regarded as a particular case of the most general relativistic force field.
In particular, at first it is shown that the structure of the Lorentz force
comes directly from the structure of the three-force transformation law, and
that E and B fields can be defined, which in general will depend not only on
the space-time coordinate, but also on the velocity of the body acted upon.
Then it is proved that these fields become independent from the body velocity
if the force field propagates throughout space at the relativistic speed limit
c. Finally, field sources are introduced by defining them as perturbations of
the field itself, obtaining a generalization of Maxwell equations which allow
to express the field in terms sources, if these last are known a priori.
Electrodynamics follows simply assuming, in addition to the postulate of field
propagation at the speed c, that scalar sources of the B field are missing and
that electric charge is an invariant characteristic of matter, acting both as
source of the field and sensing property. The presented approach may have
valuable didactic effectiveness in showing that space-time structure, as
defined by Special Relativity postulates, is the ultimate responsible of the
structure of electrodynamics laws. | gr-qc |
Canonical D = 1 supergravity framework for FLRW cosmology: We construct an extension of standard flat FLRW cosmology with matter,
possessing local D = 1, N = 1 proper-time supersymmetry. The fundamental
equation for the resulting mini-superspace models of quantum universes is a
Dirac-like analogue of the Friedmann and Wheeler-DeWitt equations. We provide
solutions of this equation for specific matter configurations based on the
supersymmetric O(3) and O(2, 1) sigma-models. It turns out that in the compact
model the volume rate of growth of the universe is quantized and non-vanishing
due to the zero-point energy of the scalar fields. In the non-compact model the
spectrum of the growth rates is continuous but subject to an uncertainty
relation involving the scale and the growth factor. | gr-qc |
CMB power spectrum for emergent scenario and slow expansion in
scalar-tensor theory of gravity: We analyze the stability of the Einstein static universe in scalar-tensor
theory of gravity, and find it can be stable against both scalar and tensor
perturbations under certain conditions. By assuming the emergent scenario
originating from an Einstein static state, followed by an instantaneous
transition to an inflationary phase, we study and obtain the analytical
approximations of the primordial power spectrum for the emergent scenario.
Then, we plot the primordial power spectrum and CMB TT-spectrum of the emergent
scenario and the slow expansion scenario. These figures show that both of these
spectra for the slow expansion scenario are the same as that for $\Lambda$CDM,
and the spectra of the emergent scenario are lower than that for $\Lambda$CDM
at large scales. | gr-qc |
Three little paradoxes: making sense of semiclassical gravity: I review the arguments most often raised against a fundamental coupling of
classical spacetime to quantum matter. I show that an experiment by Page and
Geilker does not exclude such a semiclassical theory but mandates an inclusion
of an objective mechanism for wave function collapse. In this regard, I present
a classification of semiclassical models defined by the way in which the wave
function collapse is introduced. Two related types of paradoxes that have been
discussed in the context of the necessity to quantize the gravitational field
can be shown to not constrain the possibility of a semiclassical coupling. A
third paradox, the possibility to signal faster than light via semiclassical
gravity, is demonstrably avoided if certain conditions are met by the
associated wave function collapse mechanism. In conclusion, all currently
discussed models of semiclassical gravity can be made consistent with
observation. Their internal theoretical consistency remains an open question. | gr-qc |
Impact of multiple modes on the black-hole superradiant instability: Ultralight bosonic fields in the mass range $\sim (10^{-20}-10^{-11})\,{\rm
eV}$ can trigger a superradiant instability that extracts energy and angular
momentum from an astrophysical black hole with mass $M\sim(5,10^{10})M_\odot$,
forming a nonspherical, rotating condensate around it. So far, most studies of
the evolution and end-state of the instability have been limited to initial
data containing only the fastest growing superradiant mode. By studying the
evolution of multimode data in a quasi-adiabatic approximation, we show that
the dynamics is much richer and depend strongly on the energy of the seed, on
the relative amplitude between modes, and on the gravitational coupling. If the
seed energy is a few percent of the black-hole mass, a black hole surrounded by
a mixture of superradiant and nonsuperradiant modes with comparable amplitudes
might not undergo a superradiant unstable phase, depending on the value of the
boson mass. If the seed energy is smaller, as in the case of an instability
triggered by quantum fluctuations, the effect of nonsuperradiant modes is
negligible. We discuss the implications of these findings for current
constraints on ultralight fields with electromagnetic and gravitational-wave
observations. | gr-qc |
Studying strong phase transitions in neutron stars with gravitational
waves: The composition of neutron stars at the extreme densities reached in their
cores is currently unknown. Besides nuclear matter of normal neutrons and
protons, the cores of neutron stars might harbor exotic matter such as
deconfined quarks. In this paper we study strong hadron-quark phase transitions
in the context of gravitational wave observations of inspiraling neutron stars.
We consider upcoming detections of neutron star coalescences and model the
neutron star equations of state with phase transitions through the
Constant-Speed-of-Sound parametrization. We use the fact that neutron star
binaries with one or more hadron-quark hybrid stars can exhibit qualitatively
different tidal properties than binaries with hadronic stars of the same mass,
and hierarchically model the masses and tidal properties of simulated
populations of binary neutron star inspiral signals. We explore the parameter
space of phase transitions and discuss under which conditions future
observations of binary neutron star inspirals can identify this effect and
constrain its properties, in particular the threshold density at which the
transition happens and the strength of the transition. We find that if the
detected population of binary neutron stars contains both hadronic and hybrid
stars, the onset mass and strength of a sufficiently strong phase transition
can be constrained with 50-100 detections. If the detected neutron stars are
exclusively hadronic or hybrid, then it is possible to place lower or upper
limits on the transition density and strength. | gr-qc |
Gauge-invariant Formulation of the Second-order Cosmological
Perturbations: Gauge invariant treatments of the second order cosmological perturbation in a
four dimensional homogeneous isotropic universe filled with the perfect fluid
are completely formulated without any gauge fixing. We derive all components of
the Einstein equations in the case where the first order vector and tensor
modes are negligible. These equations imply that the tensor and the vector mode
of the second order metric perturbations may be generated by the scalar-scalar
mode coupling of the linear order perturbations as the result of the non-linear
effects of the Einstein equations. | gr-qc |
A Normal Coordinate Expansion for the Gauge Potential: In this pedagogical note, I present a method for constructing a fully
covariant normal coordinate expansion of the gauge potential on a curved
space-time manifold. Although the content of this paper is elementary, the
results may prove useful in some applications and have not, to the best of my
knowledge, been discussed explicitly in the literature. | gr-qc |
A LISA Data-Analysis Primer: This article is an introduction for the nonpractitioner to the ideas and
issues of LISA data analysis, as reflected in the explorations and experiments
of the participants in the Mock LISA Data Challenges. In particular, I discuss
the methods and codes that have been developed for the detection and parameter
estimation of supermassive black-hole binaries, extreme mass-ratio inspirals,
and Galactic binaries. | gr-qc |
On the variable-charged black holes in General Relativity: Hawking's
radiation and naked singularities: In this paper we study the Hawking radiation in Reissner-Nordstrom and
Kerr-Newman black holes by considering the charge to be the function of radial
coordinate. | gr-qc |
Scalar-Tensor Gravitational Strain Field Equations and the Longitudinal
Wave Form: From modern observations of gravitational interactions, it can be inferred
that there is much left to discover about the fundamental gravitational field.
Since the advent of the General Theory of Relativity over a century ago, we
have come to make exotic assumptions pertaining to the inner workings of an
associated field theory. One of which is an elastic nature to spacetime and the
behavior of gravity for strong and weak fields. In this work we investigate a
more \textit{physical} nature, expanding upon general relativity led by
observations of strong sources. We introduce a candidate Lorentz-invariant
field theory that employs an \textit{elastic} and \textit{pseudoscalar} nature
to the field interpretation and it's properties. A unique generation of the
Euler-Lagrange equations of motion is presented; resulting in a longitudinal
wave equation for the \textit{Dilation} gravitational field. This provides a
modern advancement of a relativistic gravitational field theory, supported by
observation. | gr-qc |
Stability investigations of de Sitter inflationary solutions in
power-law extensions of the Starobinsky model: In this paper, we would like to examine whether stable de Sitter inflationary
solutions appear within power-law extensions of the Starobinsky model. In
particular, we will address general constraints for the existence along with
the stability of de Sitter inflationary solutions in a general case involving
not only the Starobinsky $R^2$ term but also an additional power-law $R^n$ one.
According to the obtained results, we will be able to identify which extension
is more suitable for an early inflationary phase rather than a late-time cosmic
acceleration phase. To be more specific, we will consider several values of $n$
to see whether the corresponding de Sitter inflationary solutions are stable or
not. | gr-qc |
Cosmic voids and induced hyperbolicity. II. Sensitivity to void/wall
scales: Cosmic voids as typical under-density regions in the large scale Universe are
known for their hyperbolic properties as an ability to deviate the photon
beams. The under-density then is acting as the negative curvature in the
hyperbolic spaces. The hyperbolicity of voids has to lead to distortion in the
statistical analysis at galactic surveys. We reveal the sensitivity of the
hyperbolicity and hence of the distortion with respect to the ratio of
void/wall scales which are observable parameters. This provides a principal
possibility to use the distortion in the galactic surveys in revealing the
line-of sight number of cosmic voids and their characteristic scales. | gr-qc |
Cosmological solutions in the brane-bulk system: In this work we find cosmological solutions in the brane-bulk system starting
from a 5-D line element which is a simple extension, for cosmological
applications, of the pioneering Randall-Sundrum line element. From the
knowledge of the bulk metric, assumed to have the form of plane waves
propagating in the fifth dimension, we solve the corresponding 4-D Einstein
equations on the brane with a well defined energy-momentum tensor. | gr-qc |
Highly relativistic spin-gravity-$Λ$ coupling: The effects of highly relativistic spin-gravity coupling in the
Schwarzschild-de Sitter background which follow from the Mathisson-Papapetrou
equations are investigated. The dependence of gravitoelectric and
gravitomagnetic components of gravitational field on the velocity of an
observer which is moving in Schwarzschild-de Sitter's background is estimated.
The action of gravitomagnetic components on a fast moving spinning particle is
considered. Different cases of the highly relativistic circular orbits of a
spinning particle which essentially differ from the corresponding geodesic
orbits are described. | gr-qc |
Quasinormal modes, greybody factors and thermodynamics of four
dimensional AdS black holes in Critical Gravity: In the present work, considering critical gravity as a gravity model, an
electrically charged topological Anti-de Sitter black hole with a matter source
characterized by a nonlinear electrodynamics framework is obtained. This
configuration is defined by an integration constant, three key structural
constants, and a constant that represents the topology of the event horizon.
Additionally, based on the Wald formalism, we probe that this configuration
enjoys non-trivial thermodynamic quantities, establishing the corresponding
first law of black hole thermodynamics, as well as local stability under
thermal and electrical fluctuations. Moreover, the quasinormal modes and the
greybody factor are also calculated by considering the spherical situation. We
found that the quasinormal modes exhibit a straightforward change for
variations of one of the structural constants. | gr-qc |
The Plebanski sectors of the EPRL vertex: Modern spin-foam models of four dimensional gravity are based on a discrete
version of the $Spin(4)$ Plebanski formulation. Beyond what is already in the
literature, we clarify the meaning of different Plebanski sectors in this
classical discrete model. We show that the linearized simplicity constraints
used in the EPRL and FK models are not sufficient to impose a restriction to a
single Plebanski sector, but rather, three Plebanski sectors are mixed. We
propose this as the reason for certain extra `undesired' terms in the
asymptotics of the EPRL vertex analyzed by Barrett et al. This explanation for
the extra terms is new and different from that sometimes offered in the
spin-foam literature thus far. | gr-qc |
A Yang-Mills field on the extremal Reissner-Nordström black hole: We consider a spherically symmetric (magnetic) $SU(2)$ Yang-Mills field
propagating on the exterior of the extremal Reissner-Nordstr\"om black hole.
Taking advantage of the conformal symmetry, we reduce the problem to the study
of the Yang-Mills equation in a geodesically complete spacetime with two
asymptotically flat ends. We prove the existence of infinitely many static
solutions (two of which are found in closed form) and determine the spectrum of
their linear perturbations and quasinormal modes. Finally, using the
hyperboloidal approach to the initial value problem, we describe the process of
relaxation to the static endstates of evolution for various initial data. | gr-qc |
Bounce inflation in $f(T)$ Cosmology: A unified inflaton-quintessence
field: We investigate a bounce inflation model with a graceful exit into the
Friedmann-Robertson-Walker (FRW) decelerated Universe within $f(T)$ gravity
framework, where $T$ is the torsion scalar in the teleparallelism. We study the
cosmic thermal evolution, the model predicts a supercold Universe during the
precontraction phase, which is consistent with the requirements of the
slow-roll models, while it performs a reheating period by the end of the
contraction with a maximum temperature just below the grand unified theory
(GUT) temperature. However, it matches the radiation temperature of the hot big
bang at later stages. The equation-of-state due to the effective gravitational
sector suggests that our Universe is self-accelerated by teleparallel gravity.
We assume the matter component to be a canonical scalar field. We obtain the
scalar field potential that is induced by the $f(T)$ theory. The power spectrum
of the model is nearly scale invariant. In addition, we show that the model
unifies inflaton and quintessence fields in a single model. Also, we revisited
the primordial fluctuations in $f(T)$ bounce cosmology, to study the
fluctuations that are produced at the precontraction phase. | gr-qc |
Non-minimal Gravitational Coupling of Phantom and Big Rip Singularity: We consider a non-minimal coupling of a perfect fluid matter system with
geometry, which the coupling function is taken to be an arbitrary function of
the Ricci scalar. Due to such a coupling, the matter stress tensor is no longer
conserved and there is an energy transfer between the two components. By
solving the conservation equation and applying the second law of
thermodynamics, we show that direction of the energy transfer depends on the
equation of state of the matter fluid. In particular, a phantom fluid should
loose energy with expansion of the universe. This energy reduction can avoid
the universe to end with a cosmic doomsday. | gr-qc |
On the Unruh effect, trajectories and information: We calculate the trajectories which maximize the Unruh effect, mode by mode,
when given a fixed energy budget for acceleration. We find that Unruh processes
are most likely to occur, and therefore potentially best observable, for
certain trajectories whose acceleration is not uniform. In practice, the
precise form of optimal trajectories depends on experimental bounds on how fast
the acceleration can be changed. We also show that the Unruh spectra of
arbitrarily accelerated observers contain the complete information to
reconstruct the observers' trajectories. | gr-qc |
Global stability analysis for cosmological models with non-minimally
coupled scalar fields: We explore dynamics of cosmological models with a nonminimally coupled scalar
field evolving on a spatially flat Friedmann-Lemaitre-Robertson-Walker
background. We consider cosmological models including the Hilbert-Einstein
curvature term and the $N$ degree monomial of the scalar field nonminimally
coupled to gravity. The potential of the scalar field is the $n$ degree
monomial or polynomial. We describe several qualitatively different types of
dynamics depending on values of power indices $N$ and $n$. We identify that
three main possible pictures correspond to $n<N$, $N<n<2N$ and $n>2N$ cases.
Some special features connected with the important cases of $N=n$ (including
the quadratic potential with quadratic coupling) and $n=2N$ (which shares its
asymptotics with the potential of the Higgs-driven inflation) are described
separately. A global qualitative analysis allows us to cover the most
interesting cases of small $N$ and $n$ by a limiting number of phase-space
diagrams. The influence of the cosmological constant to the global features of
dynamics is also studied. | gr-qc |
Critical Phenomena in Gravitational Collapse: The Studies So Far: Studies of black hole formation from gravitational collapse have revealed
interesting non-linear phenomena at the threshold of black hole formation. In
particular, in 1993 Choptuik studied the collapse of a massless scalar field
with spherical symmetry and found some behaviour, which is quite similar to the
critical phenomena well-known in {\em Statistical Mechanics} and {\em Quantum
Field Theory}. Universality and echoing of the critical solution and power-law
scaling of the black hole masses have given rise to the name {\em Critical
Phenomena in Gravitational Collapse}. Choptuik's results were soon confirmed
both numerically and semi-analytically, and have extended to various other
matter fields.
In this paper, we shall give a brief introduction to this fascinating and
relatively new area, and provides an updated publication list. An analytical
"toy" model of critical collapse is presented, and some current investigations
are given. | gr-qc |
Exotic Black Hole Thermodynamics in Third-Order Lovelock Gravity: The generalization of Birkhoff's theorem to higher dimensions in Lovelock
gravity allows for black hole solutions with horizon geometries of non-constant
curvature. We investigate thermodynamic aspects of these `exotic' black hole
solutions, with a particular emphasis on their phase transitions. We consider
an extended phase space where the cosmological constant acts as a thermodynamic
pressure, and examine both uncharged and $U(1)$ charged solutions. In $d=7$,
black hole solutions are restricted to having constant-curvature horizon base
manifolds. Uncharged $d=7$ black holes possess novel triple point phenomena
analogous to those recently uncovered in exotic $d=6$ black holes in
Gauss-Bonnet gravity, while their charged counterparts generically undergo
small-large black hole phase transitions. In $d=8$, we find that both charged
and uncharged black holes exhibit triple point behaviour and small-large black
hole transitions. We also show that a wide range of `exotic' horizon geometries
can be ruled out due to the appearance of naked singularities. | gr-qc |
Quasinormal modes of a Generic-class of magnetically charged regular
black hole: scalar and electromagnetic perturbations: In this contribution, we study the quasinormal modes of a Generic--class of a
regular black hole with a magnetic charge in nonlinear electrodynamics,
considering scalar and electromagnetic perturbations. The Generic--class
contains the Bardeen--class, Hayward--class, and New--class solutions. As the
Generic--class can represent a black hole with two horizons or one horizon.
First, we obtain the critical values of the magnetic charge and mass. Then,
using the third--order WKB approximation, we can determine the dependence of
the quasinormal modes with the parameters of the Generic--class. Finally, the
transmission and reflection coefficients of the scattered wave in the
third--order WKB approximation are calculated. | gr-qc |
Unveiling the Fingerprint of Eccentric Binary Black Hole Mergers: The orbital eccentricity plays a crucial role in shaping the dynamics of
binary black hole (BBH) mergers. Remarkably, our recent findings reveal a
universal oscillation in essential dynamic quantities: peak luminosity
$L_{\text{peak}}$, masses $M_f$, spins $\alpha_f$, and recoil velocity $V_f$ of
the final remnant black hole, as the initial eccentricity $e_0$ undergoes
variation. In this letter, by leveraging RIT's extensive numerical relativistic
simulations of nonspinning eccentric orbital BBH mergers, we not only confirm
the universal oscillation in peak amplitudes (including harmonic modes),
similar to the oscillations observed in $L_{\text{peak}}$, $M_f$, $\alpha_f$,
and $V_f$, but also make the first discovery of a ubiquitous spiral-like
internal fine structure that correlates $L_{\text{peak}}$, $M_f$, $\alpha_f$,
$V_f$, and peak amplitudes. This distinctive feature, which we term the
"fingerprint" of eccentric orbital BBH mergers, carries important implications
for unraveling the intricate dynamics and astrophysics associated with
eccentric orbital BBH mergers. | gr-qc |
Use and Abuse of the Model Waveform Accuracy Standards: Accuracy standards have been developed to ensure that the waveforms used for
gravitational-wave data analysis are good enough to serve their intended
purposes. These standards place constraints on certain norms of the
frequency-domain representations of the waveform errors. Examples are given
here of possible misinterpretations and misapplications of these standards,
whose effect could be to vitiate the quality control they were intended to
enforce. Suggestions are given for ways to avoid these problems. | gr-qc |
Nonlinear Yang-Mills black holes: This paper is devoted to investigating the nonlinear non-abelian Yang-Mills
black holes. We consider three Born-Infeld, exponential, and logarithmic
nonlinear Yang-Mills theories with $SO(n-1)$ and $SO(n-2,1)$ semi-simple
groups, which n is the dimension of spacetime, and obtain a new class of
nonlinear Yang-Mills (NYM) black hole solutions. Depending on the values of
dimension $n$, Yang-Mills charge $e$ and the mass $m$ and nonlinear parameters
$\beta$, our solutions can lead to a naked singularity, a black hole with two
horizons, an extreme or a Schwarzschild-type black hole. We also investigate
the thermodynamic behaviors of the NYM black holes. For small charge values,
the NYM solutions may be thermally stable in the canonical ensemble, if we
consider an AdS spacetime with spherical $k=+1$ and hyperbolic $k=-1$
coordinates or a flat one with $k=+1$. However, there are no stable regions in
the grand canonical ensemble in higher dimensions. For the NYM black hole, we
observe a reentrant phase transition between large and small black holes in the
BI-branch with small $\beta$, which cannot be visible for the nonlinear
Reissner-Nordstrom AdS black hole in the higher dimension. For the limit
$\beta\rightarrow\infty$, the critical ratio $\frac{P_{c} v_{c}}{T_{c}}$ tends
to the constant value $3/8$ for each dimension $n$, while it depends on the
dimension for the case of nonlinear electrodynamics black holes. | gr-qc |
Extended bodies with quadrupole moment interacting with gravitational
monopoles: reciprocity relations: An exact solution of Einstein's equations representing the static
gravitational field of a quasi-spherical source endowed with both mass and mass
quadrupole moment is considered. It belongs to the Weyl class of solutions and
reduces to the Schwarzschild solution when the quadrupole moment vanishes. The
geometric properties of timelike circular orbits (including geodesics) in this
spacetime are investigated. Moreover, a comparison between geodesic motion in
the spacetime of a quasi-spherical source and non-geodesic motion of an
extended body also endowed with both mass and mass quadrupole moment as
described by Dixon's model in the gravitational field of a Schwarzschild black
hole is discussed. Certain "reciprocity relations" between the source and the
particle parameters are obtained, providing a further argument in favor of the
acceptability of Dixon's model for extended bodies in general relativity. | gr-qc |
Static Spherically Symmetric Wormholes in $f(R,T)$ Gravity: In this work, we explore wormhole solutions in $f(R,T)$ theory of gravity,
where $R$ is the scalar curvature and $T$ is the trace of stress-energy tensor
of matter. To investigate this, we consider static spherically symmetric
geometry with matter contents as anisotropic, isotropic and barotropic fluids
in three separate cases. By taking into account Starobinsky $f(R)$ model , we
analyze the behavior of energy conditions for these different kind of fluids.
It is shown that the wormhole solutions can be constructed without exotic
matter in few regions of spacetime. We also give the graphical illustration of
obtained results and discuss the equilibrium picture for anisotropic case only.
It is concluded that the wormhole solutions with anisotropic matter are
realistic and stable in this gravity. | gr-qc |
From Analogue Models to Gravitating Vacuum: We discuss phenomenology of quantum vacuum. Phenomenology of macroscopic
systems has three sources: thermodynamics, topology and symmetry. Momentum
space topology determines the universality classes of fermionic vacua. The
vacuum in its massless state belongs to the Fermi-point universality class,
which has topologically protected fermionic quasiparticles. At low energy they
behave as relativistic massless Weyl fermions. Gauge fields and gravity emerge
together with Weyl fermions at low energy. Thermodynamics of the self-sustained
vacuum allows us to treat the problems related to the vacuum energy: the
cosmological constant problems. The natural value of the energy density of the
equilibrium the self-sustained vacuum is zero. Cosmology is the process of
relaxation of vacuum towards the equilibrium state. The present value of the
cosmological constant is very small compared to the Planck scale, because the
present Universe is very old and thus is close to equilibrium. | gr-qc |
On cosmic hair and "de Sitter breaking" in linearized quantum gravity: We quantize linearized Einstein-Hilbert gravity on de Sitter backgrounds in a
covariant gauge. We verify the existence of a maximally-symmetric (i.e.de
Sitter-invariant) Hadamard state $\Omega$ for all globally hyperbolic de Sitter
backgrounds in all spacetime dimensions $D \ge 4$ by constructing the state's
2-point function in closed form. This 2-pt function is explicitly maximally
symmetric. We prove an analogue of the Reeh-Schlieder theorem for linearized
gravity. Using these results we prove a cosmic no-hair theorem for linearized
gravitons: for any state in the Hilbert space constructed from $\Omega$, the
late-time behavior of local observable correlation functions reduces to those
of $\Omega$ at an exponential rate with respect to proper time. We also provide
the explicitly maximally-symmetric graviton 2-pt functions in a class of
generalized de Donder gauges suitable for use in non-linear perturbation
theory. Along the way we clarify a few technical aspects which led previous
authors to conclude that these 2-pt functions do not exist. | gr-qc |
Quantum thermodynamics in the interior of a Reissner-Nordström
black-hole: We study the interior of a Reissner-Nordstr\"om Black-Hole (RNBH) using
Relativistic Quantum Geometry, which was introduced in some previous works. We
found discrete energy levels for a scalar field from a polynomial condition for
the Heun Confluent functions expanded around the effective causal radius $r_*$.
From the solutions it is obtained that the uncertainty principle is valid for
each energy level of space-time, in the form: $E_n\, r_{*,n}=\hbar/2$, and the
charged mass is discretized and distributed in a finite number of states. The
classical RNBH entropy is recovered as the limit case where the number of
states is very large, and the RNBH quantum temperature depends on the number of
states in the interior of the RNBH. This temperature, depending of the number
of states of the RNBH, is related with the Bekeinstein-Hawking (BH)
temperature: $T_{BH} \leq T_{N} < 2\,T_{BH}$. | gr-qc |
On collisions with unlimited energies in the vicinity of Kerr and
Schwarzschild black hole horizons: Two particle collisions close to the horizon of the rotating nonextremal
Kerr's and Schwarzschild black holes are analyzed. For the case of multiple
collisions it is shown that high energy in the centre of mass frame occurs due
to a great relative velocity of two particles and a large Lorentz factor. The
dependence of the relative velocity on the distance to horizon is analyzed, the
time of movement from the point in the accretion disc to the point of
scattering with large energy as well as the time of back movement to the Earth
are calculated. It is shown that they have reasonable order. | gr-qc |
Non-singular Universes a la Palatini: It has recently been shown that f(R) theories formulated in the Palatini
variational formalism are able to avoid the big bang singularity yielding
instead a bouncing solution. The mechanism responsible for this behavior is
similar to that observed in the effective dynamics of loop quantum cosmology
and an f(R) theory exactly reproducing that dynamics has been found. I will
show here that considering more general actions, with quadratic contributions
of the Ricci tensor, results in a much richer phenomenology that yields
bouncing solutions even in anisotropic (Bianchi I) scenarios. Some implications
of these results are discussed. | gr-qc |
Construction of a traversable wormhole from a suitable embedding
function: In this work, we construct a traversable wormhole by providing a suitable
embedding function ensuring the fulfilling of the flaring--out condition. The
solution contains free parameters that are reduced through the study of the
acceptable conditions of a traversable wormhole. We compute both the quantifier
of exotic matter and the quasi--normal modes through the $13^{th}$ order WKB as
a function of the remaining free parameters. We obtain that the wormhole
geometry can be sustained by a finite amount of exotic matter and seems to be
stable under scalar perturbations. | gr-qc |
Density-metric unimodular gravity:vacuum spherical symmetry: We analyze an alternative theory of gravity characterized by metrics that are
tensor density of rank(0,2)and weight-1/2.The metric compatibility condition is
supposed to hold. The simplest expression for the action of gravitational field
is used.Taking the metric and trace of connections as dynamical variables,the
field equations in the absence of matter and other kinds of sources are
derived.The solutions of these equations are obtained for the case of vacuum
static spherical symmetric spacetime.The null geodesics and advance of
perihelion of ellipes are discussed.We confirm a subclass of solutions is
regular for r>0 and there is no event horizon while it is singular at r=0. | gr-qc |
Generalized Brans-Dicke inflation with a quartic potential: Within the framework of Brans-Dicke gravity, we investigate inflation with
the quartic potential, $\lambda\varphi^4/4$, in the presence of generalized
Brans-Dicke parameter $\omega_{\rm GBD}(\varphi)$. We obtain the inflationary
observables containing the scalar spectral index, the tensor-to-scalar ratio,
the running of the scalar spectral index and the equilateral non-Gaussianity
parameter in terms of general form of the potential $U(\varphi)$ and
$\omega_{\rm GBD}(\varphi)$. For the quartic potential, our results show that
the predictions of the model are in well agreement with the Planck 2015 data
for the generalized Brans-Dicke parameters $\omega_{\rm
GBD}(\varphi)=\omega_0\varphi^{n}$ and $\omega_0e^{b\varphi}$. This is in
contrast with both the Einstein and standard Brans-Dicke gravity, in which the
result of quartic potential is disfavored by the Planck data. | gr-qc |
Effects of the circularly polarized beam of linearized gravitational
waves: Solutions of the linearized Einstein equations are found that describe a
transversely confined beam of circularly polarized gravitational waves on a
Minkowski backdrop. By evaluating the cycle-averaged stress-energy-momentum
pseudotensor of Landau & Lifshitz it is found that the angular momentum density
is concentrated in the 'skin' at the edge of the beam where the intensity
falls, and that the ratio of angular momentum to energy per unit length of the
beam is $2/\omega$, where $\omega$ is the wave frequency, as expected for a
beam of spin-$2$ gravitons. For sharply-defined, uniform, axisymmetric beams,
the induced background metric is shown to produce the gravomagnetic field and
frame-dragging effects of a gravitational solenoid, whilst the angular momentum
current additionally twists the contained space helically. | gr-qc |
Gravitational time advancement and its possible detection: The gravitational time advancement is a natural but a consequence of curve
space-time geometry. In the present work the expressions of gravitational time
advancement have been obtained for geodesic motions. The situation when the
distance of signal travel is small in comparison to the distance of closest
approach has also been considered. The possibility of experimental detection of
time advancement effect has been explored. | gr-qc |
Towards mitigating the effect of sine-Gaussian noise transients on
searches for gravitational waves from compact binary coalescences: Gravitational wave (GW) signals were recently detected directly by LIGO from
the coalescences of two black hole pairs. These detections have strengthened
our belief that compact binary coalescences (CBCs) are the most promising GW
detection prospects accessible to ground-based interferometric detectors. For
detecting CBC signals it is of vital importance to characterize and identify
non-Gaussian and non-stationary noise in these detectors. In this work we model
two important classes of transient artifacts that contribute to this noise and
adversely affect the detector sensitivity to CBC signals. One of them is the
sine-Gaussian glitch, characterized by a central frequency $f_0$ and a quality
factor $Q$ and the other is the chirping sine-Gaussian glitch, which is
characterized by $f_0$, $Q$ as well as a chirp parameter. We study the response
a bank of compact binary inspiral templates has to these two families of
glitches when they are used to match-filter data containing any of these
glitches. Two important characteristics of this response are the distributions
of the signal-to-noise ratio and the timelag of individual templates. We show
how these distributions differ from those when the detector data has a real CBC
signal instead of a glitch. We argue that these distinctions can be utilized to
develop useful signal-artifact vetos that add negligibly to the computational
cost of a CBC search. Specifically, we show how $f_0$ of a glitch can be used
to set adaptive time-windows around it so that any template trigger occurring
in that window can be quarantined for further vetting of its supposed
astrophysical nature. Second, we recommend focusing efforts on reducing the
incidence of glitches with low $f_0$ values because they create CBC triggers
with the longest timelags. This work allows us to associate such triggers with
the glitches which otherwise would have escaped attention. | gr-qc |
The universe formation by a space reduction cascade with random initial
parameters: In this paper we discuss the creation of our universe using the idea of extra
dimensions. The initial, multidimensional Lagrangian contains only metric
tensor. We have found many sets of the numerical values of the Lagrangian
parameters corresponding to the observed low-energy physics of our universe.
Different initial parameters can lead to the same values of fundamental
constants by the appropriate choice of a dimensional reduction cascade. This
result diminishes the significance of the search for the 'unique' initial
Lagrangian. We also have obtained a large number of low-energy vacua, which is
known as a 'landscape' in the string theory. | gr-qc |
Deep exploration for continuous gravitational waves at 171--172 Hz in
LIGO second observing run data: We pursue a novel strategy towards a first detection of continuous
gravitational waves from rapidly-rotating deformed neutron stars. Computational
power is focused on a narrow region of signal parameter space selected by a
strategically-chosen benchmark. We search data from the 2nd observing run of
the LIGO Observatory with an optimised analysis run on graphics processing
units. While no continuous waves are detected, the search achieves a
sensitivity to gravitational wave strain of $h_0 = 1.01{\times}10^{-25}$ at 90%
confidence, 24% to 69% better than past searches of the same parameter space.
Constraints on neutron star deformity are within theoretical maxima, thus a
detection by this search was not inconceivable. | gr-qc |
Persistent gravitational wave observables: general framework: The gravitational wave memory effect is characterized by the permanent
relative displacement of a pair of initially comoving test particles that is
caused by the passage of a burst of gravitational waves. Recent research on
this effect has clarified the physical origin and the interpretation of this
gravitational phenomenon in terms of conserved charges at null infinity and
"soft theorems." In this paper, we describe a more general class of effects
than the gravitational wave memory that are not necessarily associated with
these charges and soft theorems, but that are, in principle, measurable. We
shall refer to these effects as persistent gravitational wave observables.
These observables vanish in non-radiative regions of a spacetime, and their
effects "persist" after a region of spacetime which is radiating. We give three
examples of such persistent observables, as well as general techniques to
calculate them. These examples, for simplicity, restrict the class of
non-radiative regions to those which are exactly flat. Our first example is a
generalization of geodesic deviation that allows for arbitrary acceleration.
The second example is a holonomy observable, which is defined in terms of a
closed loop. It contains the usual "displacement" gravitational wave memory;
three previously identified, though less well known memory effects (the proper
time, velocity, and rotation memories); and additional new observables.
Finally, the third example we give is an explicit procedure by which an
observer could measure a persistent effect using a spinning test particle. We
briefly discuss the ability of gravitational wave detectors (such as LIGO and
Virgo) to measure these observables. | gr-qc |
Will Einstein Have the Last Word on Gravity?: This is a whitepaper submitted to the 2010 Astronomy Decadal Review process,
addressing the potential tests of gravity theory that could be made by
observations of gravitational waves in the milliHertz frequency band by the
proposed ESA-NASA gravitational wave observatory LISA. A key issue is that
observations in this band of binary systems consisting of black holes offer
very clean tests with high signal-to-noise ratios. Gravitational waves would
probe nonlinear gravity and could reveal small corrections, such as extra
long-range fields that arise in unified theories, deviations of the metric
around massive black holes from the Kerr solution, massive gravitons, chiral
effects, and effects of extra dimensions. The availability of strong signals
from massive black hole binaries as well as complex signals from extreme
mass-ratio binaries is unique to the milliHertz waveband and makes LISA a
particularly sensitive probe of the validity of general relativity. | gr-qc |
Shadow and Deflection Angle of Rotating Black Holes in Perfect Fluid
Dark Matter with a Cosmological Constant: The presence of dark matter around a black hole remarkably affects its
spacetime. We consider the effects of dark matter on the shadow of a new
solution to the Einstein equations that describes a rotating black hole in the
background of perfect dark matter fluid (PFDM), along with its extension to
nonzero cosmological constant $\Lambda$. Working in Boyer-Lindquist
coordinates, we consider the effects of the PFDM parameter $\alpha$ on the
shadow cast by a black hole with respect to an observer at position
$(r_o,\theta_o)$. By applying the Gauss-Bonnet theorem to the optical geometry
we find that notable distortions from a Kerr black hole can occur. We describe
their dependence on $\alpha$ and $\Lambda$. | gr-qc |
Scalar-metric quantum cosmology with Chaplygin gas and perfect fluid: In this paper we consider the flat FRW cosmology with a scalar field coupled
with the metric along with generalized Chaplygin gas and perfect fluid
comprising the matter sector. We use the Schutz's formalism to deal with the
generalized Chaplygin gas sector. The full theory is then quantized canonically
using the Wheeler-DeWitt Hamiltonian formalism. We then solve the WD equation
with appropriate boundary conditions. Then by defining a proper completeness
relation for the self-adjointness of the WD equation we arrive at the wave
packet for the universe. It is observed that the peak in the probability
density gets affected due to both fluids in the matter sector, namely, the
Chaplygin gas and perfect fluid. | gr-qc |
Melvin Universe in Born-Infeld gravity: We consider a magnetic flux pointing in the $z$ direction of an axially
symmetric space-time (Melvin Universe) in a Born-Infeld-type extension of
General Relativity (GR) formulated in the Palatini approach. Large magnetic
fields could have been produced in the early Universe, and given rise to
interesting phenomenology regarding wormholes and black hole remnants. We find
a formal analytic solution to this problem that recovers the GR result in the
appropriate limits. Our results set the basis for further extensions that could
allow the embedding of pairs of black hole remnants in geometries with intense
magnetic fields. | gr-qc |
Holographic dark energy in the DGP model: The braneworld model proposed by Dvali, Gabadadze and Porrati leads to an
accelerated universe without cosmological constant or other form of dark
energy. Nevertheless, we have investigated the consequences of this model when
an holo- graphic dark energy is included, taken the Hubble scale as IR cutoff.
We have found that the holographic dark energy leads to an accelerated universe
flat (de Sitter like expansion) for the two branch: {\ko} = \pm1 of the DGP
model. Nevertheless, in universes with no null curvature the dark energy
presents an EoS corresponding to a phantom fluid during the present era and
evolving to a de Sitter like phase for future cosmic time. In the special case
in which the holographic parameter c is equal to one we have found a sudden
singularity in closed universes. In this case the expansion is decelerating.
Manuscript | gr-qc |
The no-defect conjecture in cosmic crystallography: The topology of space is usually assumed simply connected, but could be
multi-connected. We review in the latter case the possibility that topological
defects arising at high energy phase transitions might still be present and
find that either they are very unlikely to form at all, or space is effectively
simply connected on scales up to the horizon size. | gr-qc |
Symmetric non-expanding horizons: Symmetric non-expanding horizons are studied in arbitrary dimension. The
global properties -as the zeros of infinitesimal symmetries- are analyzed
particularly carefully. For the class of NEH geometries admitting helical
symmetry a quasi-local analog of Hawking's rigidity theorem is formulated and
proved: the presence of helical symmetry implies the presence of two
symmetries: null, and cyclic.
The results valid for arbitrary-dimensional horizons are next applied in a
complete classification of symmetric NEHs in 4-dimensional space-times (the
existence of a 2-sphere crossection is assumed). That classification divides
possible NEH geometries into classes labeled by two numbers - the dimensions
of, respectively, the group of isometries induced in the horizon base space and
the group of null symmetries of the horizon. | gr-qc |
Bar-mode instability of rapidly spinning black hole in higher
dimensions: Numerical simulation in general relativity: Numerical-relativity simulation is performed for rapidly spinning black holes
(BHs) in a higher-dimensional spacetime of special symmetries for the
dimensionality $6 \leq d \leq 8$. We find that higher-dimensional BHs, spinning
rapidly enough, are dynamically unstable against nonaxisymmetric bar-mode
deformation and spontaneously emit gravitational waves, irrespective of $d$ as
in the case $d=5$ \cite{SY09}. The critical values of a nondimensional spin
parameter for the onset of the instability are $q:=a/\mu^{1/(d-3)} \approx
0.74$ for $d=6$, $\approx 0.73$ for $d=7$, and $\approx 0.77$ for $d=8$ where
$\mu$ and $a$ are mass and spin parameters. Black holes with a spin smaller
than these critical values ($q_{\rm crit}$) appear to be dynamically stable for
any perturbation. Longterm simulations for the unstable BHs are also performed
for $d=6$ and 7. We find that they spin down as a result of gravitational-wave
emission and subsequently settle to a stable stationary BH of a spin smaller
than $q_{\rm crit}$. For more rapidly spinning unstable BHs, the timescale, for
which the new state is reached, is shorter and fraction of the spin-down is
larger. Our findings imply that a highly rapidly spinning BH with $q > q_{\rm
crit}$ cannot be a stationary product in the particle accelerators, even if it
would be formed as a consequence of a TeV-gravity hypothesis. Its implications
for the phenomenology of a mini BH are discussed. | gr-qc |
Quantum massive conformal gravity: We first find the linear approximation of the second plus fourth order
derivative massive conformal gravity action. Then we reduce the linearized
action to separated second order derivative terms, which allows us to quantize
the theory by using the standard first order canonical quantization method. It
is shown that quantum massive conformal gravity is renormalizable but has ghost
states. A possible decoupling of these ghost states at high energies is
discussed. | gr-qc |
Twisted Lorentzian manifolds, a characterization with torse-forming
time-like unit vectors: Robertson-Walker and Generalized Robertson-Walker spacetimes may be
characterized by the existence of a time-like unit torse-forming vector field,
with other constrains. We show that Twisted manifolds may still be
characterized by the existence of such (unique) vector field, with no other
constrain. Twisted manifolds generalize RW and GRW spacetimes by admitting a
scale function that depends both on time and space. We obtain the Ricci tensor,
corresponding to the stress-energy tensor of an imperfect fluid. | gr-qc |
Non-perturbative stabilization of two Kähler moduli in type-IIB/F
theory and the inflaton potential: We consider a combination of perturbative and non-perturbative corrections in
K\"ahler moduli stabilizations in the configuration of three magnetised
intersecting D7 branes in the type-IIB/F theory, compactified on the 6d T^6/Z_N
orbifold of Calabi-Yau three-fold (CY_3). Two of the K\"ahler moduli are
stabilized non-perturbatively, out of the three which get perturbative
corrections up to one-loop-order multi-graviton scattering amplitudes in the
large volume scenario. In this framework, the dS vacua are achieved through all
K\"ahler moduli stabilizations by considering the D-term. We obtain inflaton
potentials of slow-roll plateau-type, which are expected by recent cosmological
observations. Calculations of cosmological parameters with the potentials yield
experimentally favoured values. | gr-qc |
Odd-parity stability of hairy black holes in $U(1)$ gauge-invariant
scalar-vector-tensor theories: In scalar-vector-tensor theories with $U(1)$ gauge invariance, it was
recently shown that there exists a new type of hairy black hole (BH) solutions
induced by a cubic-order scalar-vector interaction. In this paper, we derive
conditions for the absence of ghosts and Laplacian instabilities against
odd-parity perturbations on a static and spherically symmetric background for
most general $U(1)$ gauge-invariant scalar-vector-tensor theories with
second-order equations of motion. We apply those conditions to hairy BH
solutions arising from the cubic-order coupling and show that the odd-parity
stability in the gravity sector is always ensured outside the event horizon
with the speed of gravity equivalent to that of light. We also study the case
in which quartic-order interactions are present in addition to the cubic
coupling and obtain conditions under which black holes are stable against
odd-parity perturbations. | gr-qc |
Does quantum gravity relate the constants of nature?: The central equation of quantum gravity is the Wheeler-DeWitt equation. We
give an argument suggesting that exact solutions of this equation give a
surface in the space of coupling constants. This provides a mechanism for
determining the cosmological constant as a function of the gravitational and
other interaction constants. We demonstrate the idea by computing one such
surface in a cosmological model. | gr-qc |
Decoherence in quantum cosmology at the onset of inflation: We calculate the reduced density matrix for the inflaton field in a model of
chaotic inflation by tracing out degrees of freedom corresponding to various
bosonic fields. We find a qualitatively new contribution to the density matrix
given by the Euclidean effective action of quantum fields. We regularise the
ultraviolet divergences in the decoherence factor. Dimensional regularisation
is shown to violate the consistency conditions for a density matrix as a
bounded operator. A physically motivated conformal redefinition of the
environmental fields leads to well-defined expressions. They show that due to
bosonic fields the Universe acquires classical properties near the onset of
inflation. | gr-qc |
Can General Relativity play a role in galactic dynamics?: We use the gravitoelectromagnetic approach to the solutions of Einstein's
equations in the weak-field and slow-motion approximation to investigate the
impact of General Relativity on galactic dynamics. In particular, we focus on a
class of the solutions for the gravitomagnetic field, and show that, contrary
to what is expected, they may introduce non negligible corrections to the
Newtonian velocity profile. These are the homogeneous solutions (HS) for the
gravitomagnetic field, i.e. solutions with vanishing matter currents. We show
how recent results about galactic dynamics are connected to this class of
solutions. | gr-qc |
Nonlinear Spinor Fields in Bianchi type-III spacetime: Within the scope of Bianchi type-III spacetime we study the role of spinor
field on the evolution of the Universe as well as the influence of gravity on
the spinor field. In doing so we have considered a polynomial type of
nonlinearity. In this case the spacetime remains locally rotationally symmetric
and anisotropic all the time. It is found that depending on the sign of
nonlinearity the models allows both accelerated and oscillatory modes of
expansion. The non-diagonal components of energy-momentum tensor though impose
some restrictions on metric functions and components of spinor field, unlike
Bianchi type I, V and $VI_0$ cases, they do not lead to vanishing mass and
nonlinear terms of the spinor field. | gr-qc |
On the instability of ultracompact horizonless spacetimes: Motivated by recent results reporting the instability of horizonless objects
with stable light rings, we revisit the linearized stability of such
structures. In particular, we consider an exterior Kerr spacetime truncated at
a surface where Dirichlet conditions on a massless scalar are imposed.This
spacetime has ergoregions and light rings when the surface is placed
sufficiently deep in the gravitational potential. We establish that the
spacetime is linearly, mode-unstable when it is sufficiently compact, in a
mechanism associated with the ergoregion. In particular, such instability has
associated zero-modes. At large multipole number the critical surface location
for zero modes to exist is precisely the location of the ergosurface along the
equator. We show that such modes don't exist when the surface is outside the
ergoregion, and that any putative linear instability mechanism acts on
timescales $\tau \gtrsim 10^5 M$, where $M$ is the black hole mass. Our results
indicate therefore that at least certain classes of objects are linearly stable
in the absence of ergoregions, even if rotation and light rings are present. | gr-qc |
New parameterization for unified dark matter and dark energy: In this paper we investigate a new phenomenological parameterization for
unified dark matter and dark energy based on the polynomial expansion of the
barotropic equation of state parameter $w$. Our parameterization provides
well-behaving evolution of $w$ for both small and big redshifts as well as in
the far future. The dark fluid described by our parameterization behaves for
big redshifts like a dark matter. Therefore one can parameterize dark energy
and dark matter using a single dark fluid, like in the case of the Chaplygin
gas. Within this parameterization we consider 2 models: one with DE barotropic
parameter fixed to be $-1$ and the second one, where $w \neq -1$ is chosen to
match the best fit to the data. We study main cosmological properties of these
models at the expansion and perturbation levels. Based on Markov chain Monte
Carlo method with currently available cosmic observational data sets, we
constrain these models to determine the cosmological parameters at the level of
background and clustering of matter. We consider the interaction between DM and
DE which directly affects the evolution of matter and its clustering. Our model
appears to be perfectly consistent with the $\Lambda$CDM model, while providing
unification of DE and DM. | gr-qc |
Octonionic Physics: The physical solutions of Lagrangian of octonionics are researched in the
paper. It is shown, the gravitational interaction in Friedmann space and in
spherically symmetric space in such model is to be described by pair of charged
massless vectorial D-bosons of Minkowski space. It is proposed to use the
formalism for the description of jet and supernova. | gr-qc |
Three Principles for Quantum Gravity: We postulate that the fundamental principles of Quantum Gravity are
diffeomorphism symmetry, unitarity, and locality. Local observables are
compatible with diffeomorphism symmetry in the presence of diff anomalies,
which modify the symmetry algebra upon quantization. We describe the
generalization of the Virasoro extension to the diffeomorphism algebra in
several dimensions, and its off-shell representations. These anomalies can not
arise in QFT, because the Virasoro-like cocycles are functionals of the
observer's spacetime trajectory, which is not present in QFT. Possible
implications for physics are discussed. | gr-qc |
Dark Entropy: We examine the consequences of a universe with a non-constant cosmological
term in Einstein's equations and find that the Bianchi identities reduce to the
first law of thermodynamics when cosmological term is identified as being
proportional to the entropy density of the universe. This means that
gravitating dark energy can be viewed as entropy, but more, the holographic
principle along with the known expansion of the universe indicates that the
entropy of the universe is growing with time and this leads to a cosmic
repulsion that also grows with time. Direct implications of this result are
calculated and shown to be in good accord with recent observational data. | gr-qc |
Tracking the Long-Term GW Phase Evolution for HM Cancri-like Binaries
with LISA: From prolonged X-ray and optical data of the ultra-compact binary HM Cancri,
two groups recently measured the second derivative of its orbital frequency.
The space gravitational wave (GW) detector LISA will detect $\sim10^4$ Galactic
binaries and their second frequency derivatives will be interesting
observational targets for LISA. Here, we forecast the GW signal analysis for HM
Cancri, as an ideal reference system for these numerous binaries. We find that,
in its nominal operation period $T\sim4$yr, LISA is unlikely to realize a
sufficient measurement precision for the reported second frequency derivative
of this binary. However, because of a strong dependence on the time baseline,
the precision will be drastically improved by extending the operation period of
LISA or combining it with other missions (e.g., Taiji and TianQin) in a
sequential order. | gr-qc |
Construction of anti-de Sitter-like spacetimes using the metric
conformal Einstein field equations: the vacuum case: We make use of the metric version of the conformal Einstein field equations
to construct anti-de Sitter-like spacetimes by means of a suitably posed
initial-boundary value problem. The evolution system associated to this
initial-boundary value problem consists of a set of conformal wave equations
for a number of conformal fields and the conformal metric. This formulation
makes use of generalised wave coordinates and allows the free specification of
the Ricci scalar of the conformal metric via a conformal gauge source function.
We consider Dirichlet boundary conditions for the evolution equations at the
conformal boundary and show that these boundary conditions can, in turn, be
constructed from the 3-dimensional Lorentzian metric of the conformal boundary
and a linear combination of the incoming and outgoing radiation as measured by
certain components of the Weyl tensor. To show that a solution to the conformal
evolution equations implies a solution to the Einstein field equations we also
provide a discussion of the propagation of the constraints for this
initial-boundary value problem. The existence of local solutions to the
initial-boundary value problem in a neighbourhood of the corner where the
initial hypersurface and the conformal boundary intersect is subject to
compatibility conditions between the initial and boundary data. The
construction described is amenable to numerical implementation and should allow
the systematic exploration of boundary conditions. | gr-qc |
Exact form of the generalized Lorentz force in Fock's nonlinear
relativity: This work completes a serie of two papers devoted to the extension of the
fundamental laws of electrodynamics in the context of Fock's nonlinear
relativity (FNLR). Indeed, after having established in the previous study the
exact generalizations of both Maxwell's equations and Dirac magnetic monopole,
we present here the remaining exact Lorentz force. As in k-Minkowski spacetime,
two different nature contributions appear in the corresponding equation of
motion where the new effect is interpreted as the gravitational-type Lorentz
force. This common point separately induced by the radius of the universe in
our case or Planck energy in other works, reinforces once more the analogy
between electromagnetism and gravity in two different scientific approaches. As
a relative difference, it is very important to highlight that more homogeneity
characterizes our results where each effect is exclusively generated by mass or
charge but not both at the same time. Even more, the new effect emerges as the
result of the triple effect of R-deformation, mass and the square of velocities
but completely independent of electromagnetic field. | gr-qc |
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