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The Black Holes of a General Two-Dimensional Dilaton Gravity Theory: A general dilaton gravity theory in 1+1 spacetime dimensions with a
cosmological constant $\lambda$ and a new dimensionless parameter $\omega$,
contains as special cases the constant curvature theory of Teitelboim and
Jackiw, the theory equivalent to vacuum planar General Relativity, the first
order string theory, and a two-dimensional purely geometrical theory. The
equations of this general two-dimensional theory admit several different black
holes with various types of singularities. The singularities can be spacelike,
timelike or null, and there are even cases without singularities. Evaluation of
the ADM mass, as a charge density integral, is possible in some situations, by
carefully subtrating the black hole solution from the corresponding linear
dilaton at infinity. | gr-qc |
Improved methods for simulating nearly extremal binary black holes: Astrophysical black holes could be nearly extremal (that is, rotating nearly
as fast as possible); therefore, nearly extremal black holes could be among the
binaries that current and future gravitational-wave observatories will detect.
Predicting the gravitational waves emitted by merging black holes requires
numerical-relativity simulations, but these simulations are especially
challenging when one or both holes have mass $m$ and spin $S$ exceeding the
Bowen-York limit of $S/m^2=0.93$. We present improved methods that enable us to
simulate merging, nearly extremal black holes more robustly and more
efficiently. We use these methods to simulate an unequal-mass, precessing
binary black hole coalescence, where the larger black hole has $S/m^2=0.99$. We
also use these methods to simulate a non-precessing binary black hole
coalescence, where both black holes have $S/m^2=0.994$, nearly reaching the
Novikov-Thorne upper bound for holes spun up by thin accretion disks. We
demonstrate numerical convergence and estimate the numerical errors of the
waveforms; we compare numerical waveforms from our simulations with
post-Newtonian and effective-one-body waveforms; we compare the evolution of
the black-hole masses and spins with analytic predictions; and we explore the
effect of increasing spin magnitude on the orbital dynamics (the so-called
"orbital hangup" effect). | gr-qc |
Directed search for continuous gravitational-wave signals from the
Galactic Center in the Advanced LIGO second observing run: In this work we present the results of a search for continuous gravitational
waves from the Galactic Center using LIGO O2 data. The search uses the
Band-Sampled-Data directed search pipeline, which performs a semi-coherent
wide-parameter-space search, exploiting the robustness of the FrequencyHough
transform algorithm. The search targets signals emitted by isolated asymmetric
spinning neutron stars, located within 25-150 parsecs from the Galactic Center.
The frequencies covered in this search range between 10 and 710 Hz with a
spin-down range from $-1.8\times10^{-9}$ to $3.7\times10^{-11}$ Hz/s. No
continuous wave signal has been detected and upper limits on the gravitational
wave amplitude are presented. The most stringent upper limit at $95\%$
confidence level, for the Livingston detector, is $\sim 1.4 \times 10^{-25}$ at
frequencies near 160 Hz. To date, this is the most sensitive directed search
for continuous gravitational-wave signals from the Galactic Center and the
first search of this kind using the LIGO second observing run. | gr-qc |
Constraining $f(Q,T)$ gravity from energy conditions: We are living in a golden age for experimental cosmology. New experiments
with high accuracy precision are been used to constrain proposals of several
theories of gravity, as it has been never done before. However, important roles
to constrain new theories of gravity in a theoretical perspective are the
energy conditions. Throughout this work, we carefully constrained some free
parameters of two different families of $f(Q,T)$ gravity using different energy
conditions. This theory of gravity combines the gravitation effects through the
non-metricity scalar function $Q$, and manifestations from the quantum era of
the Universe in the classical theory (due to the presence of the trace of the
energy-momentum tensor $T$). Our investigation unveils the viability of
$f(Q,T)$ gravity to describe the accelerated expansion our Universe passes
through. Besides, one of our models naturally provides a phantom regime for
dark energy and satisfies the dominant energy condition. The results here
derived strength the viability of $f(Q,T)$ as a promising complete theory of
gravity, lighting a new path towards the description of the dark sector of the
Universe. | gr-qc |
Crossing the Phantom Divide Line in a DGP-Inspired $F(R,φ)$-Gravity: We study possible crossing of the phantom divide line in a DGP-inspired
$F(R,\phi)$ braneworld scenario where scalar field and curvature quintessence
are treated in a unified framework. With some specific form of $F(R,\phi)$ and
by adopting a suitable ansatz, we show that there are appropriate regions of
the parameters space which account for late-time acceleration and admit
crossing of the phantom divide line. | gr-qc |
Global Portraits of Nonminimal Inflation: Metric and Palatini: In this paper, we study the global phase space dynamics of single
nonminimally coupled scalar field inflation models in the metric and Palatini
formalisms. Working in the Jordan frame, we derive the scalar-tensor general
field equations and flat FLRW cosmological equations, and present the Palatini
and metric equations in a common framework. We show that inflation is
characterized by a "master" trajectory from a saddle-type de Sitter fixed point
to a stable node fixed point, approximated by slow roll conditions (presented
for the first time in the Palatini formalism). We show that, despite different
underlying equations, the fixed point structure and properties of many models
are congruent in metric and Palatini, which explains their qualitative
similarities and their suitability for driving inflation. On the other hand,
the global phase portraits reveal how even models which predict the same values
for observable perturbations differ, both to the extent of the phase space
physically available to their trajectories, as well as their past asymptotic
states. We also note how the slow roll conditions tend to underestimate the end
of inflationary accelerated expansion experienced by the true nonlinear
"master" solution. The explicit examples we consider range from the metric and
Palatini induced gravity quintic potential with a Coleman-Weinberg correction
factor to Starobinsky, metric and Palatini nonminimal Higgs, second order pole,
and several nontrivial Palatini models. | gr-qc |
Spin fluids in Bianchi-I f(R)-cosmology with torsion: We study Weyssenhoff spin fluids in Bianchi type-I cosmological models,
within the framework of torsional f(R)-gravity; the resulting field equations
are derived and discussed in both Jordan and Einstein frames, clarifying the
role played by the spin and the non-linearity of the gravitational Lagrangian
f(R) in generating the torsional dynamical contributions. The general
conservation laws holding for f(R)-gravity with torsion are employed to provide
the conditions needed to ensure the preservation of the Hamiltonian constraint
and the consequent correct formulation of the associated initial value problem.
Examples are eventually given. | gr-qc |
A theorem on topologically massive gravity: We show that for three dimensional space-times admitting a hypersurface
orthogonal Killing vector field Deser, Jackiw and Templeton's vacuum field
equations of topologically massive gravity allow only the trivial flat
space-time solution. Thus spin is necessary to support topological mass. | gr-qc |
Higher dimensional thin-shell wormholes in
Einstein-Yang-Mills-Gauss-Bonnet gravity: We present thin-shell wormhole solutions in Einstein-Yang-Mills-Gauss-Bonnet
(EYMGB) theory in higher dimensions d\geq5. Exact black hole solutions are
employed for this purpose where the radius of thin-shell lies outside the event
horizon. For some reasons the cases d=5 and d>5 are treated separately. The
surface energy-momentum of the thin-shell creates surface pressures to resist
against collapse and rendering stable wormholes possible. We test the stability
of the wormholes against spherical perturbations through a linear
energy-pressure relation and plot stability regions. Apart from this restricted
stability we investigate the possibility of normal (i.e. non-exotic) matter
which satisfies the energy conditions. For negative values of the Gauss-Bonnet
(GB) parameter we obtain such physical wormholes. | gr-qc |
Parameter estimation from Gravitational waves generated by non-spinning
binary black holes with laser interferometers: beyond the Fisher information: In this paper we apply to gravitational waves from non-spinning binary
systems a recently intro- duced frequentist methodology to calculate
analytically the error for a maximum likelihood estimate (MLE) of physical
parameters. While existing literature focuses on using the Cramer Rao Lower
bound (CRLB) and Monte Carlo simulations, we use a power expansion of the bias
and covariance in inverse powers of the signal to noise ratio. The use of
higher order derivatives of the likelihood function in the expansions makes the
prediction also sensitive to the secondary lobes of the MLE probability
distribution. We discuss conditions for validity of the CRLB and predict new
features in regions of the parameter space currently not explored. For example,
we see how the bias can become the most important contributor to the
parameters' errors for high mass systems (200M and above). | gr-qc |
Equivalence-principle Analog of the Gravitational Redshift: What happens when two synchronized clocks on a rigid beam are both given the
exact same acceleration profile? Will they remain synchronized? What if we use
a rigid-rod Rindler acceleration profile? The special relativity prediction
surprises many people. This experimental setup is the special-relativity analog
of the gravitational redshift. Just like two clocks higher and lower in a
gravitational field lose synchronization, one sees a loss of synchronization in
these clocks with `identical' acceleration profiles. To the best of our
knowledge this equivalence principle analog has never been directly measured,
and current experimental techniques are sensitive enough to measure it. We
discuss the origin of the essential physics behind this synchronization loss,
and some special conditions which simplify its experimental observation. We
discuss the origin of the essential physics behind this synchronization loss,
and some special conditions which simplify its experimental observation. If
validated this effect will not only test the equivalence principle from a new
vantage, but it may one day aid in understanding and enhancing future
ultra-precise navigation systems. | gr-qc |
Cosmological models with a Hybrid Scale Factor: In this brief review, we present some cosmological models with a Hybrid Scale
Factor (HSF) in the framework of general relativity (GR). The hybrid scale
factor fosters an early deceleration as well as a late time acceleration and
mimics the present Universe. The dynamical aspects of different cosmological
models with HSF in the presence of different matter fields have been discussed. | gr-qc |
Accretion of perfect fluids onto a class of regular black holes: We consider the stationary spherical accretion process of perfect fluids onto
a class of spherically symmetric regular black holes corresponding to
quantum-corrected Schwarzschild spacetimes. We show that the accretion rates
can differ from the Schwarzschild case, suggesting that the de Sitter core
inside these regular black holes, which indeed precludes the central
singularity, can act for some cases as a sort of antigravitational source,
decreasing the fluid's radial infall velocity in the accretion process, and for
others as a gravitational enhancer, increasing the fluid flow into the black
hole horizon. Our analysis and results can be extended and also applied to the
problem of black hole evaporation in cosmological scenarios with phantom
fluids. In particular, we show that the mass of typical regular black holes can
be used in order to constrain turnaround events in cyclic cosmologies. | gr-qc |
Finite entanglement entropy from the zero-point-area of spacetime: The calculation of entanglement entropy S of quantum fields in spacetimes
with horizon shows that, quite generically, S (a) is proportional to the area A
of the horizon and (b) is divergent. I argue that this divergence, which arises
even in the case of Rindler horizon in flat spacetime, is yet another
indication of a deep connection between horizon thermodynamics and
gravitational dynamics. In an emergent perspective of gravity, which
accommodates this connection, the fluctuations around the equipartition value
in the area elements will lead to a minimal quantum of area, of the order of
L_P^2, which will act as a regulator for this divergence. In a particular
prescription for incorporating L_P^2 as zero-point-area of spacetime, this does
happen and the divergence in entanglement entropy is regularized, leading to S
proportional to (A/L_P^2) in Einstein gravity. In more general models of
gravity, the surface density of microscopic degrees of freedom is different
which leads to a modified regularisation procedure and the possibility that the
entanglement entropy - when appropriately regularised - matches the Wald
entropy. | gr-qc |
A strongly hyperbolic and regular reduction of Einstein's equations for
axisymmetric spacetimes: This paper is concerned exclusively with axisymmetric spacetimes. We want to
develop reductions of Einstein's equations which are suitable for numerical
evolutions. We first make a Kaluza-Klein type dimensional reduction followed by
an ADM reduction on the Lorentzian 3-space, the (2+1)+1 formalism. We include
also the Z4 extension of Einstein's equations adapted to this formalism. Our
gauge choice is based on a generalized harmonic gauge condition. We consider
vacuum and perfect fluid sources.
We use these ingredients to construct a strongly hyperbolic first-order
evolution system and exhibit its characteristic structure. This enables us to
construct constraint-preserving stable outer boundary conditions. We use
cylindrical polar coordinates and so we provide a careful discussion of the
coordinate singularity on axis. By choosing our dependent variables
appropriately we are able to produce an evolution system in which each and
every term is manifestly regular on axis. | gr-qc |
Asymptotic Behavior in Polarized {\bf T}$^2$-symmetric Vacuum Spacetimes: We use Fuchsian Reduction to study the behavior near the singularity of a
class of solutions of Einstein's vacuum equations. These solutions admit two
commuting spacelike Killing fields like the Gowdy spacetimes, but their twist
does not vanish. The spacetimes are also polarized in the sense that one of the
`gravitational degrees of freedom' is turned off. Examining an analytic family
of solutions with the maximum number of arbitrary functions, we find that they
are all asymptotically velocity-term dominated as one approaches the
singularity. | gr-qc |
Impact of the wave-like nature of Proca stars on their
gravitational-wave emission: We present a systematic study of the dynamics and gravitational-wave emission
of head-on collisions of spinning vector boson stars, known as Proca stars. To
this aim we build a catalogue of about 800 numerical-relativity simulations of
such systems. We find that the wave-like nature of bosonic stars has a large
impact on the gravitational-wave emission. In particular, we show that the
initial relative phase $\Delta \epsilon =\epsilon_1-\epsilon_2$ of the two
complex fields forming the stars (or equivalently, the relative phase at
merger) strongly impacts both the emitted gravitational-wave energy and the
corresponding mode structure. This leads to a non-monotonic dependence of the
emission on the frequency of the secondary star $\omega_2$, for fixed frequency
$\omega_1$ of the primary. This phenomenology, which has not been found for the
case of black-hole mergers, reflects the distinct ability of the Proca field to
interact with itself in both constructive and destructive manners. We postulate
this may serve as a smoking gun to shed light on the possible existence of
these objects. | gr-qc |
Resonantly enhanced and diminished strong-field gravitational-wave
fluxes: The inspiral of a stellar mass ($1 - 100\,M_\odot$) compact body into a
massive ($10^5 - 10^7\,M_\odot$) black hole has been a focus of much effort,
both for the promise of such systems as astrophysical sources of gravitational
waves, and because they are a clean limit of the general relativistic two-body
problem. Our understanding of this problem has advanced significantly in recent
years, with much progress in modeling the "self force" arising from the small
body's interaction with its own spacetime deformation. Recent work has shown
that this self interaction is especially interesting when the frequencies
associated with the orbit's $\theta$ and $r$ motions are in an integer ratio:
$\Omega_\theta/\Omega_r = \beta_\theta/\beta_r$, with $\beta_\theta$ and
$\beta_r$ both integers. In this paper, we show that key aspects of the self
interaction for such "resonant" orbits can be understood with a relatively
simple Teukolsky-equation-based calculation of gravitational-wave fluxes. We
show that fluxes from resonant orbits depend on the relative phase of radial
and angular motions. The purpose of this paper is to illustrate in simple terms
how this phase dependence arises using tools that are good for strong-field
orbits, and to present a first study of how strongly the fluxes vary as a
function of this phase and other orbital parameters. Future work will use the
full dissipative self force to examine resonant and near resonant strong-field
effects in greater depth, which will be needed to characterize how a binary
evolves through orbital resonances. | gr-qc |
The black hole that went away: A purported black hole solution in (2+1)-dimensions is shown to be nothing
more than flat space viewed from an accelerated frame. | gr-qc |
Incorporating information from LIGO data quality streams into the PyCBC
search for gravitational waves: We present a new method which accounts for changes in the properties of
gravitational-wave detector noise over time in the PyCBC search for
gravitational waves from compact binary coalescences. We use information from
LIGO data quality streams that monitor the status of each detector and its
environment to model changes in the rate of noise in each detector. These data
quality streams allow candidates identified in the data during periods of
detector malfunctions to be more efficiently rejected as noise. This method
allows data from machine learning predictions of the detector state to be
included as part of the PyCBC search, increasing the the total number of
detectable gravitational-wave signals by up to 5%. When both machine learning
classifications and manually-generated flags are used to search data from
LIGO-Virgo's third observing run, the total number of detectable
gravitational-wave signals is increased by up to 20% compared to not using any
data quality streams. We also show how this method is flexible enough to
include information from large numbers of additional arbitrary data streams
that may be able to further increase the sensitivity of the search. | gr-qc |
An approximate global solution of Einstein's equations for a finite body: We obtain an approximate global stationary and axisymmetric solution of
Einstein's equations which can be considered as a simple star model: a
self-gravitating perfect fluid ball with constant mass density rotating in
rigid motion. Using the post-Minkowskian formalism (weak-field approximation)
and considering rotation as a perturbation (slow-rotation approximation), we
find approximate interior and exterior (asymptotically flat) solutions to this
problem in harmonic and quo-harmonic coordinates. In both cases, interior and
exterior solutions are matched, in the sense of Lichnerowicz, on the surface of
zero pressure to obtain a global solution. The resulting metric depends on
three arbitrary constants: mass density, rotational velocity and the star
radius at the non-rotation limit. The mass, angular momentum, quadrupole moment
and other constants of the exterior metric are determined by these three
parameters. It is easy to show that this type of fluid cannot be a source of
the Kerr metric | gr-qc |
Thermoelastic Noise and Homogeneous Thermal Noise in Finite Sized
Gravitational-Wave Test Masses: An analysis is given of thermoelastic noise (thermal noise due to
thermoelastic dissipation) in finite sized test masses of laser interferometer
gravitational-wave detectors. Finite-size effects increase the thermoelastic
noise by a modest amount; for example, for the sapphire test masses tentatively
planned for LIGO-II and plausible beam-spot radii, the increase is less than or
of order 10 per cent. As a side issue, errors are pointed out in the currently
used formulas for conventional, homogeneous thermal noise (noise associated
with dissipation which is homogeneous and described by an imaginary part of the
Young's modulus) in finite sized test masses. Correction of these errors
increases the homogeneous thermal noise by less than or of order 5 per cent for
LIGO-II-type configurations. | gr-qc |
New Signals in Precision Gravity Tests and Beyond: We review the status of tests of spacetime symmetries with gravity. Recent
theoretical and experimental work has involved gravitational wave signals,
precision solar-system tests, and sensitive laboratory tests searching for
violations of spacetime symmetries. We present some new theoretical results
relevant for short-range gravity tests, with features of multiple length
scales, and possible large non-Newtonian forces at short distances. | gr-qc |
Friedmann Thermodynamics and the Geometry of the Universe: In a recent article we have introduced Friedmann thermodynamics, where
certain geometric parameters in Friedmann models are treated like their
thermodynamic counterparts (temperature, entropy, Gibbs potential etc.). This
model has the advantage of allowing us to determine the geometry of the
universe by thermodynamic stability arguments. In this article we review
connections between thermodynamics, geometry and cosmology. | gr-qc |
On the relation between ADM and Bondi energy-momenta III -- perturbed
radiative spatial infinity: In a vacuum spacetime equipped with the Bondi's radiating metric which is
asymptotically flat at spatial infinity including gravitational radiation ({\bf
Condition D}), we establish the relation between the ADM total energy-momentum
and the Bondi energy-momentum for perturbed radiative spatial infinity. The
perturbation is given by defining the "real" time the sum of the retarded time,
the Euclidean distance and certain function $f$. | gr-qc |
Jupiter, Saturn and the Pioneer anomaly: a planetary-based independent
test: In this paper we use the ratio of the corrections to the standard
Newtonian/Einsteinian secular precessions of the longitudes of perihelia of
Jupiter and Saturn, recently estimated by the Russian astronomer E.V. Pitjeva
by fitting almost one century of data with the EPM ephemerides, to make an
independent, planetary-based test of the hypothesis that the Pioneer anomaly
(PA), as it is presently known in the 5-10 AU region, is of gravitational
origin. Accounting for the errors in the determined apsidal extra-rates and in
the values of the PA acceleration at the orbits of Jupiter and Saturn the
answer is negative. If and when the re-analysis of the entire Pioneer 10/11
will be completed more firm conclusions could be reached. Moreover, it would
also be important that other teams of astronomers estimate independently their
own corrections to the perihelion precessions. | gr-qc |
Evolution of radial profiles in regular Lemaitre-Tolman-Bondi dust
models: We undertake a comprehensive and rigorous analytic study of the evolution of
radial profiles of covariant scalars in regular Lemaitre-Tolman-Bondi dust
models. We consider specifically the phenomenon of "profile inversions" in
which an initial clump profile of density, spatial curvature or the expansion
scalar, might evolve into a void profile (and vice versa). Previous work in the
literature on models with density void profiles and/or allowing for density
profile inversions is given full generalization, with some erroneous results
corrected. We prove rigorously that if an evolution without shell crossings is
assumed, then only the 'clump to void' inversion can occur in density profiles,
and only in hyperbolic models or regions with negative spatial curvature. The
profiles of spatial curvature follow similar patterns as those of the density,
with 'clump to void' inversions only possible for hyperbolic models or regions.
However, profiles of the expansion scalar are less restrictive, with profile
inversions necessarily taking place in elliptic models. We also examine radial
profiles in special LTB configurations: closed elliptic models, models with a
simultaneous big bang singularity, as well as a locally collapsing elliptic
region surrounded by an expanding hyperbolic background. The general analytic
statements that we obtain allow for setting up the right initial conditions to
construct fully regular LTB models with any specific qualitative requirements
for the profiles of all scalars and their time evolution. The results presented
can be very useful in guiding future numerical work on these models and in
revising previous analytic work on all their applications. | gr-qc |
Is conformal symmetry really anomalous?: The conformal anomaly (also known as the stress-energy trace anomaly) of an
interacting quantum theory, associated with violation of Weyl (conformal)
symmetry by quantum effects, can be amended if one endows the theory with a
dilatation current coupled to a vector field that is the gauge connection of
local Weyl symmetry transformations. The natural candidate for this Weyl
connection is the trace of the geometric torsion tensor, especially if one
recalls that pure (Cartan-Einstein) gravity with torsion is conformal. We first
point out that both canonical and path integral quantisation respect Weyl
symmetry. The only way quantum effects can violate conformal symmetry is by the
process of regularization. However, if one calculates an effective action from
a conformally invariant classical theory by using a regularisation procedure
that is conform with Weyl symmetry, then the conformal Ward identities will be
satisfied. In this sense Weyl symmetry is not broken by quantum effects. This
work suggests that Weyl symmetry can be treated on equal footing with gauge
symmetries and gravity, for which an infinite set of Ward identities guarantees
that they remain unbroken by quantum effects. | gr-qc |
On the energy of the de Sitter-Schwarzschild black hole: Using Einstein's and Weinberg's energy complex, we evaluate the energy
distribution of the vaccum nonsingularity black hole solution. The energy
distribution is positive everywhere and be equal to zero at origin. | gr-qc |
An Interacting model of Dark Energy in Brans-Dicke theory: In this paper it is shown that in non-minimally coupled Brans-Dicke theory
containing a self-interacting potential, a suitable conformal transformation
can automatically give rise to an interaction between the normal matter and the
Brans-Dicke scalar field. Considering the scalar field in the Einstein frame as
the quintessence matter, it has been shown that such a non-minimal coupling
between the matter and the scalar field can give rise to a late time
accelerated expansion for the universe preceded by a decelerated expansion for
very high values of the Brans-Dicke parameter $\omega$. We have also studied
the observational constraints on the model parameters considering the Hubble
and Supernova data. | gr-qc |
The Quantum Echo of the Early Universe: We show that the fluctuations of quantum fields as seen by late comoving
observers are significantly influenced by the history of the early Universe,
and therefore they transmit information about the nature of spacetime in
timescales when quantum gravitational effects were non-negligible. We discuss
how this may be observable even nowadays, and thus used to build falsifiability
tests of quantum gravity theories. | gr-qc |
Cosmological Models in Lyra Geometry: Kinematics Tests: In this paper the observational consequence of the cosmological models and
the expression for the neoclassical tests, luminosity distance, angular
diameter distance and look back time are analyzed in the framework of Lyra
geometry. It is interesting to note that the space time of the universe is not
only free of Big Bang singularity but also exhibits acceleration during its
evolution. | gr-qc |
Black hole induced spins from hyperbolic encounters in dense clusters: The black holes that have been detected via gravitational waves (GW) can have
either astrophysical or primordial origin. Some GW events show significant spin
for one of the components and have been assumed to be astrophysical, since
primordial black holes are generated with very low spins. However, it is worth
studying if they can increase their spin throughout the evolution of the
universe. Possible mechanisms that have already been explored are multiple
black hole mergers and gas accretion. We propose here a new mechanism that can
occur in dense clusters of black holes: the spin-up of primordial black holes
when they are involved in close hyperbolic encounters. We explore this effect
numerically with the Einstein Toolkit for different initial conditions,
including variable mass ratios. For equal masses, there is a maximum spin that
can be induced on the black holes, $\chi = a/m \leq 0.2$. We find however that
for large mass ratios one can attain spins up to $\chi \simeq 0.8$, where the
highest spin is induced on the most massive black hole. For small induced spins
we provide simple analytical expressions that depend on the relative velocity
and impact parameter. | gr-qc |
No Regularity Singularities Exist at Points of General Relativistic
Shock Wave Interaction between Shocks from Different Characteristic Families: We give a constructive proof that coordinate transformations exist which
raise the regularity of the gravitational metric tensor from $C^{0,1}$ to
$C^{1,1}$ in a neighborhood of points of shock wave collision in General
Relativity. The proof applies to collisions between shock waves coming from
different characteristic families, in spherically symmetric spacetimes. Our
result here implies that spacetime is locally inertial and corrects an error in
our earlier RSPA-publication, which led us to the false conclusion that such
coordinate transformations, which smooth the metric to $C^{1,1}$, cannot exist.
Thus, our result implies that regularity singularities, (a type of mild
singularity introduced in our RSPA-paper), do not exist at points of
interacting shock waves from different families in spherically symmetric
spacetimes. Our result generalizes Israel's celebrated 1966 paper to the case
of such shock wave interactions but our proof strategy differs fundamentally
from that used by Israel and is an extension of the strategy outlined in our
original RSPA-publication. Whether regularity singularities exist in more
complicated shock wave solutions of the Einstein Euler equations remains open. | gr-qc |
A Physical Interpretation of Gravitational Field Equations: It is possible to provide a thermodynamic interpretation for the field
equations in any diffeomorphism invariant theory of gravity. This insight, in
turn, leads us to the possibility of deriving the gravitational field equations
from another variational principle without using the metric as a dynamical
variable. I review this approach and discuss its implications. | gr-qc |
Comments on absorption cross section for Chern-Simons black holes in
five dimensions: In this paper we study the effects of black hole mass on the absorption cross
section for a massive scalar field propagating in a 5-dimensional topological
Chern-Simons black hole at the low-frequency limit. We consider the two
branches of black hole solutions $(\alpha=\pm 1)$ and we show that, if the mass
of black hole increase the absorption cross section decreases at the
zero-frequency limit for the branch $\alpha=-1$ and for the other branch,
$\alpha=1$, the behavior is opposite, if the black hole mass increase the
absorption cross section increases. Also we find that beyond a certain
frequency value, the mass black hole does not affect the absorption cross
section. | gr-qc |
Supersymmetric double Darboux method in quantum cosmology: We briefly present the supersymmetric double Darboux method and next apply it
to the continuum of the quantum Taub cosmological model as a toy model in order
to generate a one-parameter family of bosonic Taub potentials and the
corresponding wavefunctions | gr-qc |
Dirac particle in gravitation field: Being considered is the motion of Dirac particle in gravitational field,
described by Kerr solution. It is proved, that evolution of the wave function
is determined by Hermitian Hamiltonian, if the concomitant reference frame is
involved. | gr-qc |
The influence of differential rotation on the detectability of
gravitational waves from the r-mode instability: Recently, it was shown that differential rotation is an unavoidable feature
of nonlinear r-modes. We investigate the influence of this differential
rotation on the detectability of gravitational waves emitted by a newly born,
hot, rapidly-rotating neutron star, as it spins down due to the r-mode
instability. We conclude that gravitational radiation may be detected by the
advanced laser interferometer detector LIGO if the amount of differential
rotation at the time the r-mode instability becomes active is not very high. | gr-qc |
Geodesic Congruences and a Collapsing Stellar Distribution in f (T )
Theories: Teleparallel Gravity (TG) describes gravitation as a torsional- rather than
curvature-based effect. As in curvature-based constructions of gravity, several
different formulations can be proposed, one of which is the Teleparallel
equivalent of General Relativity (TEGR) which is dynamically equivalent to GR.
In this work, we explore the evolution of a spatially homogeneous collapsing
stellar body in the context of two important modifications to TEGR, namely f
(T) gravity which is the TG analogue of f (R) gravity, and a nonminimal
coupling with a scalar field which has become popular in TG for its effects in
cosmology. We explore the role of geodesic deviation to study the congruence of
nearby particles in lieu of the Raychaudhuri equation. We find f (T) models
that satisfy the null energy condition and describe interesting collapse
profiles. In the case of a nonminimally coupled scalar field, we also find
potential collapse models with intriguing scalar field evolution profiles. | gr-qc |
Curvature invariants of static spherically symmetric geometries: We construct all independent local scalar monomials in the Riemann tensor at
arbitrary dimension, for the special regime of static, spherically symmetric
geometries. Compared to general spaces, their number is significantly reduced:
the extreme example is the collapse of all invariants ~ Weyl^k, to a single
term at each k. The latter is equivalent to the Lovelock invariant L_k.
Depopulation is less extreme for invariants involving rising numbers of Ricci
tensors, and also depends on the dimension. The corresponding local
gravitational actions and their solution spaces are discussed. | gr-qc |
Chameleon scalar fields in relativistic gravitational backgrounds: We study the field profile of a scalar field $\phi$ that couples to a matter
fluid (dubbed a chameleon field) in the relativistic gravitational background
of a spherically symmetric spacetime. Employing a linear expansion in terms of
the gravitational potential $\Phi_c$ at the surface of a compact object with a
constant density, we derive the thin-shell field profile both inside and
outside the object, as well as the resulting effective coupling with matter,
analytically. We also carry out numerical simulations for the class of inverse
power-law potentials $V(\phi)=M^{4+n} \phi^{-n}$ by employing the information
provided by our analytical solutions to set the boundary conditions around the
centre of the object and show that thin-shell solutions in fact exist if the
gravitational potential $\Phi_c$ is smaller than 0.3, which marginally covers
the case of neutron stars. Thus the chameleon mechanism is present in the
relativistic gravitational backgrounds, capable of reducing the effective
coupling. Since thin-shell solutions are sensitive to the choice of boundary
conditions, our analytic field profile is very helpful to provide appropriate
boundary conditions for $\Phi_c \lesssim O(0.1)$. | gr-qc |
Black holes in nonlinear electrodynamics: quasi-normal spectra and
parity splitting: We discuss the quasi-normal oscillations of black holes which are sourced by
a nonlinear electrodynamic field. While previous studies have focused on the
computation of quasi-normal frequencies for the wave or higher spin equation on
a fixed background geometry described by such black holes, here we compute for
the first time the quasi-normal frequencies for the coupled
electromagnetic-gravitational linear perturbations.
To this purpose, we consider a parametrized family of Lagrangians for the
electromagnetic field which contains the Maxwell Lagrangian as a special case.
In the Maxwell case, the unique spherically symmetric black hole solutions are
described by the Reissner-Nordstr\"om family and in this case it is well-known
that the quasi-normal spectra in the even- and odd-parity sectors are identical
to each other. However, when moving away from the Maxwell case, we obtain
deformed Reissner-Nordstr\"om black holes, and we show that in this case there
is a parity splitting in the quasi-normal mode spectra. A partial explanation
for this phenomena is provided by considering the eikonal (high-frequency)
limit. | gr-qc |
Dynamical Evolution of a Cylindrical Shell with Rotational Pressure: We prepare a general framework for analyzing the dynamics of a cylindrical
shell in the spacetime with cylindrical symmetry. Based on the framework, we
investigate a particular model of a cylindrical shell-collapse with rotational
pressure, accompanying the radiation of gravitational waves and massless
particles. The model has been introduced previously but has been awaiting for
proper analysis. Here the analysis is put forward: It is proved that, as far as
the weak energy condition is satisfied outside the shell, the collapsing shell
bounces back at some point irrespective of the initial conditions, and escapes
from the singularity formation.
The behavior after the bounce depends on the sign of the shell pressure in
the z-direction. When the pressure is non-negative, the shell continues to
expand without re-contraction. On the other hand, when the pressure is negative
(i.e. it has a tension), the behavior after the bounce can be more complicated
depending on the details of the model. However, even in this case, the shell
never reaches the zero-radius configuration. | gr-qc |
The running vacuum in effective quantum gravity: We briefly review the previous works on the renormalization group in quantum
general relativity with the cosmological constant, based on the Vilkovisky and
DeWitt version of effective action. On top of that, we discuss the prospects of
the applications of this version of renormalization group to the cosmological
models with a running Newton constant and vacuum energy density. | gr-qc |
Gravitational waves in scalar-tensor theory to one-and-a-half
post-Newtonian order: We compute the gravitational waves generated by compact binary systems in a
class of massless scalar-tensor (ST) theories to the 1.5 post-Newtonian (1.5PN)
order beyond the standard quadrupole radiation in general relativity (GR).
Using and adapting to ST theories the multipolar-post-Minkowskian and
post-Newtonian formalisms originally defined in GR, we obtain the tail and
non-linear memory terms associated with the dipole radiation in ST theory. The
multipole moments and GW flux of compact binaries are derived for general
orbits including the new 1.5PN contribution, and comparison is made with
previous results in the literature. In the case of quasi-circular orbits, we
present ready-to-use templates for the data analysis of detectors, and for the
first time the scalar GW modes for comparisons with numerical relativity
results. | gr-qc |
On the existence of topological dyons and dyonic black holes in anti-de
Sitter Einstein-Yang-Mills theories with compact semisimple gauge groups: Here we study the global existence of `hairy' dyonic black hole and dyon
solutions to four dimensional, anti-de Sitter Einstein-Yang-Mills theories for
a general simply-connected and semisimple gauge group $G$, for so-called
topologically symmetric systems, concentrating here on the regular case. We
generalise here cases in the literature which considered purely magnetic
spherically symmetric solutions for a general gauge group and topological
dyonic solutions for $\mathfrak{su}(N)$. We are able to establish the global
existence of non-trivial solutions to all such systems, both near existing
embedded solutions and as $|\Lambda|\rightarrow\infty$. In particular, we can
identify non-trivial solutions where the gauge field functions have no zeroes,
which in the $\mathfrak{su}(N)$ case proved important to stability. We believe
that these are the most general analytically proven solutions in 4D anti-de
Sitter Einstein-Yang-Mills systems to date. | gr-qc |
Entanglement partners and monogamy in de Sitter universes: We investigate entanglement of local spatial modes defined by a quantum field
in a de Sitter universe. The introduced modes show dis-entanglement behavior
when the separation between two regions where local modes are assigned becomes
larger than the cosmological horizon. To understand the emergence of
separability between these local modes, we apply the monogamy inequality
proposed by S. Camalet. We embed the focusing bipartite mode defined by the
quantum field in a pure four-mode Gaussian state, and identify its partner
modes. Then applying a Gaussian version of the monogamy relation, we show that
the external entanglement between the bipartite mode and its partner modes
constrains the entanglement of the bipartite mode. Thus the emergence of
separability of local modes in the de Sitter universe can be understood from
the perspective of entanglement monogamy. | gr-qc |
Dark Energy from Gauss-Bonnet and non-minimal couplings: We consider a scalar-tensor model of dark energy with Gauss-Bonnet and
non-minimal couplings. Exact cosmological solutions were found in absence of
potential, that give equations of state of dark energy consistent with current
observational constraints, but with different asymptotic behaviors depending on
the couplings of the model. A detailed reconstruction procedure is given for
the scalar potential and the Gauss-Bonnet coupling for any given cosmological
scenario. Particularly, we consider conditions for the existence of a variety
of cosmological solutions with accelerated expansion, including quintessence,
phantom, de Sitter, Little Rip. For the case of quintessence and phantom we
have found a scalar potential of the Albrecht-Skordis type, where the potential
is an exponential with a polynomial factor. | gr-qc |
Observational Exclusion of a Consistent Quantum Cosmology Scenario: It is often argued that inflation erases all the information about what took
place before it started. Quantum gravity, relevant in the Planck era, seems
therefore mostly impossible to probe with cosmological observations. In
general, only very ad hoc scenarios or hyper fine-tuned initial conditions can
lead to observationally testable theories. Here we consider a well-defined and
well motivated candidate quantum cosmology model that predicts inflation. Using
the most recent observational constraints on the cosmic microwave background B
modes, we show that the model is excluded for all its parameter space, without
any tuning. Some important consequences are drawn for the deformed algebra
approach to loop quantum cosmology. We emphasize that neither loop quantum
cosmology in general nor loop quantum gravity are disfavored by this study but
their falsifiability is established. | gr-qc |
Constraining Barrow entropy-based Cosmology with power-law inflation: We study the inflationary era of the Universe in a modified cosmological
scenario based on the gravity-thermodynamics conjecture with Barrow entropy
instead of the usual Bekenstein-Hawking one. The former arises from the effort
to account for quantum gravitational effects on the horizon surface of black
holes and, in a broader sense, of the Universe. First, we extract modified
Friedmann equations from the first law of thermodynamics applied to the
apparent horizon of a Friedmann- Robertson-Walker Universe in (n +
1)-dimensions. Assuming a power-law behavior for the scalar inflaton field, we
then investigate how the inflationary dynamics is affected in Barrow
cosmological setup. We find that the inflationary era may phenomenologically
consist of the slow-roll phase, while Barrow entropy is incompatible with
kinetic inflation. By demanding observationally consistency of the scalar
spectral index and tensor-to-scalar ratio with recent Planck data, we finally
constrain Barrow exponent to $\Delta\lesssim10^{-4}$. | gr-qc |
Spherically-symmetric gravitational fields in the metric-affine gauge
theory of gravitation: Geometric structure of spherically-symmetric space-time in metric-affine
gauge theory of gravity is studied. Restrictions on curvature tensor and
Bianchi identities are obtained. By using certain simple gravitational
Lagrangian the solution of gravitational equations for vacuum
spherically-symmetric gravitational field is obtained. | gr-qc |
Bounce inflation with a conserved frame of rest: Some form of approximately exponential inflation is generally assumed to be
the origin of our present universe. The inflation is thought to be driven by a
scalar field potential where the field first slowly slides along the potential
and then comes to a steep slope where the field rapidly falls and then
oscillates around zero transforming into particles. The slowly sliding scalar
field inflation leads to an exponentially expanding de Sitter space. A scalar
field as well as the deSitter space are both Lorentz invariant. Thus no global
frame of rest can be established in this scenario, while particle creation
requires a preferred frame of rest. Observations of the cosmic microwave
background show, when the redshift is corrected for our local velocity, a very
even temperature and redshift distribution requiring a global preferred frame
of rest. We suggest here that a density dependent equilibrium relation between
matter/radiation and a scalar energy density could maintain a preferred frame
of rest throughout the bounce and inflation and thereby solve the problem. | gr-qc |
Static spherically symmetric solutions in New General Relativity: We give a pedagogical introduction to static spherically symmetric solutions
in models of New GR, both explaining the basics and showing how all such vacuum
solutions can be obtained in elementary functions. In doing so, we coherently
introduce the full landscape of these modified teleparallel spacetimes, and
find a few special cases. The equations of motion are turned into a very nice
and compact form by using the Levi-Civita divergence of the torsion-conjugate;
and generalised Bianchi identities are briefly discussed. Another important
point we make is that a convenient choice of the radial variable might be
instrumental for success of similar studies in other modified gravity models. | gr-qc |
4D spin-2 fields from 5D Chern-Simons theory: We consider a 5-dimensional Chern-Simons gauge theory for the isometry group
of Anti-de-Sitter spacetime,
$\operatorname{AdS}_{4+1}\simeq\operatorname{SO}(4,2)$, and invoke different
dimensional reduction schemes in order to relate it to 4-dimensional spin-2
theories. The AdS gauge algebra is isomorphic to a parametrized 4-dimensional
conformal algebra, and the gauge fields corresponding to the generators of
non-Abelian translations and special conformal transformations reduce to two
vierbein fields in $D=4$. Besides these two vierbeine, our reduction schemes
leave only the Lorentz spin connection as an additional dynamical field in the
4-dimensional theories. We identify the corresponding actions as particular
generalizations of Einstein-Cartan theory, conformal gravity and ghost-free
bimetric gravity in first-order form. | gr-qc |
Euclidean and Hamiltonian thermodynamics for regular black holes: We investigate the thermodynamic properties of the Hayward regular black hole
using both Euclidean path integral and Hamiltonian methods, in asymptotically
anti-de Sitter, Minkowski, and de Sitter spacetimes. With the inclusion of
matter fields which act as a source for the regular black hole geometry, an
effective temperature emerges that differs from the conventional definition
related to the Killing surface gravity. We posit that this temperature is the
appropriate choice for studying thermodynamic phenomena, by demonstrating
consistency between the Euclidean and Hamiltonian formulations in the
appropriate limits. We examine the thermodynamic properties and phase structure
of the Hayward black hole in the canonical ensemble and show that, counter to
some earlier indications, standard mean-field theory critical behavior is
observed when the cosmological constant is treated as a thermodynamic pressure.
We note the absence of a Hawking-Page transition, and conjecture that quantum
gravity corrections which are suitably strong to regulate the Schwarzschild
singularity generically prevent the transition from occurring. We also show
that the Smarr relation remains linear in all cases, despite the absence of a
linearity proof for nonlinear electrodynamic theories with nonsymmetry
inheriting fields. | gr-qc |
Thermodynamics of Spherically Symmetric Spacetimes in Loop Quantum
Gravity: The choice of the area operator in loop quantum gravity is by no means
unique. In addition to the area operator commonly used in loop quantum gravity
there is also an area operator introduced by Krasnov in 1998, which gives
uniformly spaced area spectra for the horizons of spacetime. Using Krasnov's
area operator we consider the thermodynamics of spherically symmetric
spacetimes equipped with horizons in loop quantum gravity. Among other things,
our approach implies, in a pretty simple manner, that every horizon of
spacetime emits thermal radiation and possesses emtropy which, in the natural
units, is one-quarter of its area. When applied to the de Sitter spacetime loop
quantum gravity provides an explanation both to the presence and the smallness
of the cosmological constant. | gr-qc |
Quantized Intrinsic Redshift in Cosmological General Relativity: There are now several analyses reporting quantized differences in the
redshifts between pairs of galaxies. In the simplest cases, these differential
redshifts are found to be harmonics of fundamental periods of approximately 72
km/s and 37.5 km/s. In this paper a wave equation is derived based on
cosmological general relativity, which is a space-velocity theory of the
expanding Universe. The wave equation is approximated to first order and
comparisons are made between the quantized solutions and the reported
observations. | gr-qc |
Features of gravity-Yang-Mills hierarchies in d-dimensions: Higher dimensional, direct analogues of the usual d=4 Einstein--Yang-Mills
(EYM) systems are studied. These consist of the gravitational and Yang-Mills
hierarchies in d=4p dimensional spacetimes, both consisting of 2p-form
curvature terms only. Regular and black hole solutions are constructed in
$2p+2\le d \le 4p$, in which dimensions the total mass-energy is finite,
generalising the familiar Bartnik-McKinnon solutions in EYM theory for p=1. In
d=4p, this similarity is complete. In the special case of d=2p+1, just beyond
the finite energy range of d, exact solutions in closed form are found.
Finally, d=2p+1 purely gravitational systems, whose solutions generalise the
static d=3 BTZ solutions, are discussed. | gr-qc |
Is the Strong Anthropic Principle Too Weak?: We discuss the Carter's formula about the mankind evolution probability
following the derivation proposed by Barrow and Tipler. We stress the relation
between the existence of billions of galaxies and the evolution of at least one
intelligent life, whose living time is not trivial, all over the Universe. We
show that the existence probability and the lifetime of a civilization depend
not only on the evolutionary critical steps, but also on the number of places
where the life can arise. In the light of these results, we propose a stronger
version of Anthropic Principle. | gr-qc |
A clarification on a common misconception about interferometric
detectors of gravitational waves: The aims of this letter are two. First, to show the angular gauge-invariance
on the response of interferometers to gravitational waves (GWs). In this
process, after resuming for completeness results on the Transverse-Traceless
(TT) gauge, where, in general, the theoretical computations on GWs are
performed, we analyse the gauge of the local observer, which represents the
gauge of a laboratory environment on Earth. The gauge-invariance between the
two gauges is shown in its full angular and frequency dependences. In previous
works in the literature this gauge-invariance was shown only in the low
frequencies approximation or in the simplest geometry of the interferometer
with respect to the propagating GW (i.e. both of the arms of the interferometer
are perpendicular to the propagating GW). Second, as far as the computation of
the response functions in the gauge of the local observer is concerned, a
common misconception about interferometers is also clarified. Such a
misconception purports that, as the wavelength of laser light and the length of
an interferometer's arm are both stretched by a GW, no effect should be
visible, invoking an analogy with cosmological redshift in an expanding
universe. | gr-qc |
Mimetic Compact Stars: Modified gravity models have been constantly proposed with the purpose of
evading some standard gravity shortcomings. Recently proposed by A.H.
Chamseddine and V. Mukhanov, the Mimetic Gravity arises as an optimistic
alternative. Our purpose in this work is to derive Tolman-Oppenheimer-Volkoff
equations and solutions for such a gravity theory. We solve them numerically
for quark star and neutron star cases. The results are carefully discussed. | gr-qc |
On the possible sources of gravitational wave bursts detectable today: We discuss the possibility that galactic gravitational wave sources might
give burst signals at a rate of several events per year, detectable by
state-of-the-art detectors. We are stimulated by the results of the data
collected by the EXPLORER and NAUTILUS bar detectors in the 2001 run, which
suggest an excess of coincidences between the two detectors, when the resonant
bars are orthogonal to the galactic plane. Signals due to the coalescence of
galactic compact binaries fulfill the energy requirements but are problematic
for lack of known candidates with the necessary merging rate. We examine the
limits imposed by galactic dynamics on the mass loss of the Galaxy due to GW
emission, and we use them to put constraints also on the GW radiation from
exotic objects, like binaries made of primordial black holes. We discuss the
possibility that the events are due to GW bursts coming repeatedly from a
single or a few compact sources. We examine different possible realizations of
this idea, such as accreting neutron stars, strange quark stars, and the highly
magnetized neutron stars (``magnetars'') introduced to explain Soft Gamma
Repeaters. Various possibilities are excluded or appear very unlikely, while
others at present cannot be excluded. | gr-qc |
General relativistic hydrodynamics in curvilinear coordinates: In this paper we report on what we believe is the first successful
implementation of relativistic hydrodynamics, coupled to dynamical spacetimes,
in spherical polar coordinates without symmetry assumptions. We employ a
high-resolution shock-capturing scheme, which requires that the equations be
cast in flux-conservative form. One example of such a form is the :Valencia"
formulation, which has been adopted in numerous applications, in particular in
Cartesian coordinates. Here we generalize this formulation to allow for a
reference-metric approach, which provides a natural framework for calculations
in curvilinear coordinates. In spherical polar coordinates, for example, it
allows for an analytical treatment of the singular r and sin(\theta) terms that
appear in the equations. We experiment with different versions of our
generalized Valencia formulation in numerical implementations of relativistic
hydrodynamics for both fixed and dynamical spacetimes. We consider a number of
different tests -- non-rotating and rotating relativistic stars, as well as
gravitational collapse to a black hole -- to demonstrate that our formulation
provides a promising approach to performing fully relativistic astrophysics
simulations in spherical polar coordinates. | gr-qc |
Seeing through the cosmological bounce: Footprints of the contracting
phase and luminosity distance in bouncing models: The evolution of the luminosity distance in a contracting universe is
studied. It is shown that for quite a lot of natural dynamical evolutions, its
behavior is far from trivial and its value can even decrease with an increasing
time interval between events. The consequences are investigated and it is
underlined that this could both put stringent consistency conditions on
bouncing models and open a new observational window on "pre Big Bang" physics
using standard gravitational waves. | gr-qc |
Diamond-Shaped Regions as Microcosmoi: We give a geometrically intrinsic construction of a global time function for
relatively compact diamond-shaped regions in arbitrary spacetimes. In the case
of Minkowski spacetime, the flow of diffeomorphisms associated to a suitably
normalized gradient of this time function becomes the conformal isotropy
subgroup of the diamond. In full generality, this time function is elegantly
expressed in terms of the Lorentzian distance function, and it has an
asymptotic behavior at large absolute times similar to the one in Minkowski
spacetime. | gr-qc |
Universe acceleration and nonlinear electrodynamics: A new model of nonlinear electrodynamics with a dimensional parameter $\beta$
coupled to gravity is considered. We show that an accelerated expansion of the
universe takes place if the nonlinear electromagnetic field is the source of
the gravitational field. A pure magnetic universe is investigated and the
magnetic field drives the universe to accelerate. In this model, after the big
bang, the universe undergoes inflation, and the accelerated expansion and then
decelerates approaching Minkowski spacetime asymptotically. We demonstrate the
causality of the model and a classical stability at the deceleration phase. | gr-qc |
Observational constraints on varying fundamental constants in a minimal
CPC model: A minimal model based on the Co-varying Physical Couplings (CPC) framework
for gravity is proposed. The CPC framework is based on the assumptions of a
metric-compatible four-dimensional Riemannian manifold where a covariantly
conserved stress-energy tensor acts as source of the field equations which are
formally the same as Einstein field equations, but where the couplings $\{ G,
c,\Lambda \}$ are allowed to vary simultaneously. The minimal CPC model takes
$\Lambda$ as a genuine constant while $c$ and $G$ vary in an entangled way that
is consistent with Bianchi identity and the aforementioned assumptions. The
model is constrained using the most recent galaxy cluster gas mass fraction
observational data. Our result indicates that the functions $c(z)$ and
$G\left(z\right)=G_{0}\left(c/c_{0}\right)^{4}$ are compatible with constant
couplings for the three different parameterizations of $c=c(z)$ adopted here. | gr-qc |
Locally Anisotropic Black Holes in Einstein Gravity: By applying the method of moving frames modelling one and two dimensional
local anisotropies we construct new solutions of Einstein equations on
pseudo-Riemannian spacetimes. The first class of solutions describes
non-trivial deformations of static spherically symmetric black holes to locally
anisotropic ones which have elliptic (in three dimensions) and ellipsoidal,
toroidal and elliptic and another forms of cylinder symmetries (in four
dimensions). The second class consists from black holes with oscillating
elliptic horizons. | gr-qc |
Double Images from a Single Black Hole: In the simulations of the multi-black holes and merging black holes a larger
primary image and a secondary smaller image which looks like an eyebrow and the
deformation of the shadows have been observed. However, this kind of
eyebrow-like structure was considered as unique feature of multi black hole
systems. In this paper, we illustrate the new result that in the case of
octupole distortions of a Schwarzschild black hole the local observer sees two
shadows or two images for this single black hole, i.e., also an eyebrow-like
structure. Presence of two images in our case is remarkable, as we have only
one black hole, however, the observer sees two dark images of this single black
hole. | gr-qc |
Imprints of dark matter on gravitational ringing of supermassive black
holes: Gravitational waves emitted from the gravitational ringing of supermassive
black holes are important targets to test general relativity and probe the
matter environment surrounding such black holes. The main components of the
ringing waveform are black hole quasi-normal modes. In this paper, we study the
effects of the dark matter halos with three different density profiles on the
gravitational polar (even-parity) perturbations of a supermassive black hole.
For this purpose, we first consider modified Schwarzschild spacetime with three
different dark matter profiles and derive the equation of motion of the polar
perturbations of the supermassive black hole. It is shown that by ignoring the
dark matter perturbations, a Zerilli-like master equation with a modified
potential for the polar perturbation can be obtained explicitly. Then we
calculate the complex frequencies of the quasi-normal modes of the supermassive
black hole in the dark matter halos. The corresponding gravitational wave
spectra with the effects of the dark matter halos and their detectability have
also been discussed. | gr-qc |
Excision boundary conditions for the conformal metric: Shibata, Ury\=u and Friedman recently suggested a new decomposition of
Einstein's equations that is useful for constructing initial data. In contrast
to previous decompositions, the conformal metric is no longer treated as a
freely-specifiable variable, but rather is determined as a solution to the
field equations. The new set of freely-specifiable variables includes only
time-derivatives of metric quantities, which makes this decomposition very
attractive for the construction of quasiequilibrium solutions. To date, this
new formalism has only been used for binary neutron stars. Applications
involving black holes require new boundary conditions for the conformal metric
on the domain boundaries. In this paper we demonstrate how these boundary
conditions follow naturally from the conformal geometry of the boundary
surfaces and the inherent gauge freedom of the conformal metric. | gr-qc |
Study of Embedded Class-I Fluid Spheres in $f(R,T)$ Gravity with
Karmarkar Condition: In this article, we explore some emerging properties of the stellar objects
in the frame of the $f(R,T)$ gravity by employing the well-known Karmarkar
condition, where $R$ and $T$ represent Ricci scalar and trace of energy
momentum tensor respectively. It is worthy to highlight here that we assume the
exponential type model of $f(R,T)$ theory of gravity $f(R,T)=R+\alpha(e^{-\beta
R}-1)+\gamma T$ along with the matter Lagrangian
$\mathcal{L}_{m}=-\frac{1}{3}(p_{r}+2 p_{t})$ to classify the complete set of
modified field equations. We demonstrate the embedded class-I technique by
using the static spherically symmetric line element along with anisotropic
fluid matter distribution. Further, to achieve our goal, we consider a specific
expression of metric potential $g_{rr}$, already presented in literature, and
proceed by using the Karmarkar condition to obtain the second metric potential.
In particular, we use four different compact stars, namely $LMC~X-4,$
$EXO~1785-248,$ $Cen~X-3$ and $4U~1820-30$ and compute the corresponding values
of the unknown parameters appearing in metric potentials. Moreover, we conduct
various physical evolutions such as graphical nature of energy density and
pressure progression, energy constraints, mass function, adiabatic index,
stability and equilibrium conditions to ensure the viability and consistency of
our proposed model. Our analysis indicates that the obtained anisotropic
outcomes are physically acceptable with the finest degree of accuracy. | gr-qc |
Cosmological attractor inflation from the RG-improved Higgs sector of
finite gauge theory: The possibility to construct an inflationary scenario for
renormalization-group improved potentials corresponding to the Higgs sector of
finite gauge models is investigated. Taking into account quantum corrections to
the renormalization-group potential which sums all leading logs of perturbation
theory is essential for a successful realization of the inflationary scenario,
with very reasonable parameter values. The inflationary models thus obtained
are seen to be in good agreement with the most recent and accurate
observational data. More specifically, the values of the relevant inflationary
parameters, $n_s$ and $r$, are close to the corresponding ones in the $R^2$ and
Higgs-driven inflation scenarios. It is shown that the model here constructed
and Higgs-driven inflation belong to the same class of cosmological attractors. | gr-qc |
Higher-dimensional black holes with multiple equal rotations: We study a limit of the Kerr-(A)dS spacetime in a general dimension where an
arbitrary number of its rotational parameters is set equal. The resulting
metric after the limit formally splits into two parts - the first part has the
form of the Kerr-NUT-(A)dS metric analogous to the metric of the entire
spacetime, but only for the directions not subject to the limit, and the second
part can be interpreted as the K\"{a}hler metrics. However, this separation is
not integrable, thus it does not lead to a product of independent manifolds. We
also reconstruct the original number of explicit and hidden symmetries
associated with Killing vectors and Killing tensors. Therefore, the resulting
spacetime represents a special subcase of the generalized Kerr-NUT-(A)dS metric
that retains the full Killing tower of symmetries. In $D=6$, we present
evidence of an enhanced symmetry structure after the limit. Namely, we find
additional Killing vectors and show that one of the Killing tensors becomes
reducible as it can be decomposed into Killing vectors. | gr-qc |
Conditions for negative specific heat in systems of attracting classical
particles: We identify conditions for the presence of negative specific heat in
non-relativistic self-gravitating systems and similar systems of attracting
particles.
The method used, is to analyse the Virial theorem and two soluble models of
systems of attracting particles, and to map the sign of the specific heat for
different combinations of the number of spatial dimensions of the system,
$D$($\geq 2$), and the exponent, $\nu$($\neq 0$), in the force potential,
$\phi=Cr^\nu$. Negative specific heat in such systems is found to be present
exactly for $\nu=-1$, at least for $D \geq 3$. For many combinations of $D$ and
$\nu$ representing long-range forces, the specific heat is positive or zero,
for both models and the Virial theorem. Hence negative specific heat is not
caused by long-range forces as such. We also find that negative specific heat
appears when $\nu$ is negative, and there is no singular point in a certain
density distribution. A possible mechanism behind this is suggested. | gr-qc |
Cosmology of gravitational vacuum: Production of gravitational vacuum defects and their contribution to the
energy density of our Universe are discussed. These topological microstructures
(defects) could be produced in the result of creation of the Universe from
"nothing" when a gravitational vacuum condensate has appeared. They must be
isotropically distributed over the isotropic expanding Universe. After Universe
inflation these microdefects are smoothed, stretched and broken up. A part of
them could survive and now they are perceived as the structures of Lambda-term
and an unclustered dark matter. It is shown that the parametrization
noninvariance of the Wheeler-De Witt equation can be used to describe
phenomenologically vacuum topological defects of different dimensions
(worm-holes, micromembranes, microstrings and monopoles). The mathematical
illustration of these processes may be the spontaneous breaking of the local
Lorentz-invariance of the quasi-classical equations of gravity. Probably the
gravitational vacuum condensate has fixed time in our Universe. Besides,
3-dimensional topological defects renormalize Lambda-term. | gr-qc |
Linking Covariant and Canonical General Relativity via Local Observers: Hamiltonian gravity, relying on arbitrary choices of "space," can obscure
spacetime symmetries. We present an alternative, manifestly spacetime covariant
formulation that nonetheless distinguishes between "spatial" and "temporal"
variables. The key is viewing dynamical fields from the perspective of a field
of observers -- a unit timelike vector field that also transforms under local
Lorentz transformations. On one hand, all fields are spacetime fields,
covariant under spacetime symmeties. On the other, when the observer field is
normal to a spatial foliation, the fields automatically fall into Hamiltonian
form, recovering the Ashtekar formulation. We argue this provides a bridge
between Ashtekar variables and covariant phase space methods. We also outline a
framework where the 'space of observers' is fundamental, and spacetime geometry
itself may be observer-dependent. | gr-qc |
Asymptotically flat black holes with scalar hair: a review: We consider the status of black hole solutions with non-trivial scalar fields
but no gauge fields, in four dimensional asymptotically flat space-times,
reviewing both classical results and recent developments. We start by providing
a simple illustration on the physical difference between black holes in
electro-vacuum and scalar-vacuum. Next, we review no-scalar-hair theorems. In
particular, we detail an influential theorem by Bekenstein and stress three key
assumptions: 1) the type of scalar field equation; 2) the spacetime symmetry
inheritance by the scalar field; 3) an energy condition. Then, we list regular
(on and outside the horizon), asymptotically flat BH solutions with scalar
hair, organizing them by the assumption which is violated in each case and
distinguishing primary from secondary hair. We provide a table summary of the
state of the art. | gr-qc |
The role of elliptic integrals in calculating the gravitational lensing
of a charged Weyl black hole surrounded by plasma: In this paper, we mainly aim at highlighting the importance of
(hyper-)elliptic integrals in the study of gravitational effects caused by
strongly gravitating systems. For this, we study the application of elliptic
integrals in calculating the light deflection as it passes a plasmic medium,
surrounding a charged Weyl black hole. To proceed with this, we consider two
specific algebraic ansatzes for the plasmic refractive index, and we
characterize the photon sphere for each of the cases. This will be used further
to calculate the angular diameter of the corresponding black hole shadow. We
show that the complexity of the refractive index expressions, can result in
substantially different types of dependencies of the light behavior on the
spacetime parameters. | gr-qc |
Anisotropic dark energy stars: A model of compact object coupled to inhomogeneous anisotropic dark energy is
studied. It is assumed a variable dark energy that suffers a phase transition
at a critical density. The anisotropic Lambda-Tolman-Oppenheimer-Volkoff
equations are integrated to know the structure of these objects. The anisotropy
is concentrated on a thin shell where the phase transition takes place, while
the rest of the star remains isotropic. The family of solutions obtained
depends on the coupling parameter between the dark energy and the fermion
matter. The solutions share several features in common with the gravastar
model. There is a critical coupling parameter that gives non-singular black
hole solutions. The mass-radius relations are studied as well as the internal
structure of the compact objects. The hydrodynamic stability of the models is
analyzed using a standard test from the mass-radius relation. For each
permissible value of the coupling parameter there is a maximum mass, so the
existence of black holes is unavoidable within this model. | gr-qc |
Effect of gravitational radiation reaction on circular orbits around a
spinning black hole: The effect of gravitational radiation reaction on circular orbits around a
spinning (Kerr) black hole is computed to leading order in $S$ (the magnitude
of the spin angular momentum of the hole) and in the strength of gravity $M/r$
(where $M$ is the mass of the black hole, $r$ is the orbital radius, and
$G=c=1$). The radiation reaction makes the orbit shrink but leaves it circular,
and drives the orbital plane very slowly toward antialignment with the spin of
the hole: $\tan (\iota /2) = \tan (\iota_0 /2) [1+(61/72)(S/M^2) (M/r)^{3/2}]$,
where $\iota$ is the angle between the normal to the orbital plane and the spin
direction, and $\iota_0$ is the initial value of $\iota$, when $r$ is very
large. | gr-qc |
Compact objects in general relativity: From Buchdahl stars to quasiblack
holes: A Buchdahl star is a highly compact star for which the boundary radius $R$
obeys $R=\frac98 r_+$, where $r_+$ is the gravitational radius of the star
itself. A quasiblack hole is a maximum compact star, or more generically a
maximum compact object, for which the boundary radius $R$ obeys $R=r_+$.
Quasiblack holes are objects on the verge of becoming black holes. Continued
gravitational collapse ends in black holes and has to be handled with the
Oppenheimer-Snyder formalism. Quasistatic contraction ends in a quasiblack hole
and should be treated with appropriate techniques. Quasiblack holes, not black
holes, are the real descendants of Mitchell and Laplace dark stars. Quasiblack
holes have many interesting properties. We develop the concept of a quasiblack
hole, give several examples of such an object, define what it is, draw its
Carter-Penrose diagram, study its pressure properties, obtain its mass formula,
derive the entropy of a nonextremal quasiblack hole, and through an extremal
quasiblack hole give a solution to the puzzling entropy of extremal black
holes. | gr-qc |
On the discrete version of the Schwarzschild problem: We consider a Schwarzschild type solution in the discrete Regge calculus
formulation of general relativity quantized within the path integral approach.
Earlier, we found a mechanism of a loose fixation of the background scale of
Regge lengths. This elementary length scale is defined by the Planck scale and
some free parameter of such a quantum extension of the theory. Besides, Regge
action was reduced to an expansion over metric variations between the
tetrahedra and, in the main approximation, is a finite-difference form of the
Hilbert-Einstein action. Using for the Schwarzschild problem a priori general
non-spherically symmetrical ansatz, we get finite-difference equations for its
discrete version. This defines a solution which at large distances is close to
the continuum Schwarzschild geometry, and the metric and effective curvature at
the center are cut off at the elementary length scale. Slow rotation can also
be taken into account (Lense-Thirring-like metric). Thus we get a general
approach to the classical background in the quantum framework in zero order: it
is an optimal starting point for the perturbative expansion of the theory;
finite-difference equations are classical, the elementary length scale has
quantum origin. Singularities, if any, are resolved. | gr-qc |
Exact solutions of an anisotropic universe in a modified teleparallel
gravity model via the Noether and B.N.S. approaches: In this paper, we present the Noether symmetries of locally rotationally
symmetric Bianchi type I (LRS BI), an anisotropic model, in the context of the
teleparallel gravity. We study a certain modified teleparallel theory based on
the action that, in particular, contains a coupling between the scalar field
and field strength (magnetism part). We derive the symmetry generators and show
that, by means of cyclic variables approach, we can not obtain a suitable
solution for field equations. Hence by the use of B.N.S. approach, we solve the
equations which carry Noether currents as well. By data analysis of the
obtained results, we show compatible results with observational data at the
last half the age of universe which is accelerating. | gr-qc |
Quantum Gravity/String/M-theory/ as we approach the 3rd Millennium: I review some recent progress in String/M-theory | gr-qc |
Constructing $p,n$-forms from $p$-forms via the Hodge star operator and
the exterior derivative: In this paper, we aim to explore the properties and applications on the
operators consisting of the Hodge star operator together with the exterior
derivative, whose action on an arbitrary $p$-form field in $n$-dimensional
spacetimes makes its form degree remain invariant. Such operations are able to
generate a variety of $p$-forms with the even-order derivatives of the
$p$-form. To do this, we first investigate the properties of the operators,
such as the Laplace-de Rham operator, the codifferential and their
combinations, as well as the applications of the operators in the construction
of conserved currents. On basis of two general p-forms, then we construct a
general n-form with higher-order derivatives. Finally, we propose that such an
n-form could be applied to define a generalized Lagrangian with respect to a
p-form field according to the fact that it incudes the ordinary Lagrangians for
the $p$-form and scalar fields as special cases. | gr-qc |
Gravity: a gauge theory perspective: The evolution of a generally covariant theory is under-determined. One
hundred years ago such dynamics had never before been considered; its
ramifications were perplexing, its future important role for all the
fundamental interactions under the name gauge principle could not be foreseen.
We recount some history regarding Einstein, Hilbert, Klein and Noether and the
novel features of gravitational energy that led to Noether's two theorems.
Under-determined evolution is best revealed in the Hamiltonian formulation. We
developed a covariant Hamiltonian formulation. The Hamiltonian boundary term
gives covariant expressions for the quasi-local energy, momentum and angular
momentum. Gravity can be considered as a gauge theory of the local Poincar\'e
group. The dynamical potentials of the Poincar\'e gauge theory of gravity are
the frame and the connection. The spacetime geometry has in general both
curvature and torsion. Torsion naturally couples to spin; it could have a
significant magnitude and yet not be noticed, except on a cosmological scale
where it could have significant effects. | gr-qc |
Covariant Spin Structure: Every Dirac spin structure on a world manifold is associated with a certain
gravitational field, and is not preserved under general covariant
transformations. We construct a composite spinor bundle such that any Dirac
spin structure is its subbundle, and this bundle admits general covariant
transformations. | gr-qc |
Weak cosmic censorship conjecture in BTZ black holes with scalar fields: The weak cosmic censorship conjecture in the near-extremal BTZ black hole has
been tested by the test particles and fields. It was claimed that this black
hole could be overspun. In this paper, we review the thermodynamics and weak
cosmic censorship conjecture in BTZ black holes by the scattering of the scalar
field. The first law of thermodynamics in the non-extremal BTZ black hole is
recovered. For the extremal and near-extremal black holes, due to the
divergence of the variation of the entropy, we test the weak cosmic censorship
conjecture by evaluating the minimum values of the function $f$. Both of the
extremal and near-extremal black holes cannot be overspun. | gr-qc |
Stability and Instability of Extreme Reissner-Nordström Black Hole
Spacetimes for Linear Scalar Perturbations II: This paper contains the second part of a two-part series on the stability and
instability of extreme Reissner-Nordstrom spacetimes for linear scalar
perturbations. We continue our study of solutions to the linear wave equation
on a suitable globally hyperbolic subset of such a spacetime, arising from
regular initial data prescribed on a Cauchy hypersurface crossing the future
event horizon. We here obtain definitive energy and pointwise decay, non-decay
and blow-up results. Our estimates hold up to and including the horizon. A
hierarchy of conservations laws on degenerate horizons is also derived. | gr-qc |
Spectral Geometry and Causality: For a physical interpretation of a theory of quantum gravity, it is necessary
to recover classical spacetime, at least approximately. However, quantum
gravity may eventually provide classical spacetimes by giving spectral data
similar to those appearing in noncommutative geometry, rather than by giving
directly a spacetime manifold. It is shown that a globally hyperbolic
Lorentzian manifold can be given by spectral data. A new phenomenon in the
context of spectral geometry is observed: causal relationships. The employment
of the causal relationships of spectral data is shown to lead to a highly
efficient description of Lorentzian manifolds, indicating the possible
usefulness of this approach. Connections to free quantum field theory are
discussed for both motivation and physical interpretation. It is conjectured
that the necessary spectral data can be generically obtained from an effective
field theory having the fundamental structures of generalized quantum
mechanics: a decoherence functional and a choice of histories. | gr-qc |
Gravitation and vacuum entanglement entropy: The vacuum of quantum fields contains correlated fluctuations. When
restricted to one side of a surface these have a huge entropy of entanglement
that scales with the surface area. If UV physics renders this entropy finite,
then a thermodynamic argument implies the existence of gravity. That is, the
causal structure of spacetime must be dynamical and governed by the Einstein
equation with Newton's constant inversely proportional to the entropy density.
Conversely, the existence of gravity makes the entanglement entropy finite.
This thermodynamic reasoning is powerful despite the lack of a detailed
description of the dynamics at the cutoff scale, but it has its limitations. In
particular, we should not expect to understand corrections to Einstein gravity
in this way. | gr-qc |
Scalar field propagation in higher dimensional black holes at a Lifshitz
point: We study the complete time evolution of scalar fields propagating in
space-times of higher dimensional Lifshitz Black Holes with dynamical critical
exponent $z=2$, obtained from a theory including the most general quadratic
curvature corrections to Einstein-Hilbert gravity in $D$ dimensions. We also
computed the quasinormal spectrum after performing a numerical integration and
solving exactly the Klein-Gordon equation obeyed by the massive scalar field.
We found that quasinormal modes are purely imaginary for all dimensions. | gr-qc |
Behaviour of spin-half particles in curved space-time: We study the behaviour of spin-half particles in curved space-time. Since
Dirac equation gives the dynamics of spin-half particles, we mainly study the
Dirac equation in Schwarzschild, Kerr, Reissner-Nordstr\"om geometry. Due to
the consideration of existence of black hole in space-time (the curved
space-time), particles are influenced and equation will be modified. As a
result the solution will be changed from that due to flat space. | gr-qc |
The black hole challenge in Randall-Sundrum II model: Models postulating the existence of additional spacelike dimensions of
macroscopic or even infinite size, while viewing our observable universe as
merely a 3-brane living in a higher-dimensional bulk were a major breakthrough
when proposed some 15 years ago. The most interesting among them both in terms
of elegance of the setup and of the richness of the emerging phenomenology is
the Randall-Sundrum II model where one infinite extra spacelike dimension is
considered with an AdS topology, characterized by the warping effect caused by
the presence of a negative cosmological constant in the bulk. A major drawback
of this model is that despite numerous efforts no line element has ever been
found that could describe a stable, regular, realistic black hole. Finding a
smoothly behaved such solution supported by the presence of some more or less
conventional fields either in the bulk and/or on the brane is the core of the
black hole challenge. After a comprehensive presentation of the details of the
model and the analysis of the significance and the utility of getting a
specific analytic black hole solution, several (unsuccessful) analytic and
numerical approaches to the problem developed over the years are presented with
some discussion about their results. The chapter closes with the latest
numerical results that actually consists a major advancement in the effort to
address the challenge, the presentation of the most recent analytic work trying
(and unfortunately failing) to build a solution assuming the existence of
unconventional scalar fields and some ideas about the routes the forthcoming
analytic approaches should explore. | gr-qc |
A no-go result for covariance in models of loop quantum gravity: Based on the observation that the exterior space-times of Schwarzschild-type
solutions allow two symmetric slicings, a static spherically symmetric one and
a timelike homogeneous one, modifications of gravitational dynamics suggested
by symmetry-reduced models of quantum cosmology can be used to derive
corresponding modified spherically symmetric equations. Generally covariant
theories are much more restricted in spherical symmetry compared with
homogeneous slicings, given by $1+1$-dimensional dilaton models if they are
local. As shown here, modifications used in loop quantum cosmology do not have
a corresponding covariant spherically symmetric theory. Models of loop quantum
cosmology therefore violate general covariance in the form of slicing
independence. Only a generalized form of covariance with a non-Riemannian
geometry could consistently describe space-time in models of loop quantum
gravity. | gr-qc |
How often does the Unruh-DeWitt detector click beyond four dimensions?: We analyse the response of an arbitrarily-accelerated Unruh-DeWitt detector
coupled to a massless scalar field in Minkowski spacetimes of dimensions up to
six, working within first-order perturbation theory and assuming a smooth
switch-on and switch-off. We express the total transition probability as a
manifestly finite and regulator-free integral formula. In the sharp switching
limit, the transition probability diverges in dimensions greater than three but
the transition rate remains finite up to dimension five. In dimension six, the
transition rate remains finite in the sharp switching limit for trajectories of
constant scalar proper acceleration, including all stationary trajectories, but
it diverges for generic trajectories. The divergence of the transition rate in
six dimensions suggests that global embedding spacetime (GEMS) methods for
investigating detector response in curved spacetime may have limited validity
for generic trajectories when the embedding spacetime has dimension higher than
five. | gr-qc |
Gravitational Waves and Their Memory in General Relativity: General relativity explains gravitational radiation from binary black hole or
neutron star mergers, from core-collapse supernovae and even from the inflation
period in cosmology. These waves exhibit a unique effect called memory or
Christodoulou effect, which in a detector like LIGO or LISA shows as a
permanent displacement of test masses and in radio telescopes like NANOGrav as
a change in the frequency of pulsars' pulses. It was shown that electromagnetic
fields and neutrino radiation enlarge the memory. Recently it has been
understood that the two types of memory addressed in the literature as `linear'
and `nonlinear' are in fact two different phenomena. The former is due to
fields that do not and the latter is due to fields that do reach null infinity. | gr-qc |
Circularization vs. Eccentrification in Intermediate Mass Ratio
Inspirals inside Dark Matter Spikes: Inspirals of an Intermediate Mass Black Hole (IMBH) and a solar mass type
object will be observable by space based gravitational wave detectors such as
The Laser Interferometer Space Antenna (LISA). A dark matter overdensity around
an IMBH - a dark matter spike - can affect the orbital evolution of the system.
We consider here such Intermediate Mass Ratio Inspirals on eccentric orbits,
experiencing dynamical friction of the dark matter spike. We find that by
including the phase space distribution of the dark matter, the dynamical
friction tends to circularize the orbit, in contrast to previous inquiries. We
derive a general condition for circularization or eccentrification for any
given dissipative force. In addition to the dephasing, we suggest using the
circularization rate as another probe of the dark matter spike. Observing these
effects would be an indicator for the particle nature of dark matter. | gr-qc |
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