529 research outputs found
Approximate Waveforms for Extreme-Mass-Ratio Inspirals in Modified Gravity Spacetimes
Extreme-mass-ratio inspirals, in which a stellar-mass compact object spirals
into a supermassive black hole, are prime candidates for detection with
space-borne milliHertz gravitational wave detectors, similar to the Laser
Interferometer Space Antenna. The gravitational waves generated during such
inspirals encode information about the background in which the small object is
moving, providing a tracer of the spacetime geometry and a probe of
strong-field physics. In this paper, we construct approximate,
"analytic-kludge" waveforms for such inspirals with parameterized
post-Einsteinian corrections that allow for generic, model-independent
deformations of the supermassive black hole background away from the Kerr
metric. These approximate waveforms include all of the qualitative features of
true waveforms for generic inspirals, including orbital eccentricity and
relativistic precession. The deformations of the Kerr metric are modeled using
a recently proposed, modified gravity bumpy metric, which parametrically
deforms the Kerr spacetime while ensuring that three approximate constants of
the motion remain for geodesic orbits: a conserved energy, azimuthal angular
momentum and Carter constant. The deformations represent modified gravity
effects and have been analytically mapped to several modified gravity black
hole solutions in four dimensions. In the analytic kludge waveforms, the
conservative motion is modeled by a post-Newtonian expansion of the geodesic
equations in the deformed spacetimes, which in turn induce modifications to the
radiation-reaction force. These analytic-kludge waveforms serve as a first step
toward complete and model-independent tests of General Relativity with extreme
mass-ratio inspirals.Comment: v1: 28 pages, no figures; v2: minor changes for consistency with
accepted version, 2 figures added showing sample waveforms; accepted by Phys.
Rev.
Immune evasion of the CD1d/NKT cell axis
Many reviews on the CD1d/NKT cell axis focus on the ability of CD1d-restricted NKT cells to serve as effector cells in a variety of disorders, be they infectious diseases, cancer or autoimmunity. In contrast, here, we discuss the ways that viruses, bacteria and tumor cells can evade the CD1d/NKT cell axis. As a result, these disease states have a better chance to establish a foothold and potentially cause problems for the subsequent adaptive immune response, as the host tries to rid itself of infections or tumors
Constraining alternative theories of gravity using pulsar timing arrays
The opening of the gravitational wave window by ground-based laser
interferometers has made possible many new tests of gravity, including the
first constraints on polarization. It is hoped that within the next decade
pulsar timing will extend the window by making the first detections in the
nano-Hertz frequency regime. Pulsar timing offers several advantages over
ground-based interferometers for constraining the polarization of gravitational
waves due to the many projections of the polarization pattern provided by the
different lines of sight to the pulsars, and the enhanced response to
longitudinal polarizations. Here we show that existing results from pulsar
timing arrays can be used to place stringent limits on the energy density of
longitudinal stochastic gravitational waves. Paradoxically however, we find
that longitudinal modes will be very difficult to detect due to the large
variance in the pulsar-pulsar correlation patterns for these modes. Existing
upper limits on the power spectrum of pulsar timing residuals imply that the
amplitude of vector longitudinal and scalar longitudinal modes at frequencies
of 1/year are constrained: and , while the bounds on the energy density for a
scale invariant cosmological background are: and .Comment: 5 pages, 4 figure
Improved gravitational-wave constraints on higher-order curvature theories of gravity
Gravitational wave observations of compact binaries allow us to test general relativity (and modifications thereof) in the strong and highly-dynamical field regime of gravity. Here we confront two extensions to general relativity, dynamical Chern-Simons and Einstein-dilaton-Gauss-Bonnet theories, against the gravitational wave sources from the GWTC-1 and GWTC-2 catalogs by the LIGO-Virgo Collaboration. By stacking the posterior of individual events, we strengthen the constraint on the square root of the coupling parameter in Einstein-dilaton-Gauss-Bonnet gravity to km, but we are unable to place meaningful constraints on dynamical Chern-Simons gravity. Importantly, we also show that our bounds are robust to (i) the choice of general-relativity base waveform model, upon which we add modifications, (ii) unknown higher post-Newtonian order terms in the modifications to general relativity, (iii) the small-coupling approximation, and (iv) uncertainties on the nature of the constituent compact objects
Analysis of the Environmental Factors Affecting the Growth Traits of Iran-Black Sheep
A study was conducted to evaluate the effects of non-genetic factors on the growth behavior of Iran-Black sheep. The data of growth performances, birth weight (BW), weaning weight (W3), weight at 6, 9and 12 months of age (W6, W9 and W12, respectively), were taken from 1522 lambs belonging to data bank from Abbas Abad Sheep Breeding Station located at the North-east of Iran during a period of five years. Statistical analyses were performed using a general linear model including non-genetic factors: lamb sex, birth year and litter size as main effects, the lamb's age when weighed as covariate, and the interactions between these factors. Results showed that all traits were significantly (
Probing internal dissipative processes of neutron stars with gravitational waves during the inspiral of neutron star binaries
We study the impact of out-of-equilibrium, dissipative effects on the
dynamics of inspiraling neutron stars. We find that modeling dissipative
processes (such as those from the stars internal effective fluid viscosity)
requires that one introduce a new tidal deformability parameter--the
dissipative tidal deformability--which modifies the phase of gravitational
waves emitted during the inspiral phase of a neutron star binary. We show that
the dissipative tidal deformability corrects the gravitational-wave phase at 4
post-Newtonian order for quasi-circular binaries. This correction receives a
large finite-size enhancement by the stellar compactness, analogous to the case
of the tidal deformability. Moreover, the correction is not degenerate with the
time of coalescence, which also enters at 4PN order, because it contains a
logarithmic frequency-dependent contribution. Using a simple Fisher analysis,
we show that physically allowed values for the dissipative tidal deformability
may be constrained by measurements of the phase of emitted gravitational waves
to roughly the same extent as the (electric-type, quadrupolar) tidal
deformability. Finally, we show that there are no out-of-equilibrium,
dissipative corrections to the tidal deformability itself. We conclude that
there are at least two relevant tidal deformability parameters that can be
constrained with gravitational-wave phase measurements during the late inspiral
of a neutron star binary: one which characterizes the adiabatic tidal response
of the star, and another which characterizes the leading-order
out-of-equilibrium, dissipative tidal response. These findings open a window to
probe dissipative processes in the interior of neutron stars with gravitational
waves.Comment: 24 pages, 1 figur
How Do Axisymmetric Black Holes Grow Monopole and Dipole Hair?
We study the dynamical formation of scalar monopole and dipole hair in scalar
Gauss-Bonnet theory and dynamical Chern-Simons theory. We prove that the
spherically-symmetric mode of the dipole hair is completely determined by the
product of the mass of the spacetime and the value of the monopole hair. We
then show that the dynamics of the mode of the dipole hair is
intimately tied to the appearance of the event horizon during axisymmetric
collapse, which results in the radiation of certain modes that could have been
divergent in the future of the collapse. We confirm these analytical
predictions by simulating the gravitational collapse of a rapidly rotating
neutron star in the decoupling limit, both in scalar Gauss-Bonnet and dynamical
Chern-Simons theory. Our results, combined with those of Ref.~\cite{R:2022cwe},
provide a clear physical picture of the dynamics of scalar monopole and dipole
radiation in axisymmetric and spherical gravitational collapse in these
theories.Comment: v2-matches published version in PR
Metric of a tidally perturbed spinning black hole
We explicitly construct the metric of a Kerr black hole that is tidally
perturbed by the external universe in the slow-motion approximation. This
approximation assumes that the external universe changes slowly relative to the
rotation rate of the hole, thus allowing the parameterization of the
Newman-Penrose scalar by time-dependent electric and magnetic tidal
tensors. This approximation, however, does not constrain how big the spin of
the background hole can be and, in principle, the perturbed metric can model
rapidly spinning holes. We first generate a potential by acting with a
differential operator on . From this potential we arrive at the metric
perturbation by use of the Chrzanowski procedure in the ingoing radiation
gauge. We provide explicit analytic formulae for this metric perturbation in
spherical Kerr-Schild coordinates, where the perturbation is finite at the
horizon. This perturbation is parametrized by the mass and Kerr spin parameter
of the background hole together with the electric and magnetic tidal tensors
that describe the time evolution of the perturbation produced by the external
universe. In order to take the metric accurate far away from the hole, these
tidal tensors should be determined by asymptotically matching this metric to
another one valid far from the hole. The tidally perturbed metric constructed
here could be useful in initial data constructions to describe the metric near
the horizons of a binary system of spinning holes. This perturbed metric could
also be used to construct waveforms and study the absorption of mass and
angular momentum by a Kerr black hole when external processes generate
gravitational radiation.Comment: 17 pages, 3 figures. Final PRD version, minor typos, etc corrected.
v3: corrected typo in Eq. (35) and (57
Spin-induced scalarized black holes
It was recently shown that a scalar field suitably coupled to the
Gauss-Bonnet invariant can undergo a spin-induced linear
tachyonic instability near a Kerr black hole. This instability appears only
once the dimensionless spin is sufficiently large, that is, . A tachyonic instability is the hallmark of spontaneous scalarization.
Focusing, for illustrative purposes, on a class of theories that do exhibit
this instability, we show that stationary, rotating black hole solutions do
indeed have scalar hair once the spin-induced instability threshold is
exceeded, while black holes that lie below the threshold are described by the
Kerr solution. Our results provide strong support for spin-induced black hole
scalarization.publishe
Observable Signatures of EMRI Black Hole Binaries Embedded in Thin Accretion Disks
We examine the electromagnetic (EM) and gravitational wave (GW) signatures of
stellar-mass compact objects (COs) spiraling into a supermassive black hole
(extreme mass-ratio inspirals or EMRIs), embedded in a thin, radiation-pressure
dominated, accretion disk. At large separations, the tidal effect of the
secondary CO clears a gap. We show that the gap refills during the late
GW-driven phase of the inspiral, leading to a sudden EM brightening of the
source. The accretion disk leaves an imprint on the GW through its angular
momentum exchange with the binary, the mass increase of the binary members due
to accretion, and its gravity. We compute the disk-modified GWs both in an
analytical Newtonian approximation and in a numerical effective-one-body
approach. We find that disk-induced migration provides the dominant
perturbation to the inspiral, with weaker effects from the mass accretion onto
the CO and hydrodynamic drag. Depending on whether a gap is present, the
perturbation of the GW phase is between 10 and 1000 radians per year,
detectable with the future Laser Interferometer Space Antenna (LISA) at high
significance. The Fourier transform of the disk-modified GW in the stationary
phase approximation is sensitive to disk parameters with a frequency trend
different from post-Newtonian vacuum corrections. Our results suggest that
observations of EMRIs may place new sensitive constraints on the physics of
accretion disks.Comment: 42 pages, 8 figures, 3 tables, submitted to Phys. Rev.
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