268 research outputs found
The adiabatic evolution of orbital parameters in the Kerr spacetime
We investigate the adiabatic orbital evolution of a point particle in the
Kerr spacetime due to the emission of gravitational waves. In the case that the
timescale of the orbital evolution is enough smaller than the typical timescale
of orbits, the evolution of orbits is characterized by the change rates of
three constants of motion, the energy , the azimuthal angular momentum ,
and the Carter constant . For and , we can evaluate their change
rates from the fluxes of the energy and the angular momentum at infinity and on
the event horizon according to the balance argument. On the other hand, for the
Carter constant, we cannot use the balance argument because we do not know the
conserved current associated with it. %and the corresponding conservation law.
Recently, Mino proposed a new method of evaluating the averaged change rate of
the Carter constant by using the radiative field. In our previous paper we
developed a simplified scheme for practical evaluation of the evolution of the
Carter constant based on the Mino's proposal. In this paper we describe our
scheme in more detail, and derive explicit analytic formulae for the change
rates of the energy, the angular momentum and the Carter constant.Comment: 34 pages, no figur
Self-force Regularization in the Schwarzschild Spacetime
We discuss the gravitational self-force on a particle in a black hole
space-time. For a point particle, the full (bare) self-force diverges. The
metric perturbation induced by a particle can be divided into two parts, the
direct part (or the S part) and the tail part (or the R part), in the harmonic
gauge, and the regularized self-force is derived from the R part which is
regular and satisfies the source-free perturbed Einstein equations. But this
formulation is abstract, so when we apply to black hole-particle systems, there
are many problems to be overcome in order to derive a concrete self-force.
These problems are roughly divided into two parts. They are the problem of
regularizing the divergent self-force, i.e., ``subtraction problem'' and the
problem of the singularity in gauge transformation, i.e., ``gauge problem''. In
this paper, we discuss these problems in the Schwarzschild background and
report some recent progress.Comment: 34 pages, 2 figures, submitted to CQG, special volume for Radiation
Reaction (CAPRA7
Geometrical locus of massive test particle orbits in the space of physical parameters in Kerr space-time
Gravitational radiation of binary systems can be studied by using the
adiabatic approximation in General Relativity. In this approach a small
astrophysical object follows a trajectory consisting of a chained series of
bounded geodesics (orbits) in the outer region of a Kerr Black Hole,
representing the space time created by a bigger object. In our paper we study
the entire class of orbits, both of constant radius (spherical orbits), as well
as non-null eccentricity orbits, showing a number of properties on the physical
parameters and trajectories. The main result is the determination of the
geometrical locus of all the orbits in the space of physical parameters in Kerr
space-time. This becomes a powerful tool to know if different orbits can be
connected by a continuous change of their physical parameters. A discussion on
the influence of different values of the angular momentum of the hole is given.
Main results have been obtained by analytical methods.Comment: 26 pages, 12 figure
Detecting gravitational waves from inspiraling binaries with a network of detectors : coherent versus coincident strategies
We compare two strategies of multi-detector detection of compact binary
inspiral signals, namely, the coincidence and the coherent. For simplicity we
consider here two identical detectors having the same power spectral density of
noise, that of initial LIGO, located in the same place and having the same
orientation. We consider the cases of independent noise as well as that of
correlated noise. The coincident strategy involves separately making two
candidate event lists, one for each detector, and from these choosing those
pairs of events from the two lists which lie within a suitable parameter
window, which then are called as coincidence detections. The coherent strategy
on the other hand involves combining the data phase coherently, so as to obtain
a single network statistic which is then compared with a single threshold. Here
we attempt to shed light on the question as to which strategy is better. We
compare the performances of the two methods by plotting the Receiver Operating
Characteristics (ROC) for the two strategies. Several of the results are
obtained analytically in order to gain insight. Further we perform numerical
simulations in order to determine certain parameters in the analytic formulae
and thus obtain the final complete results. We consider here several cases from
the relatively simple to the astrophysically more relevant in order to
establish our results. The bottom line is that the coherent strategy although
more computationally expensive in general than the coincidence strategy, is
superior to the coincidence strategy - considerably less false dismissal
probability for the same false alarm probability in the viable false alarm
regime.Comment: 18 pages, 10 figures, typo correcte
Prospects for improving the sensitivity of KAGRA gravitational wave detector
KAGRA is a new gravitational wave detector which aims to begin joint observation with Advanced LIGO and Advanced Virgo from late 2019. Here, we present KAGRA's possible upgrade plans to improve the sensitivity in the decade ahead. Unlike other state-of-the-art detectors, KAGRA requires different investigations for the upgrade since it is the only detector which employs cryogenic cooling of the test mass mirrors. In this paper, investigations on the upgrade plans which can be realized by changing the input laser power, increasing the mirror mass, and injecting frequency dependent squeezed vacuum are presented. We show how each upgrade affects to the detector frequency bands and also discuss impacts on gravitational-wave science. We then propose an effective progression of upgrades based on technical feasibility and scientific scenarios
Braneworld reheating in the bulk inflaton model
In the context of the braneworld inflation driven by a bulk scalar field, we
study the energy dissipation from the bulk scalar field into the matter on the
brane in order to understand the reheating after inflation. Deriving the
late-time behavior of the bulk field with dissipation by using the Green's
function method, we give a rigorous justification of the statement that the
standard reheating process is reproduced in this bulk inflaton model as long as
the Hubble parameter on the brane and the mass of the bulk scalar field are
much smaller than the 5-dimensional inverse curvature scale. Our result
supports the idea that the brane inflation model caused by a bulk scalar field
is expected to be a viable alternative scenario of the early universe.Comment: 5 pages, no figures, final version to be published in PR
Primordial fluctuations in bulk inflaton model
An inflationary brane model driven by a bulk inflaton with exponential
potential is proposed. We find a family of exact solutions that describe
power-law inflation on the brane. These solutions enable us to derive exact
solutions for metric perturbations analytically. By calculating scalar and
tensor perturbations, we obtain a spectrum of primordial fluctuations at the
end of the inflation. The amplitudes of scalar and tensor perturbations are
enhanced in the same way if the energy scale of the inflation is sufficiently
higher than the tension of the brane. Then the relative amplitude of scalar and
tensor perturbations is not suppressed even for high-energy inflation. This is
a distinguishable feature from the inflation model driven by inflaton on the
brane where tensor perturbations are suppressed for high-energy inflation. We
also point out that massive Kaluza-Klein modes are not negligible at
high-frequencies on 3-space of our brane.Comment: 16 pages, 3 figures, reference adde
Exactly solvable model for cosmological perturbations in dilatonic brane worlds
We construct a model where cosmological perturbations are analytically solved
based on dilatonic brane worlds. A bulk scalar field has an exponential
potential in the bulk and an exponential coupling to the brane tension. The
bulk scalar field yields a power-law inflation on the brane. The exact
background metric can be found including the back-reaction of the scalar field.
Then exact solutions for cosmological perturbations which properly satisfy the
junction conditions on the brane are derived. These solutions provide us an
interesting model to understand the connection between the behavior of
cosmological perturbations on the brane and the geometry of the bulk. Using
these solutions, the behavior of an anisotropic stress induced on the
inflationary brane by bulk gravitational fields is investigated.Comment: 30 pages, typos corrected, reference adde
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