1,223 research outputs found
A light-cone gauge for black-hole perturbation theory
The geometrical meaning of the Eddington-Finkelstein coordinates of
Schwarzschild spacetime is well understood: (i) the advanced-time coordinate v
is constant on incoming light cones that converge toward r=0, (ii) the angles
theta and phi are constant on the null generators of each light cone, (iii) the
radial coordinate r is an affine-parameter distance along each generator, and
(iv) r is an areal radius, in the sense that 4 pi r^2 is the area of each
two-surface (v,r) = constant. The light-cone gauge of black-hole perturbation
theory, which is formulated in this paper, places conditions on a perturbation
of the Schwarzschild metric that ensure that properties (i)--(iii) of the
coordinates are preserved in the perturbed spacetime. Property (iv) is lost in
general, but it is retained in exceptional situations that are identified in
this paper. Unlike other popular choices of gauge, the light-cone gauge
produces a perturbed metric that is expressed in a meaningful coordinate
system; this is a considerable asset that greatly facilitates the task of
extracting physical consequences. We illustrate the use of the light-cone gauge
by calculating the metric of a black hole immersed in a uniform magnetic field.
We construct a three-parameter family of solutions to the perturbative
Einstein-Maxwell equations and argue that it is applicable to a broader range
of physical situations than the exact, two-parameter Schwarzschild-Melvin
family.Comment: 12 page
Mode-sum regularization of the scalar self-force: Formulation in terms of a tetrad decomposition of the singular field
We examine the motion in Schwarzschild spacetime of a point particle endowed
with a scalar charge. The particle produces a retarded scalar field which
interacts with the particle and influences its motion via the action of a
self-force. We exploit the spherical symmetry of the Schwarzschild spacetime
and decompose the scalar field in spherical-harmonic modes. Although each mode
is bounded at the position of the particle, a mode-sum evaluation of the
self-force requires regularization because the sum does not converge: the
retarded field is infinite at the position of the particle. The regularization
procedure involves the computation of regularization parameters, which are
obtained from a mode decomposition of the Detweiler-Whiting singular field;
these are subtracted from the modes of the retarded field, and the result is a
mode-sum that converges to the actual self-force. We present such a computation
in this paper. There are two main aspects of our work that are new. First, we
define the regularization parameters as scalar quantities by referring them to
a tetrad decomposition of the singular field. Second, we calculate four sets of
regularization parameters (denoted schematically by A, B, C, and D) instead of
the usual three (A, B, and C). As proof of principle that our methods are
reliable, we calculate the self-force acting on a scalar charge in circular
motion around a Schwarzschild black hole, and compare our answers with those
recorded in the literature.Comment: 38 pages, 2 figure
Static spherically symmetric scalar field spacetimes with C^0 matching
All the classes of static massless scalar field models available currently in
the Einstein theory of gravity necessarily contain a strong curvature naked
singularity. We obtain here a family of solutions for static massless scalar
fields coupled to gravity, which does not have any strong curvature
singularity. This class of models contain a thin shell of singular matter,
which has a physical interpretation. The central curvature singularity is,
however, avoided which is common to all static massless scalar field spacetimes
models known so far. Our result thus points out that the full class of
solutions in this case may contain non-singular models, which is an intriguing
possibility.Comment: revised version, 10 pages, no figures, accepted in Mod. Phys. Let.
On the fate of singularities and horizons in higher derivative gravity
We study static spherically symmetric solutions of high derivative gravity
theories, with 4, 6, 8 and even 10 derivatives. Except for isolated points in
the space of theories with more than 4 derivatives, only solutions that are
nonsingular near the origin are found. But these solutions cannot smooth out
the Schwarzschild singularity without the appearance of a second horizon. This
conundrum, and the possibility of singularities at finite r, leads us to study
numerical solutions of theories truncated at four derivatives. Rather than two
horizons we are led to the suggestion that the original horizon is replaced by
a rapid nonsingular transition from weak to strong gravity. We also consider
this possibility for the de Sitter horizon.Comment: 15 pages, 3 figures, improvements and references added, to appear in
PR
Self force in 2+1 electrodynamics
The radiation reaction problem for an electric charge moving in flat
space-time of three dimensions is discussed. The divergences stemming from the
pointness of the particle are studied. A consistent regularization procedure is
proposed, which exploits the Poincar\'e invariance of the theory. Effective
equation of motion of radiating charge in an external electromagnetic field is
obtained via the consideration of energy-momentum and angular momentum
conservation. This equation includes the effect of the particle's own field.
The radiation reaction is determined by the Lorentz force of point-like charge
acting upon itself plus a non-local term which provides finiteness of the
self-action.Comment: 20 pages, 3 figure
Absorption of mass and angular momentum by a black hole: Time-domain formalisms for gravitational perturbations, and the small-hole/slow-motion approximation
The first objective of this work is to obtain practical prescriptions to
calculate the absorption of mass and angular momentum by a black hole when
external processes produce gravitational radiation. These prescriptions are
formulated in the time domain within the framework of black-hole perturbation
theory. Two such prescriptions are presented. The first is based on the
Teukolsky equation and it applies to general (rotating) black holes. The second
is based on the Regge-Wheeler and Zerilli equations and it applies to
nonrotating black holes. The second objective of this work is to apply the
time-domain absorption formalisms to situations in which the black hole is
either small or slowly moving. In the context of this small-hole/slow-motion
approximation, the equations of black-hole perturbation theory can be solved
analytically, and explicit expressions can be obtained for the absorption of
mass and angular momentum. The changes in the black-hole parameters can then be
understood in terms of an interaction between the tidal gravitational fields
supplied by the external universe and the hole's tidally-induced mass and
current quadrupole moments. For a nonrotating black hole the quadrupole moments
are proportional to the rate of change of the tidal fields on the hole's world
line. For a rotating black hole they are proportional to the tidal fields
themselves.Comment: 36 pages, revtex4, no figures, final published versio
Upper limits of particle emission from high-energy collision and reaction near a maximally rotating Kerr black hole
The center-of-mass energy of two particles colliding near the horizon of a
maximally rotating black hole can be arbitrarily high if the angular momentum
of either of the incident particles is fine-tuned, which we call a critical
particle. We study particle emission from such high-energy collision and
reaction in the equatorial plane fully analytically. We show that the
unconditional upper limit of the energy of the emitted particle is given by
218.6% of that of the injected critical particle, irrespective of the details
of the reaction and this upper limit can be realized for massless particle
emission. The upper limit of the energy extraction efficiency for this emission
as a collisional Penrose process is given by 146.6%, which can be realized in
the collision of two massive particles with optimized mass ratio. Moreover, we
analyze perfectly elastic collision, Compton scattering, and pair annihilation
and show that net positive energy extraction is really possible for these three
reactions. The Compton scattering is most efficient among them and the
efficiency can reach 137.2%. On the other hand, our result is qualitatively
consistent with the earlier claim that the mass and energy of the emitted
particle are at most of order the total energy of the injected particles and
hence we can observe neither super-heavy nor super-energetic particles.Comment: 22 pages, 3 figures, typos corrected, reference updated, accepted for
publication in Physical Review D, typos correcte
Dirty rotating black holes: regularity conditions on stationary horizons
We consider generic, or "dirty" (surrounded by matter), stationary rotating
black holes with axial symmetry. The restrictions are found on the asymptotic
form of metric in the vicinity of non-extremal, extremal and ultra-extremal
horizons, imposed by the conditions of regularity of increasing strength:
boundedness on the horizon of the Ricci scalar, of scalar quadratic curvature
invariants, and of the components of the curvature tensor in the tetrad
attached to a falling observer. We show, in particular, that boundedness of the
Ricci scalar implies the "rigidity" of the horizon's rotation in all cases,
while the finiteness of quadratic invariants leads to the constancy of the
surface gravity. We discuss the role of quasiglobal coordinate r that is
emphasized by the conditions of regularity. Further restrictions on the metric
are formulated in terms of subsequent coefficients of expansion of metric
functions by r. The boundedness of the tetrad components of curvature tensor
for an observer crossing the horizon is shown to lead in the horizon limit to
diagonalization of Einstein tensor in the frame of zero angular momentum
observer on a circular orbit (ZAMO frame) for horizons of all degrees of
extremality.Comment: 31 pages. Misprints correcte
Gravitational waves from inspiraling compact binaries: Second post-Newtonian waveforms as search templates
We ascertain the effectiveness of the second post-Newtonian approximation to
the gravitational waves emitted during the adiabatic inspiral of a compact
binary system as templates for signal searches with kilometer-scale
interferometric detectors. The reference signal is obtained by solving the
Teukolsky equation for a small mass moving on a circular orbit around a large
nonrotating black hole. Fitting factors computed from this signal and these
templates, for various types of binary systems, are all above the 90% mark.
According to Apostolatos' criterion, second post-Newtonian waveforms should
make acceptably effective search templates.Comment: LaTeX, one eps figure. Hires and color versions are available from
http://jovian.physics.uoguelph.ca/~droz/uni/papers/search.htm
Quadrupole moments of rotating neutron stars
Numerical models of rotating neutron stars are constructed for four equations
of state using the computer code RNS written by Stergioulas. For five selected
values of the star's gravitational mass (in the interval between 1.0 and 1.8
solar masses) and for each equation of state, the star's angular momentum is
varied from J=0 to the Keplerian limit J=J_{max}. For each neutron-star
configuration we compute Q, the quadrupole moment of the mass distribution. We
show that for given values of M and J, |Q| increases with the stiffness of the
equation of state. For fixed mass and equation of state, the dependence on J is
well reproduced with a simple quadratic fit, Q \simeq - aJ^2/M c^2, where c is
the speed of light, and a is a parameter of order unity depending on the mass
and the equation of state.Comment: ReVTeX, 7 pages, 5 figures, additional material, and references adde
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