150 research outputs found
Comment on ``Perturbative Method to solve fourth-order Gravity Field Equations"
We reconsider the cosmic string perturbative solution to the classical
fourth-order gravity field equations, obtained in Ref.\cite{CLA94}, and we
obtain that static, cylindricaly symmetric gauge cosmic strings, with constant
energy density, can contain only -terms in the first order corrections
to the interior gravitational field, while the exact exterior solution is a
conical spacetime with deficit angle .Comment: 6 pages, Revte
Black hole collisions: how far can perturbation theory go?
The computation of gravitational radiation generated by the coalescence of
inspiralling binary black holes is nowdays one of the main goals of numerical
relativity. Perturbation theory has emerged as an ubiquitous tool for all those
dynamical evolutions where the two black holes start close enough to each
other, to be treated as single distorted black hole (close limit
approximation), providing at the same time useful benchmarks for full numerical
simulations. Here we summarize the most recent developments to study evolutions
of perturbations around rotating (Kerr) black holes. The final aim is to
generalize the close limit approximation to the most general case of two
rotating black holes in orbit around each other, and thus provide reliable
templates for the gravitational waveforms in this regime. For this reason it
has become very important to know if these predictions can actually be trusted
to larger separation parameters (even in the region where the holes have
distinct event horizons). The only way to extend the range of validity of the
linear approximation is to develop the theory of second order perturbations
around a Kerr hole, by generalizing the Teukolsky formalism.Comment: 6 pages, Latex, uses moriond.sty, proceedings of the talk given at
the Moriond 99' euroconferenc
Semiclassical models for uniform-density Cosmic Strings and Relativistic Stars
In this paper we show how quantum corrections, although perturbatively small,
may play an important role in the analysis of the existence of some classical
models. This, in fact, appears to be the case of static, uniform--density
models of the interior metric of cosmic strings and neutron stars. We consider
the fourth order semiclassical equations and first look for perturbative
solutions in the coupling constants and of the quadratic
curvature terms in the effective gravitational Lagrangian. We find that there
is not a consistent solution; neither for strings nor for spherical stars. We
then look for non--perturbative solutions and find an explicit approximate
metric for the case of straight cosmic strings. We finally analyse the
contribution of the non--local terms to the renormalized energy--momentum
tensor and the possibility of this terms to allow for a perturbative solution.
We explicitly build up a particular renormalized energy--momentum tensor to
fulfill that end. These state--dependent corrections are found by simple
considerations of symmetry, conservation law and trace anomaly, and are chosen
to compensate for the local terms. However, they are not only ad hoc, but have
to depend on and , what is not expected to first perturbative
order. We then conclude that non--perturbative solutions are valuable for
describing certain physical situations.Comment: 19 pages, REVTEX, no figure
Algebraic Classification of Numerical Spacetimes and Black-Hole-Binary Remnants
In this paper we develop a technique for determining the algebraic
classification of a numerical spacetime, possibly resulting from a generic
black-hole-binary merger, using the Newman-Penrose Weyl scalars. We demonstrate
these techniques for a test case involving a close binary with arbitrarily
oriented spins and unequal masses. We find that, post merger, the spacetime
quickly approaches Petrov type II, and only approaches type D on much longer
timescales. These techniques allow us to begin to explore the validity of the
"no-hair theorem" for generic merging-black-hole spacetimes.Comment: published versio
Comparison of Post-Newtonian and Numerical Evolutions of Black-Hole Binaries
In this paper, we compare the waveforms from the post-Newtonian (PN) approach
with the numerical simulations of generic black-hole binaries which have mass
ratio , arbitrarily oriented spins with magnitudes
and , and orbit 9 times from an initial orbital separation of
prior to merger. We observe a reasonably good agreement between
the PN and numerical waveforms, with an overlap of over 98% for the first six
cycles of the mode and over 90% for the and
modes.Comment: 4 pages, 2 figures, prepared for the proceedings of the 18th workshop
on general relativity and gravitation, Hiroshima, Japan, Nov.17 - Nov.21,
200
Intermediate Mass Ratio Black Hole Binaries: Numerical Relativity meets Perturbation Theory
We study black-hole binaries in the intermediate-mass-ratio regime 0.01 < q <
0.1 with a new technique that makes use of nonlinear numerical trajectories and
efficient perturbative evolutions to compute waveforms at large radii for the
leading and nonleading modes. As a proof-of-concept, we compute waveforms for
q=1/10. We discuss applications of these techniques for LIGO/VIRGO data
analysis and the possibility that our technique can be extended to produce
accurate waveform templates from a modest number of fully-nonlinear numerical
simulations.Comment: 4 pages, 5 figures, revtex
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