5,661 research outputs found
Making use of geometrical invariants in black hole collisions
We consider curvature invariants in the context of black hole collision
simulations. In particular, we propose a simple and elegant combination of the
Weyl invariants I and J, the {\sl speciality index} . In the context
of black hole perturbations provides a measure of the size of the
distortions from an ideal Kerr black hole spacetime. Explicit calculations in
well-known examples of axisymmetric black hole collisions demonstrate that this
quantity may serve as a useful tool for predicting in which cases perturbative
dynamics provide an accurate estimate of the radiation waveform and energy.
This makes particularly suited to studying the transition from
nonlinear to linear dynamics and for invariant interpretation of numerical
results.Comment: 4 pages, 3 eps figures, Revte
Sparse spectral-tau method for the three-dimensional helically reduced wave equation on two-center domains
We describe a multidomain spectral-tau method for solving the
three-dimensional helically reduced wave equation on the type of two-center
domain that arises when modeling compact binary objects in astrophysical
applications. A global two-center domain may arise as the union of Cartesian
blocks, cylindrical shells, and inner and outer spherical shells. For each such
subdomain, our key objective is to realize certain (differential and
multiplication) physical-space operators as matrices acting on the
corresponding set of modal coefficients. We achieve sparse banded realizations
through the integration "preconditioning" of Coutsias, Hagstrom, Hesthaven, and
Torres. Since ours is the first three-dimensional multidomain implementation of
the technique, we focus on the issue of convergence for the global solver, here
the alternating Schwarz method accelerated by GMRES. Our methods may prove
relevant for numerical solution of other mixed-type or elliptic problems, and
in particular for the generation of initial data in general relativity.Comment: 37 pages, 3 figures, 12 table
The close limit from a null point of view: the advanced solution
We present a characteristic algorithm for computing the perturbation of a
Schwarzschild spacetime by means of solving the Teukolsky equation. We
implement the algorithm as a characteristic evolution code and apply it to
compute the advanced solution to a black hole collision in the close
approximation. The code successfully tracks the initial burst and quasinormal
decay of a black hole perturbation through 10 orders of magnitude and tracks
the final power law decay through an additional 6 orders of magnitude.
Determination of the advanced solution, in which ingoing radiation is absorbed
by the black hole but no outgoing radiation is emitted, is the first stage of a
two stage approach to determining the retarded solution, which provides the
close approximation waveform with the physically appropriate boundary condition
of no ingoing radiation.Comment: Revised version, published in Phys. Rev. D, 34 pages, 13 figures,
RevTe
Accelerating dark energy models in bianchi Type-V space-time
Some new exact solutions of Einstein's field equations in a spatially
homogeneous and anisotropic Bianchi type-V space-time with minimally
interaction of perfect fluid and dark energy components have been obtained. To
prevail the deterministic solution we choose the scale factor , which yields a time dependent deceleration parameter (DP),
representing a model which generates a transition of the universe from the
early decelerating phase to the recent accelerating phase. We find that for , the quintessence model is reproducible with present and expected
future evolution of the universe. The other models (for ), we observe
the phantom scenario. The quintessence as well as phantom models approach to
isotropy at late time. For different values of , we can generate a class of
physically viable DE models. The cosmic jerk parameter in our descended model
is also found to be in good concordance with the recent data of astrophysical
observations under appropriate condition. The physical and geometric properties
of spatially homogeneous and anisotropic cosmological models are discussed.Comment: 12 pages, 6 figure
Probing the QCD vacuum with an abelian chromomagnetic field: A study within an effective model
We study the response of the QCD vacuum to an external abelian chromomagnetic
field in the framework of a non local Nambu-Jona Lasinio model with the
Polyakov loop. We use the Lattice results on the deconfinement temperature of
the pure gauge theory to compute the same quantity in the presence of dynamical
quarks. We find a linear relationship between the deconfinement temperature
with quarks and the squared root of the applied field strength, , in
qualitative (and to some extent also quantitative) agreement with existing
Lattice calculations. On the other hand, we find a discrepancy on the
approximate chiral symmetry restoration: while Lattice results suggest the
deconfinement and the chiral restoration remain linked even at non-zero value
of , our results are consistent with a scenario in which the two
transitions are separated as is increased.Comment: 14 pages, 7 figures, RevTeX4. Published version, with enlarged
abstract and minor changes in the main tex
A perturbative solution for gravitational waves in quadratic gravity
We find a gravitational wave solution to the linearized version of quadratic
gravity by adding successive perturbations to the Einstein's linearized field
equations. We show that only the Ricci squared quadratic invariant contributes
to give a different solution of those found in Einstein's general relativity.
The perturbative solution is written as a power series in the
parameter, the coefficient of the Ricci squared term in the quadratic
gravitational action. We also show that, for monochromatic waves of a given
angular frequency , the perturbative solution can be summed out to give
an exact solution to linearized version of quadratic gravity, for
.
This result may lead to implications to the predictions for gravitational
wave backgrounds of cosmological origin.Comment: 9 pages, to appear in CQ
Maximum gravitational recoil
Recent calculations of gravitational radiation recoil generated during
black-hole binary mergers have reopened the possibility that a merged binary
can be ejected even from the nucleus of a massive host galaxy. Here we report
the first systematic study of gravitational recoil of equal-mass binaries with
equal, but anti-aligned, spins parallel to the orbital plane. Such an
orientation of the spins is expected to maximize the recoil. We find that
recoil velocity (which is perpendicular to the orbital plane) varies
sinusoidally with the angle that the initial spin directions make with the
initial linear momenta of each hole and scales up to a maximum of ~4000 km/s
for maximally-rotating holes. Our results show that the amplitude of the recoil
velocity can depend sensitively on spin orientations of the black holes prior
to merger.Comment: 4 pages, 4 figs, revtex
Universality of massive scalar field late-time tails in black-hole spacetimes
The late-time tails of a massive scalar field in the spacetime of black holes
are studied numerically. Previous analytical results for a Schwarzschild black
hole are confirmed: The late-time behavior of the field as recorded by a static
observer is given by , where
depends weakly on time. This result is carried over to the case of
a Kerr black hole. In particular, it is found that the power-law index of -5/6
depends on neither the multipole mode nor on the spin rate of the black
hole . In all black hole spacetimes, massive scalar fields have the same
late-time behavior irrespective of their initial data (i.e., angular
distribution). Their late-time behavior is universal.Comment: 11 pages, 14 figures, published versio
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