5,824 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
Perturbations of the Kerr spacetime in horizon penetrating coordinates
We derive the Teukolsky equation for perturbations of a Kerr spacetime when
the spacetime metric is written in either ingoing or outgoing Kerr-Schild form.
We also write explicit formulae for setting up the initial data for the
Teukolsky equation in the time domain in terms of a three metric and an
extrinsic curvature. The motivation of this work is to have in place a
formalism to study the evolution in the ``close limit'' of two recently
proposed solutions to the initial value problem in general relativity that are
based on Kerr-Schild slicings. A perturbative formalism in horizon penetrating
coordinates is also very desirable in connection with numerical relativity
simulations using black hole ``excision''.Comment: 8 pages, RevTex, 2 figures, final version to appear in CQ
Close encounters of three black holes
We present the first fully relativistic longterm numerical evolutions of
three equal-mass black holes in a system consisting of a third black hole in a
close orbit about a black-hole binary. We find that these
close-three-black-hole systems have very different merger dynamics from
black-hole binaries. In particular, we see complex trajectories, a
redistribution of energy that can impart substantial kicks to one of the holes,
distinctive waveforms, and suppression of the emitted gravitational radiation.
We evolve two such configurations and find very different behaviors. In one
configuration the binary is quickly disrupted and the individual holes follow
complicated trajectories and merge with the third hole in rapid succession,
while in the other, the binary completes a half-orbit before the initial merger
of one of the members with the third black hole, and the resulting
two-black-hole system forms a highly elliptical, well separated binary that
shows no significant inspiral for (at least) the first t~1000M of evolution.Comment: 4 pages, 5 figure
The Lazarus project: A pragmatic approach to binary black hole evolutions
We present a detailed description of techniques developed to combine 3D
numerical simulations and, subsequently, a single black hole close-limit
approximation. This method has made it possible to compute the first complete
waveforms covering the post-orbital dynamics of a binary black hole system with
the numerical simulation covering the essential non-linear interaction before
the close limit becomes applicable for the late time dynamics. To determine
when close-limit perturbation theory is applicable we apply a combination of
invariant a priori estimates and a posteriori consistency checks of the
robustness of our results against exchange of linear and non-linear treatments
near the interface. Once the numerically modeled binary system reaches a regime
that can be treated as perturbations of the Kerr spacetime, we must
approximately relate the numerical coordinates to the perturbative background
coordinates. We also perform a rotation of a numerically defined tetrad to
asymptotically reproduce the tetrad required in the perturbative treatment. We
can then produce numerical Cauchy data for the close-limit evolution in the
form of the Weyl scalar and its time derivative
with both objects being first order coordinate and tetrad invariant. The
Teukolsky equation in Boyer-Lindquist coordinates is adopted to further
continue the evolution. To illustrate the application of these techniques we
evolve a single Kerr hole and compute the spurious radiation as a measure of
the error of the whole procedure. We also briefly discuss the extension of the
project to make use of improved full numerical evolutions and outline the
approach to a full understanding of astrophysical black hole binary systems
which we can now pursue.Comment: New typos found in the version appeared in PRD. (Mostly found and
collected by Bernard Kelly
Dynamics of Ferromagnetic Walls: Gravitational Properties
We discuss a new mechanism which allows domain walls produced during the
primordial electroweak phase transition. We show that the effective surface
tension of these domain walls can be made vanishingly small due to a peculiar
magnetic condensation induced by fermion zero modes localized on the wall. We
find that in the perfect gas approximation the domain wall network behaves like
a radiation gas. We consider the recent high-red shift supernova data and we
find that the corresponding Hubble diagram is compatible with the presence in
the Universe of a ideal gas of ferromagnetic domain walls. We show that our
domain wall gas induces a completely negligible contribution to the large-scale
anisotropy of the microwave background radiation.Comment: Replaced with revised version, accepted for publication in IJMP
Lorentz Symmetry Violation and Galactic Magnetism
We analyze the generation of primordial magnetic fields during de Sitter
inflation in a Lorentz-violating theory of Electrodynamics containing a
Chern-Simons term which couples the photon to an external four-vector. We find
that, for appropriate magnitude of the four-vector, the generated field is
maximally helical and, through an inverse cascade caused by turbulence of
primeval plasma, reaches at the time of protogalactic collapse an intensity and
correlation length such as to directly explain galactic magnetism.Comment: 5 pages, minor revisions, version published in Phys. Lett.
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
Accurate Evolutions of Orbiting Black-Hole Binaries Without Excision
We present a new algorithm for evolving orbiting black-hole binaries that
does not require excision or a corotating shift. Our algorithm is based on a
novel technique to handle the singular puncture conformal factor. This system,
based on the BSSN formulation of Einstein's equations, when used with a
`pre-collapsed' initial lapse, is non-singular at the start of the evolution,
and remains non-singular and stable provided that a good choice is made for the
gauge. As a test case, we use this technique to fully evolve orbiting
black-hole binaries from near the Innermost Stable Circular Orbit (ISCO)
regime. We show fourth order convergence of waveforms and compute the radiated
gravitational energy and angular momentum from the plunge. These results are in
good agreement with those predicted by the Lazarus approach.Comment: 4 pages, revtex4, 3 figs, references added, typos fixe
Testing the Isotropy of the Universe with Type Ia Supernovae
We analyze the magnitude-redshift data of type Ia supernovae included in the
Union and Union2 compilations in the framework of an anisotropic Bianchi type I
cosmological model and in the presence of a dark energy fluid with anisotropic
equation of state. We find that the amount of deviation from isotropy of the
equation of state of dark energy, the skewness \delta, and the present level of
anisotropy of the large-scale geometry of the Universe, the actual shear
\Sigma_0, are constrained in the ranges -0.16 < \delta < 0.12 and -0.012 <
\Sigma_0 < 0.012 (1\sigma C.L.) by Union2 data. Supernova data are then
compatible with a standard isotropic universe (\delta = \Sigma_0 = 0), but a
large level of anisotropy, both in the geometry of the Universe and in the
equation of state of dark energy, is allowed.Comment: 12 pages, 7 figures, 2 tables. Union2 analysis added. New references
added. To appear in Phys. Rev.
Large Merger Recoils and Spin Flips From Generic Black-Hole Binaries
We report the first results from evolutions of a generic black-hole binary,
i.e. a binary containing unequal mass black holes with misaligned spins. Our
configuration, which has a mass ratio of 2:1, consists of an initially
non-spinning hole orbiting a larger, rapidly spinning hole (specific spin a/m =
0.885), with the spin direction oriented -45 degrees with respect to the
orbital plane. We track the inspiral and merger for ~2 orbits and find that the
remnant receives a substantial kick of 454 km/s, more than twice as large as
the maximum kick from non-spinning binaries. The remnant spin direction is
flipped by 103 degrees with respect to the initial spin direction of the larger
hole. We performed a second run with anti-aligned spins, a/m = +-0.5 lying in
the orbital plane that produces a kick of 1830 km/s off the orbital plane. This
value scales to nearly 4000 km/s for maximally spinning holes. Such a large
recoil velocity opens the possibility that a merged binary can be ejected even
from the nucleus of a massive host galaxy.Comment: 4 pages. Accepted for publication in ApJ
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