3,954 research outputs found
On the definition of matter collineations
It is shown that when the stress-energy tensor of a spacetime is diagonal and
is written in the mixed form, its collineations admit infinite dimensional Lie
algebras except possibly in the case when the tensor depends on all the
spacetime coordinates. The result can be extended for more general second rank
tensors
Radiation generated by accelerating and rotating charged black holes in (anti-)de Sitter space
Asymptotic behaviour of gravitational and electromagnetic fields of exact
type D solutions from the large Plebanski-Demianski family of black hole
spacetimes is analyzed. The amplitude and directional structure of radiation is
evaluated in cases when the cosmological constant is non-vanishing, so that the
conformal infinities have either de Sitter-like or anti-de Sitter-like
character. In particular, explicit relations between the parameters that
characterize the sources (that is their mass, electric and magnetic charges,
NUT parameter, rotational parameter, and acceleration) and properties of the
radiation generated by them are presented. The results further elucidate the
physical interpretation of these solutions and may help to understand radiative
characteristics of more general spacetimes than those that are asymptotically
flat.Comment: 24 pages, 18 figures. To appear in Classical and Quantum Gravit
Prima Facie Questions in Quantum Gravity
The long history of the study of quantum gravity has thrown up a complex web
of ideas and approaches. The aim of this article is to unravel this web a
little by analysing some of the {\em prima facie\/} questions that can be asked
of almost any approach to quantum gravity and whose answers assist in
classifying the different schemes. Particular emphasis is placed on (i) the
role of background conceptual and technical structure; (ii) the role of
spacetime diffeomorphisms; and (iii) the problem of time.Comment: 20,IC/TP/0
The Final Remnant of Binary Black Hole Mergers: Multipolar Analysis
Methods are presented to define and compute source multipoles of dynamical
horizons in numerical relativity codes, extending previous work from the
isolated and dynamical horizon formalisms in a manner that allows for the
consideration of horizons that are not axisymmetric. These methods are then
applied to a binary black hole merger simulation, providing evidence that the
final remnant is a Kerr black hole, both through the (spatially)
gauge-invariant recovery of the geometry of the apparent horizon, and through a
detailed extraction of quasinormal ringing modes directly from the strong-field
region.Comment: 12 pages, 13 figures. Published version. Some references have been
added and reordered, and the figures cleaned up
Thermal gravity, black holes and cosmological entropy
Taking seriously the interpretation of black hole entropy as the logarithm of
the number of microstates, we argue that thermal gravitons may undergo a phase
transition to a kind of black hole condensate. The phase transition proceeds
via nucleation of black holes at a rate governed by a saddlepoint configuration
whose free energy is of order the inverse temperature in Planck units. Whether
the universe remains in a low entropy state as opposed to the high entropy
black hole condensate depends sensitively on its thermal history. Our results
may clarify an old observation of Penrose regarding the very low entropy state
of the universe.Comment: 5 pages, 2 figures, RevTex. v4: to appear in Phys. Rev.
Isotropic cosmological singularities 1: Polytropic perfect fluid spacetimes
We consider the conformal Einstein equations for polytropic perfect fluid
cosmologies which admit an isotropic singularity. For the polytropic index
gamma strictly greater than 1 and less than or equal to 2 it is shown that the
Cauchy problem for these equations is well-posed, that is to say that solutions
exist, are unique and depend smoothly on the data, with data consisting of
simply the 3-metric of the singularity. The analogous result for gamma=1 (dust)
is obtained when Bianchi type symmetry is assumed.Comment: LaTeX, 43 pages, no figures, submitted to Ann. Phy
Isotropic cosmological singularities: other matter models
Isotropic cosmological singularities are singularities which can be removed
by rescaling the metric. In some cases already studied (gr-qc/9903008,
gr-qc/9903009, gr-qc/9903018) existence and uniqueness of cosmological models
with data at the singularity has been established. These were cosmologies with,
as source, either perfect fluids with linear equations of state or massless,
collisionless particles. In this article we consider how to extend these
results to a variety of other matter models. These are scalar fields, massive
collisionless matter, the Yang-Mills plasma of Choquet-Bruhat, or matter
satisfying the Einstein-Boltzmann equation.Comment: LaTeX, 19 pages, no figure
Horizon Mass Theorem
A new theorem for black holes is found. It is called the horizon mass
theorem. The horizon mass is the mass which cannot escape from the horizon of a
black hole. For all black holes: neutral, charged or rotating, the horizon mass
is always twice the irreducible mass observed at infinity. Previous theorems on
black holes are: 1. the singularity theorem, 2. the area theorem, 3. the
uniqueness theorem, 4. the positive energy theorem. The horizon mass theorem is
possibly the last general theorem for classical black holes. It is crucial for
understanding Hawking radiation and for investigating processes occurring near
the horizon.Comment: A new theorem for black holes is establishe
On the existence of singularity-free solutions in quartic gravity
We study a general field theory of a scalar field coupled to gravitation
through a quadratic Gauss-Bonnet term . We show that,
under mild assumptions about the function , the classical solutions
in a spatially flat FRW background include singularity - free solutions.Comment: 9 pages, LATEX, uses epsf.tex macro, (1 figure included in
uuencode+compress EPSF form), IOA-29
Non-spherical collapse of a two fluid star
We obtain the analogue of collapsing Vaidya-like solution to include both a
null fluid and a string fluid, with a linear equation of state (), in non-spherical (plane symmetric and cylindrically symmetric) anti-de
Sitter space-timess. It turns out that the non-spherical collapse of two fluid
in anti-de Sitter space-times, in accordance with cosmic censorship, proceed to
form black holes, i.e., on naked singularity ever forms, violating hoop
conjecture.Comment: 7 pages, RevTeX 4, minor correction
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