6,908 research outputs found
Physical process version of the first law of thermodynamics for black holes in Einstein-Maxwell axion-dilaton gravity
We derive general formulae for the first order variation of the ADM mass,
angular momentum for linear perturbations of a stationary background in
Einstein-Maxwell axion-dilaton gravity being the low-energy limit of the
heterotic string theory. All these variations were expressed in terms of the
perturbed matter energy momentum tensor and the perturbed charge current
density. Combining these expressions we reached to the form of the {\it
physical version} of the first law of black hole dynamics for the stationary
black holes in the considered theory being the strong support for the cosmic
censorship.Comment: 8 pages, Revte
Isolated Horizon, Killing Horizon and Event Horizon
We consider space-times which in addition to admitting an isolated horizon
also admit Killing horizons with or without an event horizon. We show that an
isolated horizon is a Killing horizon provided either (1) it admits a
stationary neighbourhood or (2) it admits a neighbourhood with two independent,
commuting Killing vectors. A Killing horizon is always an isolated horizon. For
the case when an event horizon is definable, all conceivable relative locations
of isolated horizon and event horizons are possible. Corresponding conditions
are given.Comment: 14 pages, Latex, no figures. Some arguments tightened. To appear in
Class. Quant. Gra
Development of single crystal beta-alumina membrane
Feasibility of crystal growth technique for beta alumina membrane from molybdenum, tungsten, and iridiu
Challenging the weak cosmic censorship conjecture with charged quantum particles
Motivated by the recent attempts to violate the weak cosmic censorship
conjecture for near-extreme black-holes, we consider the possibility of
overcharging a near-extreme Reissner-Nordstr\"om black hole by the quantum
tunneling of charged particles. We consider the scattering of spin-0 and
spin-1/2 particles by the black hole in a unified framework and obtain
analytically, for the first time, the pertinent reflection and transmission
coefficients without any small charge approximation. Based on these results, we
propose some gedanken experiments that could lead to the violation of the weak
cosmic censorship conjecture due to the (classically forbidden) absorption of
small energy charged particles by the black hole. As for the case of scattering
in Kerr spacetimes, our results demonstrate explicitly that scalar fields are
subject to (electrical) superradiance phenomenon, while spin-1/2 fields are
not. Superradiance impose some limitations on the gedanken experiments
involving spin-0 fields, favoring, in this way, the mechanisms for creation of
a naked singularity by the quantum tunneling of spin-1/2 charged fermions. We
also discuss the implications that vacuum polarization effects and quantum
statistics might have on these gedanken experiments. In particular, we show
that they are not enough to prevent the absorption of incident small energy
particles and, consequently, the formation of a naked singularity.Comment: 9 pages; Final version to appear in PR
Electromagnetic Energy for a Charged Kerr Black Hole in a Uniform Magnetic Field
With the Komar mass formula we calculate the electromagnetic energy for a
charged Kerr black hole in a uniform magnetic field. We find that the total
electromagnetic energy takes the minimum when the Kerr black hole possesses a
non-zero net charge where is the strength of the
magnetic field, is the angular momentum of the black hole, is a
dimensionless parameter determined by the spin of the black hole.Comment: 9 pages, 1 figur
Regular phantom black holes
For self-gravitating, static, spherically symmetric, minimally coupled scalar
fields with arbitrary potentials and negative kinetic energy (favored by the
cosmological observations), we give a classification of possible regular
solutions to the field equations with flat, de Sitter and AdS asymptotic
behavior. Among the 16 presented classes of regular rsolutions are traversable
wormholes, Kantowski-Sachs (KS) cosmologies beginning and ending with de Sitter
stages, and asymptotically flat black holes (BHs). The Penrose diagram of a
regular BH is Schwarzschild-like, but the singularity at is replaced by a
de Sitter infinity, which gives a hypothetic BH explorer a chance to survive.
Such solutions also lead to the idea that our Universe could be created from a
phantom-dominated collapse in another universe, with KS expansion and
isotropization after crossing the horizon. Explicit examples of regular
solutions are built and discussed. Possible generalizations include -essence
type scalar fields (with a potential) and scalar-tensor theories of gravity.Comment: revtex4, 4 pages, no figure
Conformally related massless fields in dS, AdS and Minkowski spaces
In this paper we write down the equation for a scalar conformally coupled
field simultaneously for de Sitter (dS), anti-de Sitter (AdS) and Minkowski
spacetime in d-dimensions. The curvature dependence appears in a very simple
way through a conformal factor. As a consequence the process of curvature free
limit, including wave functions limit and two-points functions, turns to be a
straightforward issue. We determine a set of modes, that we call de Sitter
plane waves, which become ordinary plane waves when the curvature vanishes.Comment: 7 pages, 1 figur
Dynamics in Stationary, Non-Globally Hyperbolic Spacetimes
Classically, the dynamics in a non-globally hyperbolic spacetime is ill
posed. Previously, a prescription was given for defining dynamics in static
spacetimes in terms of a second order operator acting on a Hilbert space
defined on static slices. The present work extends this result by giving a
similar prescription for defining dynamics in stationary spacetimes obeying
certain mild assumptions. The prescription is defined in terms of a first order
operator acting on a different Hilbert space from the one used in the static
prescription. It preserves the important properties of the earlier one: the
formal solution agrees with the Cauchy evolution within the domain of
dependence, and smooth data of compact support always give rise to smooth
solutions. In the static case, the first order formalism agrees with second
order formalism (using specifically the Friedrichs extension). Applications to
field quantization are also discussed.Comment: 18 pages, 1 figure, AMSLaTeX; v2: expanded discussion of field
quantization, new Proposition 3.1, revised Theorem 4.2, corrected typos, and
updated reference
The speed of gravity in general relativity
The question is discussed of what is the speed of gravity (at the fundamental
non-perturbative level). The question is important, if nowhere else, in
discussing the problem of information "lost" in black holes. It turns out that
the duly defined "gravitational signal" generally may be causal, superluminal
and "semi-superluminal". In the class of globally hyperbolic spacetimes the two
last varieties coincide. And if some (often imposed, but not always satisfied)
conditions hold, the signals may be \emph{only} causal. In this sense the speed
of gravity does not exceed the speed of light.Comment: typos corrected, et
On the Noether charge form of the first law of black hole mechanics
The first law of black hole mechanics was derived by Wald in a general
covariant theory of gravity for stationary variations around a stationary black
hole. It is formulated in terms of Noether charges, and has many advantages. In
this paper several issues are discussed to strengthen the validity of the
Noether charge form of the first law. In particular, a gauge condition used in
the derivation is justified. After that, we justify the generalization to
non-stationary variations done by Iyer-Wald.Comment: Latex, 16 pages, arguments on gauge conditions and near-stationary
entropy are added, accepted for publication in Physical Review
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