9,948 research outputs found
A Cosmological Constant Limits the Size of Black Holes
In a space-time with cosmological constant and matter satisfying
the dominant energy condition, the area of a black or white hole cannot exceed
. This applies to event horizons where defined, i.e. in an
asymptotically deSitter space-time, and to outer trapping horizons (cf.
apparent horizons) in any space-time. The bound is attained if and only if the
horizon is identical to that of the degenerate `Schwarzschild-deSitter'
solution. This yields a topological restriction on the event horizon, namely
that components whose total area exceeds cannot merge. We
discuss the conjectured isoperimetric inequality and implications for the
cosmic censorship conjecture.Comment: 10 page
Generalized inverse mean curvature flows in spacetime
Motivated by the conjectured Penrose inequality and by the work of Hawking,
Geroch, Huisken and Ilmanen in the null and the Riemannian case, we examine
necessary conditions on flows of two-surfaces in spacetime under which the
Hawking quasilocal mass is monotone. We focus on a subclass of such flows which
we call uniformly expanding, which can be considered for null as well as for
spacelike directions. In the null case, local existence of the flow is
guaranteed. In the spacelike case, the uniformly expanding condition leaves a
1-parameter freedom, but for the whole family, the embedding functions satisfy
a forward-backward parabolic system for which local existence does not hold in
general. Nevertheless, we have obtained a generalization of the weak
(distributional) formulation of this class of flows, generalizing the
corresponding step of Huisken and Ilmanen's proof of the Riemannian Penrose
inequality.Comment: 21 pages, 1 figur
Hamiltonians for Reduced Gravity
A generalised canonical formulation of gravity is devised for foliations of
spacetime with codimension . The new formalism retains n-dimensional
covariance and is especially suited to 2+2 decompositions of spacetime. It is
also possible to use the generalised formalism to obtain boundary contributions
to the 3+1 Hamiltonian.Comment: 18 pages, revtex, 3 postscript figures include
Construction and enlargement of traversable wormholes from Schwarzschild black holes
Analytic solutions are presented which describe the construction of a
traversable wormhole from a Schwarzschild black hole, and the enlargement of
such a wormhole, in Einstein gravity. The matter model is pure radiation which
may have negative energy density (phantom or ghost radiation) and the
idealization of impulsive radiation (infinitesimally thin null shells) is
employed.Comment: 22 pages, 7 figure
Quasi-local first law of black-hole dynamics
A property well known as the first law of black hole is a relation among
infinitesimal variations of parameters of stationary black holes. We consider a
dynamical version of the first law, which may be called the first law of black
hole dynamics. The first law of black hole dynamics is derived without assuming
any symmetry or any asymptotic conditions. In the derivation, a definition of
dynamical surface gravity is proposed. In spherical symmetry it reduces to that
defined recently by one of the authors (SAH).Comment: Latex, 8 pages; version to appear in Class. Quantum Gra
Is the gravitational action additive?
The gravitational action is not always additive in the usual sense. We
provide a general prescription for the change in action that results when
different portions of the boundary of a spacetime are topologically identified.
We discuss possible implications for the superposition law of quantum gravity.
We present a definition of `generalized additivity' which does hold for
arbitrary spacetime composition.Comment: 20 pages LaTeX file, report numbers UMD-PP 94-100 and Alberta Thy
10-9
Note on Signature Change and Colombeau Theory
Recent work alludes to various `controversies' associated with signature
change in general relativity. As we have argued previously, these are in fact
disagreements about the (often unstated) assumptions underlying various
possible approaches. The choice between approaches remains open.Comment: REVTex, 3 pages; to appear in GR
On the variational principle for dust shells in General Relativity
The variational principle for a thin dust shell in General Relativity is
constructed. The principle is compatible with the boundary-value problem of the
corresponding Euler-Lagrange equations, and leads to ``natural boundary
conditions'' on the shell. These conditions and the gravitational field
equations which follow from an initial variational principle, are used for
elimination of the gravitational degrees of freedom. The transformation of the
variational formula for spherically-symmetric systems leads to two natural
variants of the effective action. One of these variants describes the shell
from a stationary interior observer's point of view, another from the exterior
one. The conditions of isometry of the exterior and interior faces of the shell
lead to the momentum and Hamiltonian constraints. The canonical equivalence of
the mentioned systems is shown in the extended phase space. Some particular
cases are considered.Comment: 25 pages, RevTeX, no figures, revised version, typos corrected,
accepted for publication in Journal of Mathematical Physic
Quasilocal Thermodynamics of Dilaton Gravity coupled to Gauge Fields
We consider an Einstein-Hilbert-Dilaton action for gravity coupled to various
types of Abelian and non-Abelian gauge fields in a spatially finite system.
These include Yang-Mills fields and Abelian gauge fields with three and
four-form field strengths. We obtain various quasilocal quantities associated
with these fields, including their energy and angular momentum, and develop
methods for calculating conserved charges when a solution possesses sufficient
symmetry. For stationary black holes, we find an expression for the entropy
from the micro-canonical form of the action. We also find a form of the first
law of black hole thermodynamics for black holes with the gauge fields of the
type considered here.Comment: 41 pages, latex, uses fonts provided by AMSTe
Reception of laser generated ultrasound from a CFRP plate by an air matched piezoelectric composite transducer
Laser generated ultrasound is being investigated [1,2] for testing structures made of both conventional metals and carbon fibre reinforced polymer (CFRP). Laser interferometers are widely used in such work to detect the normal surface motion caused by ultrasonic pulses. Interferometers offer non-contact, remote and high-fidelity detection, together with a potential to cover large areas rapidly by optical scanning. However their cost is high and only in testing large and/or expensive structures may the cost be justified. A lower cost alternative, but with some compromise on the virtues of an interferometer, would be to use an air transducer as a receiver
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