2,209 research outputs found
Weak Cosmic Censorship: As Strong as Ever
Spacetime singularities that arise in gravitational collapse are always
hidden inside of black holes. This is the essence of the weak cosmic censorship
conjecture. The hypothesis, put forward by Penrose 40 years ago, is still one
of the most important open questions in general relativity. In this Letter, we
reanalyze extreme situations which have been considered as counterexamples to
the weak cosmic censorship conjecture. In particular, we consider the
absorption of scalar particles with large angular momentum by a black hole.
Ignoring back reaction effects may lead one to conclude that the incident wave
may overspin the black hole, thereby exposing its inner singularity to distant
observers. However, we show that when back reaction effects are properly taken
into account, the stability of the black-hole event horizon is irrefutable. We
therefore conclude that cosmic censorship is actually respected in this type of
gedanken experiments.Comment: 4 page
The frictional Schr\"odinger-Newton equation in models of wave function collapse
Replacing the Newtonian coupling G by -iG, the Schrodinger-Newton equation
becomes ``frictional''. Instead of the reversible Schrodinger-Newton equation,
we advocate its frictional version to generate the set of pointer states for
macroscopic quantum bodies.Comment: 6pp LaTeX for J.Phys.Conf.Ser.+2 figs. Talk given at the Int.
Workshop DICE2006 "Quantum Mechanics between Decoherence and Determinism: new
aspects from particle physics to cosmology" Piombino, Sept 11-15, 200
Linearized gravity and gauge conditions
In this paper we consider the field equations for linearized gravity and
other integer spin fields on the Kerr spacetime, and more generally on
spacetimes of Petrov type D. We give a derivation, using the GHP formalism, of
decoupled field equations for the linearized Weyl scalars for all spin weights
and identify the gauge source functions occuring in these. For the spin weight
0 Weyl scalar, imposing a generalized harmonic coordinate gauge yields a
generalization of the Regge-Wheeler equation. Specializing to the Schwarzschild
case, we derive the gauge invariant Regge-Wheeler and Zerilli equation directly
from the equation for the spin 0 scalar.Comment: 24 pages, corresponds to published versio
On trapped surface formation in gravitational collapse II
Further to our consideration on trapped surfaces in gravitational collapse,
where pressures were allowed to be negative while satisfying weak energy
condition to avoid trapped surface formation, we discuss here several other
attempts of similar nature in this direction. Certain astrophysical aspects are
pointed out towards examining the physical realization of such a possibility in
realistic gravitational collapse
Naked Singularity of the Vaidya-deSitter Spacetime and Cosmic Censorship Hypothesis
We investigate the formation of a locally naked singularity in the collapse
of radiation shells in an expanding Vaidya-deSitter background. This is
achieved by considering the behaviour of non-spacelike and radial geodesics
originating at the singularity. A specific condition is determined for the
existence of radially outgoing, null geodesics originating at the singularity
which, when this condition is satisfied, becomes locally naked. This condition
turns out to be the same as that in the collapse of radiation shells in an
asymptotically flat background. Therefore, we have, at least for the case
considered here, established that the asymptotic flatness of the spacetime is
not essential for the development of a locally naked singularity. Our result
then unequivocally supports the view that no special role be given to
asymptotic observers (or, for that matter, any set of observers) in the
formulation of the Cosmic Censorship Hypothesis.Comment: submitted Oct. 1997, Revised Version, to be published Gen. Rel.
Grav., Latex file, 9 page
Numerical treatment of the hyperboloidal initial value problem for the vacuum Einstein equations. III. On the determination of radiation
We discuss the issue of radiation extraction in asymptotically flat
space-times within the framework of conformal methods for numerical relativity.
Our aim is to show that there exists a well defined and accurate extraction
procedure which mimics the physical measurement process. It operates entirely
intrisically within \scri^+ so that there is no further approximation
necessary apart from the basic assumption that the arena be an asymptotically
flat space-time. We define the notion of a detector at infinity by idealising
local observers in Minkowski space. A detailed discussion is presented for
Maxwell fields and the generalisation to linearised and full gravity is
performed by way of the similar structure of the asymptotic fields.Comment: LaTeX2e,13 pages,2 figure
A Maximally Symmetric Vector Propagator
We derive the propagator for a massive vector field on a de Sitter background
of arbitrary dimension. This propagator is de Sitter invariant and possesses
the proper flat spacetime and massless limits. Moreover, the retarded Green's
function inferred from it produces the correct classical response to a test
source. Our result is expressed in a tensor basis which is convenient for
performing quantum field theory computations using dimensional regularization.Comment: 21 pages, no figures, uses LaTeX 2 epsilon, version 2 has an error in
eqn (86) corrected and an updated reference lis
Clifford-Finsler Algebroids and Nonholonomic Einstein-Dirac Structures
We propose a new framework for constructing geometric and physical models on
nonholonomic manifolds provided both with Clifford -- Lie algebroid symmetry
and nonlinear connection structure. Explicit parametrizations of generic
off-diagonal metrics and linear and nonlinear connections define different
types of Finsler, Lagrange and/or Riemann-Cartan spaces. A generalization to
spinor fields and Dirac operators on nonholonomic manifolds motivates the
theory of Clifford algebroids defined as Clifford bundles, in general, enabled
with nonintegrable distributions defining the nonlinear connection. In this
work, we elaborate the algebroid spinor differential geometry and formulate the
(scalar, Proca, graviton, spinor and gauge) field equations on Lie algebroids.
The paper communicates new developments in geometrical formulation of physical
theories and this approach is grounded on a number of previous examples when
exact solutions with generic off-diagonal metrics and generalized symmetries in
modern gravity define nonholonomic spacetime manifolds with uncompactified
extra dimensions.Comment: The manuscript was substantially modified following recommendations
of JMP referee. The former Chapter 2 and Appendix were elliminated. The
Introduction and Conclusion sections were modifie
Accretion with back reaction
We calculate analytically a back reaction of the stationary spherical
accretion flow near the event horizon and near the inner Cauchy horizon of the
charged black hole. It is shown that corresponding back-reaction corrections to
the black hole metric depend only on the fluid accretion rate and diverge in
the case of an extremely charged black hole. In result, the test fluid
approximation for stationary accretion is violated for extreme black holes.
This behavior of the accreting black hole is in accordance with the third law
of black hole thermodynamics, forbidding the practical attainability of the
extreme state.Comment: 5 pages, 2 figures; new figure and references adde
Note on (conformally) semi-symmetric spacetimes
We provide a simple proof that conformally semi-symmetric spacetimes are
actually semi-symmetric. We also present a complete refined classification of
the semi-symmetric spacetimes.Comment: 5 pages, no figure
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