17,653 research outputs found
On the genericity of spacetime singularities
We consider here the genericity aspects of spacetime singularities that occur
in cosmology and in gravitational collapse. The singularity theorems (that
predict the occurrence of singularities in general relativity) allow the
singularities of gravitational collapse to be either visible to external
observers or covered by an event horizon of gravity. It is shown that the
visible singularities that develop as final states of spherical collapse are
generic. Some consequences of this fact are discussed.Comment: 19 pages, To be published in the Raychaudhuri Volume, eds. Naresh
Dadhich, Pankaj Joshi and Probir Ro
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
A survey of polarization in the JVAS/CLASS flat-spectrum radio source surveys: I. The data and catalogue production
We have used the very large JVAS/CLASS 8.4-GHz surveys of flat-spectrum radio
sources to obtain a large, uniformly observed and calibrated, sample of radio
source polarizations. These are useful for many investigations of the
properties of radio sources and the interstellar medium. We discuss comparisons
with polarization measurements from this survey and from other large-scale
surveys of polarization in flat-spectrum sources.Comment: Accepted by MNRAS. 8 pages, 5 figures. Full version of Table 2
available at http://www.jb.man.ac.uk/~njj/classqu_po
Final fate of the spherically symmetric collapse of a perfect fluid
The final fate of the spherically symmetric collapse of a perfect fluid which
follows the -law equation of state and adiabatic condition is
investigated. Full general relativistic hydrodynamics is solved numerically
using a retarded time coordinate, the so-called observer time coordinate.
Thanks to this coordinate, the causal structure of the resultant space-time is
automatically constructed. Then, it is found that a globally naked,
shell-focusing singularity can occur at the center from relativistically
high-density, isentropic and time symmetric initial data if \gamma \alt 1.01
within the numerical accuracy. The result is free from the assumption of
self-similarity. The upper limit of with which a naked singularity can
occur from generic initial data is consistent with the result of Ori and Piran
based on the assumption of self-similarity.Comment: 17 pages, including 21 ps figures. Accepted for publication in
Physical Review D, Typos corrected, References update
Cosmic Censorship in Higher dimension II
Generalizing earlier results on dust collapse in higher dimensions, we show
here that cosmic censorship can be restored in gravitational collapse with
tangential pressure present if we take the spacetime dimension to be .
This is under conditions to be motivated physically, such as the smoothness of
initial data from which the collapse develops. The models considered here
incorporating a non-zero tangential pressure include the Einstein cluster
spacetime.Comment: 7 pages,1 figure,revtex
Divergence of the Quantum Stress Tensor on the Cauchy Horizon in 2-d Dust Collapse
We prove that the quantum stress tensor for a massless scalar field in two
dimensional non-selfsimilar Tolman Bondi dust collapse and Vaidya radiation
collapse models diverges on the Cauchy horizon, if the latter exists. The two
dimensional model is obtained by suppressing angular co-ordinates in the
corresponding four dimensional spherical model.Comment: 16 pages, no figures, LaTeX fil
Gravitational collapse in asymptotically Anti-de Sitter/de Sitter backgrounds
We study here the gravitational collapse of a matter cloud with a
non-vanishing tangential pressure in the presence of a non-zero cosmological
term. Conditions for bounce and singularity formation are derived for the
model. It is also shown that when the tangential pressures vanish, the bounce
and singularity conditions reduce to that of the dust case studied earlier. The
collapsing interior is matched with an exterior which is asymptotically de
Sitter or anti de Sitter, depending on the sign of cosmological constant. The
junction conditions for matching the cloud to exterior are specified. The
effect of the cosmological term on apparent horizons is studied in some detail,
and the nature of central singularity is analyzed. We also discuss here the
visibility of the singularity and implications for the cosmic censorship
conjecture.Comment: 11 pages, 1 figure, Revtex
Spherical gravitational collapse in N-dimensions
We investigate here spherically symmetric gravitational collapse in a
spacetime with an arbitrary number of dimensions and with a general {\it type
I} matter field, which is a broad class that includes most of the physically
reasonable matter forms. We show that given the initial data for matter in
terms of the initial density and pressure profiles at an initial surface
from which the collapse evolves, there exist rest of the initial data
functions and classes of solutions of Einstein equations which we construct
here, such that the spacetime evolution goes to a final state which is either a
black hole or a naked singularity, depending on the nature of initial data and
evolutions chosen, and subject to validity of the weak energy condition. The
results are discussed and analyzed in the light of the cosmic censorship
hypothesis in black hole physics. The formalism here combines the earlier
results on gravitational collapse in four dimensions in a unified treatment.
Also the earlier work is generalized to higher dimensional spacetimes to allow
a study of the effect of number of dimensions on the possible final outcome of
the collapse in terms of either a black hole or naked singularity. No
restriction is adopted on the number of dimensions, and other limiting
assumptions such as self-similarity of spacetime are avoided, in order to keep
the treatment general. Our methodology allows to consider to an extent the
genericity and stability aspects related to the occurrence of naked
singularities in gravitational collapse.Comment: Revtex4, The replaced version matches the published on
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