7,371 research outputs found
Collapsing shells of radiation in anti-de Sitter spacetimes and the hoop and cosmic censorship conjectures
Gravitational collapse of radiation in an anti-de Sitter background is
studied. For the spherical case, the collapse proceeds in much the same way as
in the Minkowski background, i.e., massless naked singularities may form for a
highly inhomogeneous collapse, violating the cosmic censorship, but not the
hoop conjecture. The toroidal, cylindrical and planar collapses can be treated
together. In these cases no naked singularity ever forms, in accordance with
the cosmic censorship. However, since the collapse proceeds to form toroidal,
cylindrical or planar black holes, the hoop conjecture in an anti-de Sitter
spacetime is violated.Comment: 4 pages, Revtex Journal: to appear in Physical Review
Exact General Relativistic Perfect Fluid Disks with Halos
Using the well-known ``displace, cut and reflect'' method used to generate
disks from given solutions of Einstein field equations, we construct static
disks made of perfect fluid based on vacuum Schwarzschild's solution in
isotropic coordinates. The same method is applied to different exactsolutions
to the Einstein'sequations that represent static spheres of perfect fluids. We
construct several models of disks with axially symmetric perfect fluid halos.
All disks have some common features: surface energy density and pressures
decrease monotonically and rapidly with radius. As the ``cut'' parameter
decreases, the disks become more relativistic, with surface energy density and
pressure more concentrated near the center. Also regions of unstable circular
orbits are more likely to appear for high relativistic disks. Parameters can be
chosen so that the sound velocity in the fluid and the tangential velocity of
test particles in circular motion are less then the velocity of light. This
tangential velocity first increases with radius and reaches a maximum.Comment: 22 pages, 25 eps.figs, RevTex. Phys. Rev. D to appea
The Two-Dimensional Analogue of General Relativity
General Relativity in three or more dimensions can be obtained by taking the
limit in the Brans-Dicke theory. In two dimensions
General Relativity is an unacceptable theory. We show that the two-dimensional
closest analogue of General Relativity is a theory that also arises in the
limit of the two-dimensional Brans-Dicke theory.Comment: 8 pages, LaTeX, preprint DF/IST-17.9
Rotating Relativistic Thin Disks
Two families of models of rotating relativistic disks based on Taub-NUT and
Kerr metrics are constructed using the well-known "displace, cut and reflect"
method. We find that for disks built from a generic stationary axially
symmetric metric the "sound velocity", , is equal to
the geometric mean of the prograde and retrograde geodesic circular velocities
of test particles moving on the disk. We also found that for generic disks we
can have zones with heat flow. For the two families of models studied the
boundaries that separate the zones with and without heat flow are not stable
against radial perturbations (ring formation).Comment: 18 eps figures, to be published PR
Local conditions for the generalized covariant entropy bound
A set of sufficient conditions for the generalized covariant entropy bound
given by Strominger and Thompson is as follows: Suppose that the entropy of
matter can be described by an entropy current . Let be any null
vector along and . Then the generalized bound can be
derived from the following conditions: (i) , where
s'=k^a\grad_a s and is the stress energy tensor; (ii) on the initial
2-surface , , where is the expansion of
. We prove that condition (ii) alone can be used to divide a spacetime
into two regions: The generalized entropy bound holds for all light sheets
residing in the region where and fails for those in the region
where . We check the validity of these conditions in FRW flat
universe and a scalar field spacetime. Some apparent violations of the entropy
bounds in the two spacetimes are discussed. These holographic bounds are
important in the formulation of the holographic principle.Comment: 10 pages, 7 figure
Gravitational Collapse of Perfect Fluid in Self-Similar Higher Dimensional Space-Times
We investigate the occurrence and nature of naked singularities in the
gravitational collapse of an adiabatic perfect fluid in self-similar higher
dimensional space-times. It is shown that strong curvature naked singularities
could occur if the weak energy condition holds. Its implication for cosmic
censorship conjecture is discussed. Known results of analogous studies in four
dimensions can be recovered.Comment: 11 Pages, Latex, no figures, Accepted in Int. J. Mod. Phys.
Does a relativistic metric generalization of Newtonian gravity exist in 2+1 dimensions?
It is shown that, contrary to previous claims, a scalar tensor theory of
Brans-Dicke type provides a relativistic generalization of Newtonian gravity in
2+1 dimensions. The theory is metric and test particles follow the space-time
geodesics. The static isotropic solution is studied in vacuum and in regions
filled with an incompressible perfect fluid. It is shown that the solutions can
be consistently matched at the matter vacuum interface, and that the Newtonian
behavior is recovered in the weak field regime.Comment: 6 pages, no figures, Revtex4. Some discussions on the physical nature
of the interior solution and on the omega->infinity limit and some references
added. Version to appear in Phys. Rev.
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