6,497 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
Cylindrical wormholes
It is shown that the existence of static, cylindrically symmetric wormholes
does not require violation of the weak or null energy conditions near the
throat, and cylindrically symmetric wormhole geometries can appear with less
exotic sources than wormholes whose throats have a spherical topology. Examples
of exact wormhole solutions are given with scalar, spinor and electromagnetic
fields as sources, and these fields are not necessarily phantom. In particular,
there are wormhole solutions for a massless, minimally coupled scalar field in
the presence of a negative cosmological constant, and for an azimuthal Maxwell
electromagnetic field. All these solutions are not asymptotically flat. A no-go
theorem is proved, according to which a flat (or string) asymptotic behavior on
both sides of a cylindrical wormhole throat is impossible if the energy density
of matter is everywhere nonnegative.Comment: 13 pages, no figures. Substantial changes, a no-go theorem and 2
references adde
Two-Dimensional Black Holes and Planar General Relativity
The Einstein-Hilbert action with a cosmological term is used to derive a new
action in 1+1 spacetime dimensions. It is shown that the two-dimensional theory
is equivalent to planar symmetry in General Relativity. The two-dimensional
theory admits black holes and free dilatons, and has a structure similar to
two-dimensional string theories. Since by construction these solutions also
solve Einstein's equations, such a theory can bring two-dimensional results
into the four-dimensional real world. In particular the two-dimensional black
hole is also a black hole in General Relativity.Comment: 11 pages, plainte
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
BLACK HOLES IN THREE-DIMENSIONAL DILATON GRAVITY THEORIES
Three dimensional black holes in a generalized dilaton gravity action theory
are analysed. The theory is specified by two fields, the dilaton and the
graviton, and two parameters, the cosmological constant and the Brans-Dicke
parameter. It contains seven different cases, of which one distinguishes as
special cases, string theory, general relativity and a theory equivalent to
four dimensional general relativity with one Killing vector. We study the
causal structure and geodesic motion of null and timelike particles in the
black hole geometries and find the ADM masses of the different solutions.Comment: 19 pages, latex, 4 figures as uuencoded postscript file
The Three-Dimensional BTZ Black Hole as a Cylindrical System in Four-Dimensional General Relativity
It is shown how to transform the three dimensional BTZ black hole into a four
dimensional cylindrical black hole (i.e., black string) in general relativity.
This process is identical to the transformation of a point particle in three
dimensions into a straight cosmic string in four dimensions.Comment: Latex, 9 page
Thin-shell wormholes in d-dimensional general relativity: Solutions, properties, and stability
We construct thin-shell electrically charged wormholes in d-dimensional
general relativity with a cosmological constant. The wormholes constructed can
have different throat geometries, namely, spherical, planar and hyperbolic.
Unlike the spherical geometry, the planar and hyperbolic geometries allow for
different topologies and in addition can be interpreted as higher-dimensional
domain walls or branes connecting two universes. In the construction we use the
cut-and-paste procedure by joining together two identical vacuum spacetime
solutions. Properties such as the null energy condition and geodesics are
studied. A linear stability analysis around the static solutions is carried
out. A general result for stability is obtained from which previous results are
recovered.Comment: 16 pages, 1 figur
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
Gravitational collapse to toroidal, cylindrical and planar black holes
Gravitational collapse of non-spherical symmetric matter leads inevitably to
non-static external spacetimes. It is shown here that gravitational collapse of
matter with toroidal topology in a toroidal anti-de Sitter background proceeds
to form a toroidal black hole. According to the analytical model presented, the
collapsing matter absorbs energy in the form of radiation (be it scalar,
neutrinos, electromagnetic, or gravitational) from the exterior spacetime. Upon
decompactification of one or two coordinates of the torus one gets collapsing
solutions of cylindrical or planar matter onto black strings or black
membranes, respectively. The results have implications on the hoop conjecture.Comment: 6 pages, Revtex, modifications in the title and in the interpretation
of some results, to appear in Physical Review
Hamiltonian thermodynamics of three-dimensional dilatonic black holes
The action for a class of three-dimensional dilaton-gravity theories with a
cosmological constant can be recast in a Brans-Dicke type action, with its free
parameter. These theories have static spherically symmetric black
holes. Those with well formulated asymptotics are studied through a Hamiltonian
formalism, and their thermodynamical properties are found out. The theories
studied are general relativity (), a dimensionally reduced
cylindrical four-dimensional general relativity theory (), and a
theory representing a class of theories (). The Hamiltonian
formalism is setup in three dimensions through foliations on the right region
of the Carter-Penrose diagram, with the bifurcation 1-sphere as the left
boundary, and anti-de Sitter infinity as the right boundary. The metric
functions on the foliated hypersurfaces are the canonical coordinates. The
Hamiltonian action is written, the Hamiltonian being a sum of constraints. One
finds a new action which yields an unconstrained theory with one pair of
canonical coordinates , being the mass parameter and its
conjugate momenta The resulting Hamiltonian is a sum of boundary terms only. A
quantization of the theory is performed. The Schr\"odinger evolution operator
is constructed, the trace is taken, and the partition function of the canonical
ensemble is obtained. The black hole entropies differ, in general, from the
usual quarter of the horizon area due to the dilaton.Comment: 34 pages, 3 figures, references added, minor changes in the revised
versio
- …