6,484 research outputs found
Stationary Black Holes in a Generalized Three-Dimensional Theory of Gravity
We consider a generalized three-dimensional theory of gravity which is
specified by two fields, the graviton and the dilaton, and one parameter. This
theory contains, as particular cases, three-dimensional General Relativity and
three-dimensional String Theory. Stationary black hole solutions are generated
from the static ones using a simple coordinate transformation. The stationary
black holes solutions thus obtained are locally equivalent to the corresponding
static ones, but globally distinct. The mass and angular momentum of the
stationary black hole solutions are computed using an extension of the Regge
and Teitelboim formalism. The causal structure of the black holes is described.Comment: 12 pages, Late
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.
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
Spontaneously broken symmetry restoration of quantum fields in the vicinity of neutral and electrically charged black holes
We consider the restoration of a spontaneously broken symmetry of an
interacting quantum scalar field around neutral, i.e., Schwarzschild, and
electrically charged, i.e., Reissner-Nordstr\"om, black holes in four
dimensions. This is done through a semiclassical self-consistent procedure, by
solving the system of non-linear coupled equations describing the dynamics of
the background field and the vacuum polarization. The black hole at its own
horizon generates an indefinitely high temperature which decreases to the
Hawking temperature at infinity. Due to the high temperature in its vicinity,
there forms a bubble around the black hole in which the scalar field can only
assume a value equal to zero, a minimum of energy. Thus, in this region the
symmetry of the energy and the field is preserved. At the bubble radius, there
is a phase transition in the value of the scalar field due to a spontaneous
symmetry breaking mechanism. Indeed, outside the bubble radius the temperature
is low enough such that the scalar field settles with a nonzero value in a new
energy minimum, indicating a breaking of the symmetry in this outer region.
Conversely, there is symmetry restoration from the outer region to the inner
bubble close to the horizon. Specific properties that emerge from different
black hole electric charges are also noteworthy. It is found that colder black
holes, i.e., more charged ones, have a smaller bubble length of restored
symmetry. In the extremal case the bubble has zero length, i.e., there is no
bubble. Additionally, for colder black holes, it becomes harder to excite the
quantum field modes, so the vacuum polarization has smaller values. In the
extremal case, the black hole temperature is zero and the vacuum polarization
is never excited.Comment: 16 pages, 4 figure
Entropy of an extremal electrically charged thin shell and the extremal black hole
There is a debate as to what is the value of the the entropy of extremal
black holes. There are approaches that yield zero entropy , while there
are others that yield the Bekenstein-Hawking entropy , in Planck
units. There are still other approaches that give that is proportional to
or even that is a generic well-behaved function of . Here
is the black hole horizon radius and is its horizon area.
Using a spherically symmetric thin matter shell with extremal electric charge,
we find the entropy expression for the extremal thin shell spacetime. When the
shell's radius approaches its own gravitational radius, and thus turns into an
extremal black hole, we encounter that the entropy is , i.e., the
entropy of an extremal black hole is a function of alone. We speculate
that the range of values for an extremal black hole is .Comment: 11 pages, minor changes, added references, matches the published
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