290 research outputs found
p-Brane Black Holes as Stability Islands
In multidimensional gravity with an arbitrary number of internal Ricci-flat
factor spaces, interacting with electric and magnetic -branes, spherically
symmetric configurations are considered. It is shown that all single-brane
black-hole solutions are stable under spherically symmetric perturbations,
whereas similar solutions possessing naked singularities turn out to be
catastrophically unstable. The black hole stability conclusion is extended to
some classes of configurations with intersecting branes. These results do not
depend on the particular composition of the -dimensional space-time, on the
number of dilatonic scalar fields and on the values of their coupling
constants. Some examples from 11-dimensional supergravity are considered.Comment: 16 pages, Latex2
Regular black holes and black universes
We give a comparative description of different types of regular static,
spherically symmetric black holes (BHs) and discuss in more detail their
particular type, which we suggest to call black universes. The latter have a
Schwarzschild-like causal structure, but inside the horizon there is an
expanding Kantowski-Sachs universe and a de Sitter infinity instead of a
singularity. Thus a hypothetic BH explorer gets a chance to survive. Solutions
of this kind are naturally obtained if one considers static, spherically
symmetric distributions of various (but not all) kinds of phantom matter whose
existence is favoured by cosmological observations. It also looks possible that
our Universe has originated from phantom-dominated collapse in another universe
and underwent isotropization after crossing the horizon. An explicit example of
a black-universe solution with positive Schwarzschild mass is discussed.Comment: 13 pages, 1 figure. 6 referenses and some discussion added, misprints
correcte
On the stability of scalar-vacuum space-times
We study the stability of static, spherically symmetric solutions to the
Einstein equations with a scalar field as the source. We describe a general
methodology of studying small radial perturbations of scalar-vacuum
configurations with arbitrary potentials V(\phi), and in particular space-times
with throats (including wormholes), which are possible if the scalar is
phantom. At such a throat, the effective potential for perturbations V_eff has
a positive pole (a potential wall) that prevents a complete perturbation
analysis. We show that, generically, (i) V_eff has precisely the form required
for regularization by the known S-deformation method, and (ii) a solution with
the regularized potential leads to regular scalar field and metric
perturbations of the initial configuration. The well-known conformal mappings
make these results also applicable to scalar-tensor and f(R) theories of
gravity. As a particular example, we prove the instability of all static
solutions with both normal and phantom scalars and V(\phi) = 0 under spherical
perturbations. We thus confirm the previous results on the unstable nature of
anti-Fisher wormholes and Fisher's singular solution and prove the instability
of other branches of these solutions including the anti-Fisher "cold black
holes".Comment: 18 pages, 5 figures. A few comments and references added. Final
version accepted at EPJ
On a general class of brane-world black holes
We use the general solution to the trace of the 4-dimensional Einstein
equations for static, spherically symmetric configurations as a basis for
finding a general class of black hole (BH) metrics, containing one arbitrary
function which vanishes at some , the horizon
radius. Under certain reasonable restrictions, BH metrics are found with or
without matter and, depending on the boundary conditions, can be asymptotically
flat or have any other prescribed large behaviour. It is shown that this
procedure generically leads to families of solutions unifying non-extremal
globally regular BHs with a Kerr-like global structure, extremal BHs and
symmetric wormholes. Horizons in space-times with zero scalar curvature are
shown to be either simple or double. The same is generically true for horizons
inside a matter distribution, but in special cases there can be horizons of any
order. A few simple examples are discussed. A natural application of the above
results is the brane world concept, in which the trace of the 4D gravity
equations is the only unambiguous equation for the 4D metric, and its solutions
can be continued into the 5D bulk according to the embedding theorems.Comment: 9 pages, revtex
Possible wormholes in a brane world
The condition R=0, where R is the four-dimensional scalar curvature, is used
for obtaining a large class (with an arbitrary function of r) of static,
spherically symmetric Lorentzian wormhole metrics. The wormholes are globally
regular and traversable, can have throats of arbitrary size and can be both
symmetric and asymmetric. These metrics may be treated as possible wormhole
solutions in a brane world since they satisfy the vacuum Einstein equations on
the brane where effective stress-energy is induced by interaction with the bulk
gravitational field. Some particular examples are discussed.Comment: 7 pages, revtex4. Submitted to Phys. Rev.
Nonsingular multidimensional cosmologies without fine tuning
Exact cosmological solutions for effective actions in D dimensions inspired
by the tree-level superstring action are studied. For a certain range of free
parameters existing in the model, nonsingular bouncing solutions are found.
Among them, of particular interest can be open hyperbolic models, in which,
without any fine tuning, the internal scale factor and the dilaton field
(connected with string coupling in string theories) tend to constant values at
late times. A cosmological singularity is avoided due to nonminimal
dilaton-gravity coupling and, for D > 11, due to pure imaginary nature of the
dilaton, which conforms to currently discussed unification models. The
existence of such and similar solutions supports the opinion that the Universe
had never undergone a stage driven by full-scale quantum gravity.Comment: Latex 2e, 9 page
4D gravity localized in non Z_2-symmetric thick branes
We present a comparative analysis of localization of 4D gravity on a non
Z_2-symmetric scalar thick brane in both a 5-dimensional Riemannian space time
and a pure geometric Weyl integrable manifold. This work was mainly motivated
by the hypothesis which claims that Weyl geometries mimic quantum behaviour
classically. We start by obtaining a classical 4-dimensional Poincare invariant
thick brane solution which does not respect Z_2-symmetry along the
(non-)compact extra dimension. The scalar energy density of our field
configuration represents several series of thick branes with positive and
negative energy densities centered at y_0. The only qualitative difference we
have encountered when comparing both frames is that the scalar curvature of the
Riemannian manifold turns out to be singular for the found solution, whereas
its Weylian counterpart presents a regular behaviour. By studying the
transverse traceless modes of the fluctuations of the classical backgrounds, we
recast their equations into a Schroedinger's equation form with a volcano
potential of finite bottom (in both frames). By solving the Schroedinger
equation for the massless zero mode m^2=0 we obtain a single bound state which
represents a stable 4-dimensional graviton in both frames. We also get a
continuum gapless spectrum of KK states with positive m^2>0 that are suppressed
at y_0, turning into continuum plane wave modes as "y" approaches spatial
infinity. We show that for the considered solution to our setup, the potential
is always bounded and cannot adopt the form of a well with infinite walls;
thus, we do not get a discrete spectrum of KK states, and we conclude that the
claim that Weylian structures mimic, classically, quantum behaviour does not
constitute a generic feature of these geometric manifolds.Comment: 13 pages, 4 figures, JHEP forma
Composite electric S-brane solutions with maximal number of branes
In this paper we consider (n+1)-dimensional cosmological model with scalar
field and antisymmetric (p+2)-form. Using an electric composite Sp-brane ansatz
the field equations for the original system reduce to the equations for a
Toda-like system with n(n-1)/2 quadratic constraints on the charge densities.
For certain odd dimensions (D = 4m+1 = 5, 9, 13, ...) and (p+2)-forms (p = 2m-1
= 1, 3, 5, ...) these algebraic constraints can be satisfied with the maximal
number of charged branes ({\it i.e.} all the branes have non-zero charge
densities). These solutions are characterized by self-dual or anti-self-dual
charge density forms Q (of rank 2m). For these algebraic solutions with the
particular D, p, Q and non-exceptional dilatonic coupling constant \lambda we
obtain general cosmological solutions to the field equations and some
properties of these solutions are highlighted (e.g. Kasner-like behavior, the
existence of attractor solutions). We prove the absence of maximal
configurations for p =1 and even D (e.g. for D =10 supergravity models and
those of superstring origin).Comment: 19 pages JHEP format, references adde
Vacuum polarization in the spacetime of charged nonlinear black hole
Building on general formulas obtained from the approximate renormalized
effective action, the approximate stress-energy tensor of the quantized massive
scalar field with arbitrary curvature coupling in the spacetime of charged
black hole being a solution of coupled equations of nonlinear electrodynamics
and general relativity is constructed and analysed. It is shown that in a few
limiting cases, the analytical expressions relating obtained tensor to the
general renormalized stress-energy tensor evaluated in the geometry of the
Reissner-Nordstr\"{o}m black hole could be derived. A detailed numerical
analysis with special emphasis put on the minimal coupling is presented and the
results are compared with those obtained earlier for the conformally coupled
field. Some novel features of the renormalized stress-energy tensor are
discussed
Extremal limit of the regular charged black holes in nonlinear electrodynamics
The near horizon limit of the extreme nonlinear black hole is investigated.
It is shown that resulting geometry belongs to the AdS2xS2 class with different
modules of curvatures of subspaces and could be described in terms of the
Lambert functions. It is demonstrated that the considered class of Lagrangians
does not admit solutions of the Bertotti-Robinson type
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