622 research outputs found

### Gravitating Brane Systems: Some General Theorems

Multidimensional gravity interacting with intersecting electric and magnetic
$p$-branes is considered for fields depending on a single variable. Some
general features of the system behaviour are revealed without solving the field
equations. Thus, essential asymptotic properties of isotropic cosmologies are
indicated for different signs of spatial curvature; a no-hair-type theorem and
a single-time theorem for black holes are proved (the latter makes sense in
models with multiple time coordinates). The validity of the general
observations is verified for a class of exact solutions known for the cases
when certain vectors, built from the input parameters of the model, are either
orthogonal in minisuperspace, or form mutually orthogonal subsystems. From the
non-existence of Lorentzian wormholes, a universal restriction is obtained,
applicable to orthogonal or block-orthogonal subsystems of any $p$-brane
system.Comment: 13 pages, Latex2e, 1 Latex figure, uses bezier.st

### Neutral and charged matter in equilibrium with black holes

We study the conditions of a possible static equilibrium between spherically
symmetric, electrically charged or neutral black holes and ambient matter. The
following kinds of matter are considered: (1) neutral and charged matter with a
linear equation of state p_r = w\rho (for neutral matter the results of our
previous work are reproduced), (2) neutral and charged matter with p_r \sim
\rho^m, m > 1, and (3) the possible presence of a "vacuum fluid" (the
cosmological constant or, more generally, anything that satisfies the equality
T^0_0 = T^1_1 at least at the horizon). We find a number of new cases of such
an equilibrium, including those generalizing the well-known Majumdar-Papapetrou
conditions for charged dust. It turns out, in particular, that ultraextremal
black holes cannot be in equilibrium with any matter in the absence of a vacuum
fluid; meanwhile, matter with w > 0, if it is properly charged, can surround an
extremal charged black hole.Comment: 12 pages, no figures, final version published in PR

### Regular phantom black holes

For self-gravitating, static, spherically symmetric, minimally coupled scalar
fields with arbitrary potentials and negative kinetic energy (favored by the
cosmological observations), we give a classification of possible regular
solutions to the field equations with flat, de Sitter and AdS asymptotic
behavior. Among the 16 presented classes of regular rsolutions are traversable
wormholes, Kantowski-Sachs (KS) cosmologies beginning and ending with de Sitter
stages, and asymptotically flat black holes (BHs). The Penrose diagram of a
regular BH is Schwarzschild-like, but the singularity at $r=0$ is replaced by a
de Sitter infinity, which gives a hypothetic BH explorer a chance to survive.
Such solutions also lead to the idea that our Universe could be created from a
phantom-dominated collapse in another universe, with KS expansion and
isotropization after crossing the horizon. Explicit examples of regular
solutions are built and discussed. Possible generalizations include $k$-essence
type scalar fields (with a potential) and scalar-tensor theories of gravity.Comment: revtex4, 4 pages, no figure

### The Birkhoff Theorem in Multidimensional Gravity

The validity conditions for the extended Birkhoff theorem in multidimensional
gravity with $n$ internal spaces are formulated, with no restriction on
space-time dimensionality and signature. Examples of matter sources and
geometries for which the theorem is valid are given. Further generalization of
the theorem is discussed.Comment: 8 page

### Stability of thin-shell wormholes with spherical symmetry

In this article, the stability of a general class of spherically symmetric
thin-shell wormholes is studied under perturbations preserving the symmetry.
For this purpose, the equation of state at the throat is linearized around the
static solutions. The formalism presented here is applied to dilaton wormholes
and it is found that there is a smaller range of possible stable configurations
for them than in the case of Reissner-Nordstrom wormholes with the same charge.Comment: 14 pages, 3 figure

### Notes on wormhole existence in scalar-tensor and F(R) gravity

Some recent papers have claimed the existence of static, spherically
symmetric wormhole solutions to gravitational field equations in the absence of
ghost (or phantom) degrees of freedom. We show that in some such cases the
solutions in question are actually not of wormhole nature while in cases where
a wormhole is obtained, the effective gravitational constant G_eff is negative
in some region of space, i.e., the graviton becomes a ghost. In particular, it
is confirmed that there are no vacuum wormhole solutions of the Brans-Dicke
theory with zero potential and the coupling constant \omega > -3/2, except for
the case \omega = 0; in the latter case, G_eff < 0 in the region beyond the
throat. The same is true for wormhole solutions of F(R) gravity: special
wormhole solutions are only possible if F(R) contains an extremum at which
G_eff changes its sign.Comment: 7 two-column pages, no figures, to appear in Grav. Cosmol. A misprint
corrected, references update

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