78 research outputs found
Levi-Civita spacetimes in multidimensional theories
We obtain the most general static cylindrically symmetric vacuum solutions of
the Einstein field equations in dimensions. Under the assumption of
separation of variables, we construct a family of Levi-Civita-Kasner vacuum
solutions in . We discuss the dimensional reduction of the static
solutions. Depending on the reduction procedure, they can be interpreted either
as a scalar-vacuum generalization of Levi-Civita spacetimes, or as the
effective 4D vacuum spacetime outside of an idealized string in braneworld
theory.Comment: 7 pages. Accepted for publication in Mod. Phys. Lett. A (MPLA
Cylindrically symmetric, static strings with a cosmological constant in Brans-Dicke theory
The static, cylindrically symmetric vacuum solutions with a cosmological
constant in the framework of the Brans-Dicke theory are investigated. Some of
these solutions admitting Lorentz boost invariance along the symmetry axis
correspond to local, straight cosmic strings with a cosmological constant. Some
physical properties of such solutions are studied. These strings apply
attractive or repulsive forces on the test particles. A smooth matching is also
performed with a recently introduced interior thick string solution with a
cosmological constant.Comment: 8 pages, Revtex; Published versio
Cylindrically Symmetric Vacuum Solutions in Higher Dimensional Brans-Dicke Theory
Higher dimensional, static, cylindrically symmetric vacuum solutions with and
without a cosmological constant in the Brans-Dicke theory are presented. We
show that, for a negative cosmological constant and for specific values of the
parameters, a particular subclass of these solutions include higher dimensional
topological black hole-type solutions with a flat horizon topology. We briefly
extend our discussion to stationary vacuum and vacuum solutions.Comment: V3: Published Versio
Multiple Photonic Shells Around a Line Singularity
Line singularities including cosmic strings may be screened by photonic
shells until they appear as a planar wall.Comment: 6 page
Gravitational hedgehog, stringy hedgehog and stringy sphere
We investigate the solutions of Einstein equations such that a hedgehog
solution is matched to different exterior or interior solutions via a spherical
shell. In the case where both the exterior and the interior regions are
hedgehog solutions or one of them is flat, the resulting spherical shell
becomes a stringy shell. We also consider more general matchings and see that
in this case the shell deviates from its stringy character.Comment: 11 page
Static Cylindrical Matter Shells
Static cylindrical shells composed of massive particles arising from matching
of two different Levi-Civita space-times are studied for the shell satisfying
either isotropic or anisotropic equation of state. We find that these solutions
satisfy the energy conditions for certain ranges of the parameters.Comment: 9 pages, 3 figures, Latex; Final version, To appear in General
Relativity and Gravitatio
Superradiant Instability of Five-Dimensional Rotating Charged AdS Black Holes
We study the instability of small AdS black holes with two independent
rotation parameters in minimal five-dimensional gauged supergravity to massless
scalar perturbations. We analytically solve the Klein-Gordon equation for
low-frequency perturbations in two regions of the spacetime of these black
holes: namely, in the region close to the horizon and in the far-region. By
matching the solutions in an intermediate region, we calculate the frequency
spectrum of quasinormal modes. We show that in the regime of superradiance only
the modes of even orbital quantum number undergo negative damping, resulting in
exponential growth of the amplitude. That is, the black holes become unstable
to these modes. Meanwhile, the modes of odd orbital quantum number do not
undergo any damping, oscillating with frequency-shifts. This is in contrast
with the case of four-dimensional small Kerr-AdS black holes which exhibit the
instability to all modes of scalar perturbations in the regime of
superradiance.Comment: 24 pages, REVTeX; Minor changes, matching published versio
A Unified Approach to Variational Derivatives of Modified Gravitational Actions
Our main aim in this paper is to promote the coframe variational method as a
unified approach to derive field equations for any given gravitational action
containing the algebraic functions of the scalars constructed from the Riemann
curvature tensor and its contractions. We are able to derive a master equation
which expresses the variational derivatives of the generalized gravitational
actions in terms of the variational derivatives of its constituent curvature
scalars. Using the Lagrange multiplier method relative to an orthonormal
coframe, we investigate the variational procedures for modified gravitational
Lagrangian densities in spacetime dimensions . We study
well-known gravitational actions such as those involving the Gauss-Bonnet and
Ricci-squared, Kretchmann scalar, Weyl-squared terms and their algebraic
generalizations similar to generic theories and the algebraic
generalization of sixth order gravitational Lagrangians. We put forth a new
model involving the gravitational Chern-Simons term and also give three
dimensional New massive gravity equations in a new form in terms of the Cotton
2-form
Higher Dimensional Cylindrical or Kasner Type Electrovacuum Solutions
We consider a D dimensional Kasner type diagonal spacetime where metric
functions depend only on a single coordinate and electromagnetic field shares
the symmetries of spacetime. These solutions can describe static cylindrical or
cosmological Einstein-Maxwell vacuum spacetimes. We mainly focus on
electrovacuum solutions and four different types of solutions are obtained in
which one of them has no four dimensional counterpart. We also consider the
properties of the general solution corresponding to the exterior field of a
charged line mass and discuss its several properties. Although it resembles the
same form with four dimensional one, there is a difference on the range of the
solutions for fixed signs of the parameters. General magnetic field vacuum
solution are also briefly discussed, which reduces to Bonnor-Melvin magnetic
universe for a special choice of the parameters. The Kasner forms of the
general solution are also presented for the cylindrical or cosmological cases.Comment: 16 pages, Revtex. Text and references are extended, Published versio
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