20,889 research outputs found
Curvature-corrected dilatonic black holes and black hole -- string transition
We investigate extremal charged black hole solutions in the four-dimensional
string frame Gauss-Bonnet gravity with the Maxwell field and the dilaton.
Without curvature corrections, the extremal electrically charged dilatonic
black holes have singular horizon and zero Bekenstein entropy. When the
Gauss-Bonnet term is switched on, the horizon radius expands to a finite value
provided curvature corrections are strong enough. Below a certain threshold
value of the Gauss-Bonnet coupling the extremal black hole solutions cease to
exist. Since decreasing Gauss-Bonnet coupling corresponds to decreasing string
coupling , the situation can tentatively be interpreted as classical
indication on the black hole -- string transition. Previously the extremal
dilaton black holes were studied in the Einstein-frame version of the
Gauss-Bonnet gravity. Here we work in the string frame version of this theory
with the S-duality symmetric dilaton function as required by the heterotic
string theory.Comment: 14 pages, 2 figure
Slow-roll inflation with a Gauss-Bonnet correction
We consider slow-roll inflation for a single scalar field with an arbitrary
potential and an arbitrary nonminimal coupling to the Gauss-Bonnet term. By
introducing a combined hierarchy of Hubble and Gauss-Bonnet flow functions, we
analytically derive the power spectra of scalar and tensor perturbations. The
standard consistency relation between the tensor-to-scalar ratio and the
spectral index of tensor perturbations is broken. We apply this formalism to a
specific model with a monomial potential and an inverse monomial Gauss-Bonnet
coupling and constrain it by the 7-year Wilkinson Microwave Anisotropy Probe
data. The Gauss-Bonnet term with a positive (or negative) coupling may lead to
a reduction (or enhancement) of the tensor-to-scalar ratio and hence may revive
the quartic potential ruled out by recent cosmological data.Comment: 7 pages, 2 figures, RevTeX, references added, published versio
NUT-Charged Black Holes in Gauss-Bonnet Gravity
We investigate the existence of Taub-NUT/bolt solutions in Gauss-Bonnet
gravity and obtain the general form of these solutions in dimensions. We
find that for all non-extremal NUT solutions of Einstein gravity having no
curvature singularity at , there exist NUT solutions in Gauss-Bonnet
gravity that contain these solutions in the limit that the Gauss-Bonnet
parameter goes to zero. Furthermore there are no NUT solutions in
Gauss-Bonnet gravity that yield non-extremal NUT solutions to Einstein gravity
having a curvature singularity at in the limit . Indeed,
we have non-extreme NUT solutions in dimensions with non-trivial
fibration only when the -dimensional base space is chosen to be
. We also find that the Gauss-Bonnet gravity has extremal NUT
solutions whenever the base space is a product of 2-torii with at most a
2-dimensional factor space of positive curvature. Indeed, when the base space
has at most one positively curved two dimensional space as one of its factor
spaces, then Gauss-Bonnet gravity admits extreme NUT solutions, even though
there a curvature singularity exists at . We also find that one can have
bolt solutions in Gauss-Bonnet gravity with any base space with factor spaces
of zero or positive constant curvature. The only case for which one does not
have bolt solutions is in the absence of a cosmological term with zero
curvature base space.Comment: 20 pages, referrence added, a few typos correcte
Scalar field evolution in Gauss-Bonnet black holes
It is presented a thorough analysis of scalar perturbations in the background
of Gauss-Bonnet, Gauss-Bonnet-de Sitter and Gauss-Bonnet-anti-de Sitter black
hole spacetimes. The perturbations are considered both in frequency and time
domain. The dependence of the scalar field evolution on the values of the
cosmological constant and the Gauss-Bonnet coupling is
investigated. For Gauss-Bonnet and Gauss-Bonnet-de Sitter black holes, at
asymptotically late times either power-law or exponential tails dominate, while
for Gauss-Bonnet-anti-de Sitter black hole, the quasinormal modes govern the
scalar field decay at all times. The power-law tails at asymptotically late
times for odd-dimensional Gauss-Bonnet black holes does not depend on ,
even though the black hole metric contains as a new parameter. The
corrections to quasinormal spectrum due to Gauss-Bonnet coupling is not small
and should not be neglected. For the limit of near extremal value of the
(positive) cosmological constant and pure de Sitter and anti-de Sitter modes in
Gauss-Bonnet gravity we have found analytical expressions.Comment: 10 pages, to be published in Phys. Rev.
Kaluza-Klein black hole with negatively curved extra dimensions in string generated gravity models
We obtain a new exact black-hole solution in Einstein-Gauss-Bonnet gravity
with a cosmological constant which bears a specific relation to the
Gauss-Bonnet coupling constant. The spacetime is a product of the usual
4-dimensional manifold with a -dimensional space of constant negative
curvature, i.e., its topology is locally {\ma M}^n \approx {\ma M}^4 \times
{\ma H}^{n-4}. The solution has two parameters and asymptotically approximates
to the field of a charged black hole in anti-de Sitter spacetime. The most
interesting and remarkable feature is that the Gauss-Bonnet term acts like a
Maxwell source for large while at the other end it regularizes the metric
and weakens the central singularity.Comment: 4 pages, 2 figures, final version to appear in Physical Review D as a
rapid communicatio
Emerging Anisotropic Compact Stars in Gravity
The possible emergence of compact stars has been investigated in the recently
introduced modified Gauss-Bonnet gravity, where
is the Gauss-Bonnet term and is the trace of the
energy-momentum tensor. Specifically, for this modified
theory, the analytic solutions of Krori and Barua have been applied to
anisotropic matter distribution. To determine the unknown constants appearing
in Krori and Barua metric, the well-known three models of the compact stars
namely 4U1820-30, Her X-I, and SAX J 1808.4-3658 have been used. The analysis
of the physical behavior of the compact stars has been presented and the
physical features like energy density and pressure, energy conditions, static
equilibrium, stability, measure of anisotropy, and regularity of the compact
stars, have been discussed.Comment: 27 pages, 43 figures, 1 table, minor change
Inhomogeneous Dust Collapse in 5D Einstein-Gauss-Bonnet Gravity
We consider a Lemaitre - Tolman - Bondi type space-time in Einstein gravity
with the Gauss-Bonnet combination of quadratic curvature terms, and present
exact solution in closed form. It turns out that the presence of the coupling
constant of the Gauss-Bonnet terms alpha > 0 completely changes the causal
structure of the singularities from the analogous general relativistic case.
The gravitational collapse of inhomogeneous dust in the five-dimensional
Gauss-Bonnet extended Einstein equations leads to formation of a massive, but
weak, timelike singularity which is forbidden in general relativity.
Interestingly, this is a counterexample to three conjecture viz. cosmic
censorship conjecture, hoop conjecture and Seifert's conjecture.Comment: 8 Latex Pages, 2 EPS figure
Discussion of "Second order topological sensitivity analysis" by J. Rocha de Faria et al
The article by J. Rocha de Faria et al. under discussion is concerned with
the evaluation of the perturbation undergone by the potential energy of a
domain (in a 2-D, scalar Laplace equation setting) when a disk
of small radius centered at a given location
\hat{\boldsymbol{x}\in\Omega is removed from , assuming either
Neumann or Dirichlet conditions on the boundary of the small `hole' thus
created. In each case, the potential energy of the
punctured domain \Omega_{\epsilon}=\Omega\setminus\B_{\epsilon} is expanded
about so that the first two terms of the perturbation are given.
The first (leading) term is the well-documented topological derivative of
. The article under discussion places, logically, its main focus on the
next term of the expansion. However, it contains incorrrect results, as shown
in this discussion. In what follows, equations referenced with Arabic numbers
refer to those of the article under discussion.Comment: International Journal of Solids and Structures (2007) to appea
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