11 research outputs found
Inflating wormholes in the braneworld models
The braneworld model, in which our Universe is a three-brane embedded in a
five-dimensional bulk, allows the existence of wormholes, without any violation
of the energy conditions. A fundamental ingredient of traversable wormholes is
the violation of the null energy condition (NEC). However, in the brane world
models, the stress energy tensor confined on the brane, threading the wormhole,
satisfies the NEC. In conventional general relativity, wormholes existing
before inflation can be significantly enlarged by the expanding spacetime. We
investigate the evolution of an inflating wormhole in the brane world scenario,
in which the wormhole is supported by the nonlocal brane world effects. As a
first step in our study we consider the possibility of embedding a
four-dimensional brane world wormhole into a five dimensional bulk. The
conditions for the embedding are obtained by studying the junction conditions
for the wormhole geometry, as well as the full set of the five dimensional bulk
field equations. For the description of the inflation we adopt the chaotic
inflation model. We study the dynamics of the brane world wormholes during the
exponential inflation stage, and in the stage of the oscillating scalar field.
A particular exact solution corresponding to a zero redshift wormhole is also
obtained. The resulting evolution shows that while the physical and geometrical
parameters of a zero redshift wormhole decay naturally, a wormhole satisfying
some very general initial conditions could turn into a black hole, and exist
forever.Comment: 30 pages, no figures, accepted for publication in CQ
Thin accretion disk signatures in dynamical Chern-Simons modified gravity
A promising extension of general relativity is Chern-Simons (CS) modified
gravity, in which the Einstein-Hilbert action is modified by adding a
parity-violating CS term, which couples to gravity via a scalar field. In this
work, we consider the interesting, yet relatively unexplored, dynamical
formulation of CS modified gravity, where the CS coupling field is treated as a
dynamical field, endowed with its own stress-energy tensor and evolution
equation. We consider the possibility of observationally testing dynamical CS
modified gravity by using the accretion disk properties around slowly-rotating
black holes. The energy flux, temperature distribution, the emission spectrum
as well as the energy conversion efficiency are obtained, and compared to the
standard general relativistic Kerr solution. It is shown that the Kerr black
hole provide a more efficient engine for the transformation of the energy of
the accreting mass into radiation than their slowly-rotating counterparts in CS
modified gravity. Specific signatures appear in the electromagnetic spectrum,
thus leading to the possibility of directly testing CS modified gravity by
using astrophysical observations of the emission spectra from accretion disks.Comment: 12 pages, 24 figures. V2: 10 pages, 13 figures, significant changes,
matches published versio
Thin accretion disk signatures of slowly rotating black holes in Ho\v{r}ava gravity
In the present work, we consider the possibility of observationally testing
Ho\v{r}ava gravity by using the accretion disk properties around slowly
rotating black holes of the Kehagias-Sfetsos solution in asymptotically flat
spacetimes. The energy flux, temperature distribution, the emission spectrum as
well as the energy conversion efficiency are obtained, and compared to the
standard slowly rotating general relativistic Kerr solution. Comparing the mass
accretion in a slowly rotating Kehagias-Sfetsos geometry in Ho\v{r}ava gravity
with the one of a slowly rotating Kerr black hole, we verify that the intensity
of the flux emerging from the disk surface is greater for the slowly rotating
Kehagias-Sfetsos solution than for rotating black holes with the same
geometrical mass and accretion rate. We also present the conversion efficiency
of the accreting mass into radiation, and show that the rotating
Kehagias-Sfetsos solution provides a much more efficient engine for the
transformation of the accreting mass into radiation than the Kerr black holes.
Thus, distinct signatures appear in the electromagnetic spectrum, leading to
the possibility of directly testing Ho\v{r}ava gravity models by using
astrophysical observations of the emission spectra from accretion disks.Comment: 12 pages, 15 figures. V2: 13 pages, clarifications and discussion
added; version accepted for publication in Classical and Quantum Gravit
New agegraphic dark energy in Horava-Lifshitz cosmology
We investigate the new agegraphic dark energy scenario in a universe governed
by Horava-Lifshitz gravity. We consider both the detailed and non-detailed
balanced version of the theory, we impose an arbitrary curvature, and we allow
for an interaction between the matter and dark energy sectors. Extracting the
differential equation for the evolution of the dark energy density parameter
and performing an expansion of the dark energy equation-of-state parameter, we
calculate its present and its low-redshift value as functions of the dark
energy and curvature density parameters at present, of the Horava-Lifshitz
running parameter , of the new agegraphic dark energy parameter ,
and of the interaction coupling . We find that
and . Although this analysis indicates that the
scenario can be compatible with observations, it does not enlighten the
discussion about the possible conceptual and theoretical problems of
Horava-Lifshitz gravity.Comment: 17 pages, no figures, version published at JCA
Comparing two approaches to Hawking radiation of Schwarzschild-de Sitter black holes
We study two different ways to analyze the Hawking evaporation of a
Schwarzschild-de Sitter black hole. The first one uses the standard approach of
surface gravity evaluated at the possible horizons. The second method derives
its results via the Generalized Uncertainty Principle (GUP) which offers a yet
different method to look at the problem. In the case of a Schwarzschild black
hole it is known that this methods affirms the existence of a black hole
remnant (minimal mass ) of the order of Planck mass
and a corresponding maximal temperature also of the order of
. The standard dispersion relation is, in the GUP
formulation, deformed in the vicinity of Planck length which is
the smallest value the horizon can take. We generalize the uncertainty
principle to Schwarzschild-de Sitter spacetime with the cosmological constant
and find a dual relation which, compared to
and , affirms the existence of a maximal mass
of the order , minimum
temperature . As compared to the standard
approach we find a deformed dispersion relation close to
and in addition at the maximally possible horizon approximately at
. agrees with the standard results at
(or equivalently at ).Comment: new references adde