20 research outputs found
How the Charge Can Affect the Formation of Gravastars
In recent work we physically interpreted a special gravastar solution
characterized by a zero Schwarzschild mass. In fact, in that case, none
gravastar was formed and the shell expanded, leaving behind a de Sitter or a
Minkowski spacetime, or collapsed without forming an event horizon, originating
what we called a massive non-gravitational object. This object has two
components of non zero mass but the exterior spacetime is Minkowski or de
Sitter. One of the component is a massive thin shell and the other one is de
Sitter spacetime inside. The total mass of this object is zero Schwarzschild
mass, which characterizes an exterior vacuum spacetime. Here, we extend this
study to the case where we have a charged shell. Now, the exterior is a
Reissner-Nordstr\"om spacetime and, depending on the parameter
of the equation of state of the shell, and the charge, a
gravastar structure can be formed. We have found that the presence of the
charge contributes to the stability of the gravastar, if the charge is greater
than a critical value. Otherwise, a massive non-gravitational object is formed
for small charges.Comment: 17 pages and 7 figures, several typos corrected, accepted for
publication in JCA
Slow-roll Inflation with the Gauss-Bonnet and Chern-Simons Corrections
We study slow-roll inflation with the Gauss-Bonnet and Chern-Simons
corrections. We obtain general formulas for the observables: spectral indices,
tensor-to-scalar ratio and circular polarization of gravitational waves. The
Gauss-Bonnet term violates the consistency relation r = -8n_T. Particularly,
blue spectrum n_T > 0 and scale invariant spectrum |8n_T|/r << 1 of tensor
modes are possible. These cases require the Gauss-Bonnet coupling function of
\xi _{,\phi } \sim 10^8/M_{Pl}. We use examples to show new-inflation-type
potential with 10M_{Pl} symmetry breaking scale and potential with flat region
in \phi \gtrsim 10M_{Pl} lead to observationally consistent blue and scale
invariant spectra, respectively. Hence, these interesting cases can actually be
realized. The Chern-Simons term produce circularly polarized tensor modes. We
show an observation of these signals supports existence of the Chern-Simons
coupling function of \omega _{,\phi } \sim 10^8/M_{Pl}. Thus, with future
observations, we can fix or constrain the value of these coupling functions, at
the CMB scale.Comment: 21 pages, 5 figure
Gravastars and Black Holes of Anisotropic Dark Energy
Dynamical models of prototype gravastars made of anisotropic dark energy are
constructed, in which an infinitely thin spherical shell of a perfect fluid
with the equation of state divides the whole spacetime
into two regions, the internal region filled with a dark energy fluid, and the
external Schwarzschild region. The models represent "bounded excursion" stable
gravastars, where the thin shell is oscillating between two finite radii, while
in other cases they collapse until the formation of black holes. Here we show,
for the first time in the literature, a model of gravastar and formation of
black hole with both interior and thin shell constituted exclusively of dark
energy. Besides, the sign of the parameter of anisotropy () seems to
be relevant to the gravastar formation. The formation is favored when the
tangential pressure is greater than the radial pressure, at least in the
neighborhood of the isotropic case ().Comment: 16 pages, 8 figures. Accepted for publication in Gen. Rel. Gra
Volume stabilization in a warped flux compactification model
We investigate the stability of the extra dimensions in a warped, codimension
two braneworld that is based upon an Einstein-Maxwell-dilaton theory with a
non-vanishing scalar field potential. The braneworld solution has two 3-branes,
which are located at the positions of the conical singularities. For this type
of brane solution the relative positions of the branes (the shape modulus) is
determined via the tension-deficit relations, if the brane tensions are fixed.
However, the volume of the extra dimensions (the volume modulus) is not fixed
in the context of the classical theory, implying we should take quantum
corrections into account. Hence, we discuss the one-loop effective potential of
the volume modulus for a massless, minimally coupled scalar field.Comment: 25 pages, 8 figures, typos correcte
6D Dyonic String With Active Hyperscalars
We derive the necessary and sufficient conditions for the existence of a
Killing spinor in N=(1,0) gauge supergravity in six dimensions coupled to a
single tensor multiplet, vector multiplets and hypermultiplets. These are shown
to imply most of the field equations and the remaining ones are determined. In
this framework, we find a novel 1/8 supersymmetric dyonic string solution with
nonvanishing hypermultiplet scalars. The activated scalars parametrize a 4
dimensional submanifold of a quaternionic hyperbolic ball. We employ an
identity map between this submanifold and the internal space transverse to the
string worldsheet. The internal space forms a 4 dimensional analog of the
Gell-Mann-Zwiebach tear-drop which is noncompact with finite volume. While the
electric charge carried by the dyonic string is arbitrary, the magnetic charge
is fixed in Planckian units, and hence necessarily non-vanishing. The source
term needed to balance a delta function type singularity at the origin is
determined. The solution is also shown to have 1/4 supersymmetric AdS_3 x S^3
near horizon limit where the radii are proportional to the electric charge.Comment: 28 pages, latex, minor corrections mad
Inhomogeneities in dusty universe - a possible alternative to dark energy?
There have been of late renewed debates on the role of inhomogeneities to
explain the observed late acceleration of the universe. We have looked into the
problem analytically with the help of the well known spherically symmetric but
inhomogeneous Lemaitre-Tolman-Bondi(LTB) model generalised to higher
dimensions. It is observed that in contrast to the claim made by Kolb et al the
presence of inhomogeneities as well as extra dimensions can not reverse the
signature of the deceleration parameter if the matter field obeys the energy
conditions. The well known Raychaudhuri equation also points to the same
result. Without solving the field equations explicitly it can, however, be
shown that although the total deceleration is positive everywhere nevertheless
it does not exclude the possibility of having radial acceleration, even in the
pure dust universe, if the angular scale factor is decelerating fast enough and
vice versa. Moreover it is found that introduction of extra dimensions can not
reverse the scenario. To the contrary it actually helps the decelerating
process.Comment: 14 pages, 4 figure
Stable Gravastars of Anisotropic Dark Energy
Dynamical models of prototype gravastars made of phantom energy are
constructed, in which an infinitely thin spherical shell of a perfect fluid
with the equation of state divides the whole spacetime
into two regions, the internal region filled with a dark energy (or phantom)
fluid, and the external Schwarzschild region. It is found that in some cases
the models represent the "bounded excursion" stable gravastars, where the thin
shell is oscillating between two finite radii, while in other cases they
collapse until the formation of black holes or normal stars. In the phase
space, the region for the "bounded excursion" gravastars is very small in
comparison to that of black holes, but not empty, as found in our previous
papers. Therefore, although the existence of gravastars can not be completely
excluded from current analysis, the opposite is not possible either, that is,
even if gravastars exist, they do not exclude the existence of black holes.Comment: 35 pages, 43 figures, added some clarifying texts and corrected some
typos, accepted for publication in JCA
New Agegraphic Dark Energy in Gravity
In this paper we study cosmological application of new agegraphic dark energy
density in the gravity framework. We employ the new agegraphic model of
dark energy to obtain the equation of state for the new agegraphic energy
density in spatially flat universe. Our calculation show, taking , it is
possible to have crossing -1. This implies that one can
generate phantom-like equation of state from a new agegraphic dark energy model
in flat universe in the modified gravity cosmology framework. Also we develop a
reconstruction scheme for the modified gravity with action.Comment: 8 pages, no figur
Phantom phase power-law solution in gravity
Power-law solutions for gravity coupled with perfect fluid have been
studied for spatially flat universe. It is shown that despite the matter
dominated and accelerating power-law solutions, the power-law solution exists
for an special form of when this universe enters a Phantom phase.Comment: 10 pages, Published online in Astrophysics and Space Scienc
Reconstruction of , and models inspired by variable deceleration parameter
We study an special law for the deceleration parameter, recently proposed by
Akarsu and Dereli, in the context of , and
theories of modified gravity. This law covers the law of Berman for obtaining
exact cosmological models to account for the current acceleration of the
universe, and also gives the opportunity to generalize many of the dark energy
models having better consistency with the cosmological observations. Our aim is
to reconstruct the , and models inspired by this
law of variable deceleration parameter. Such models may then exhibit better
consistency with the cosmological observations.Comment: 18 pages, Published online in Astrophys. Space. Sc