68 research outputs found
Phantom Energy Accretion onto Black Holes in Cyclic Universe
Black holes pose a serious problem in the cyclic or oscillating cosmology. It
is speculated that, in the cyclic universe with phantom turnarounds, black
holes will be torn apart by the phantom energy before turnaround before they
can create any problems. In this paper, using the mechanism of the phantom
accretion onto black holes, we find that black holes do not disappear before
the phantom turnaround. But the remanent black holes will not cause any
problems due to the Hawking evaporation.Comment: 8 pages, no figure; typographical errors are correcte
Exact -cosmological model coming from the request of the existence of a Noether symmetry
We present an -cosmological model with an exact analytic solution,
coming from the request of the existence of a Noether symmetry, which is able
to describe a dust-dominated decelerated phase before the current accelerated
phase of the universe.Comment: 4 pages, 2 figures, Contribution to the proceedings of Spanish
Relativity Meeting 2008, Salamanca, Sapin, 15-19 September 200
Phantom stars and topology change
In this work, we consider time-dependent dark energy star models, with an
evolving parameter crossing the phantom divide, . Once in
the phantom regime, the null energy condition is violated, which physically
implies that the negative radial pressure exceeds the energy density.
Therefore, an enormous negative pressure in the center may, in principle, imply
a topology change, consequently opening up a tunnel and converting the dark
energy star into a wormhole. The criteria for this topology change are
discussed, in particular, we consider the Morse Index analysis and a Casimir
energy approach involving quasi-local energy difference calculations that may
reflect or measure the occurrence of a topology change. We denote these exotic
geometries consisting of dark energy stars (in the phantom regime) and phantom
wormholes as phantom stars. The final product of this topological change,
namely, phantom wormholes, have far-reaching physical and cosmological
implications, as in addition to being used for interstellar shortcuts, an
absurdly advanced civilization may manipulate these geometries to induce closed
timelike curves, consequently violating causality.Comment: 19 pages, 13 figures. V2: Extended version of the paper accepted for
publication in Physical Review
Physical non-equivalence of the Jordan and Einstein frames
We show, considering a specific f(R)-gravity model, that the Jordan frame and
the Einstein frame are physically non-equivalent, although they are connected
by a conformal transformation which yields a mathematical equivalence. Since
all the calculations are performed analytically, this non-equivalence is shown
in an unambiguous way.Comment: 7 pages, 3 figure
A Completely Regular Quantum Stress Tensor with
For many quantum field theory computations in cosmology it is not possible to
use the flat space trick of obtaining full, interacting states by evolving free
states over infinite times. State wave functionals must be specified at finite
times and, although the free states suffice to obtain the lowest order effects,
higher order corrections necessarily involve changes of the initial state.
Failing to correctly change the initial state can result in effective field
equations which diverge on the initial value surface, or which contain tedious
sums of terms that redshift like inverse powers of the scale factor. In this
paper we verify a conjecture from 2004 that the lowest order initial state
correction can indeed absorb the initial value divergences and all the
redshifting terms of the two loop expectation value of the stress tensor of a
massless, minimally coupled scalar with a quartic self interaction on
nondynamical de Sitter background.Comment: 23 pages, 1 figur
Can black holes be torn up by phantom dark energy in cyclic cosmology?
Infinitely cyclic cosmology is often frustrated by the black hole problem. It
has been speculated that this obstacle in cyclic cosmology can be removed by
taking into account a peculiar cyclic model derived from loop quantum cosmology
or the braneworld scenario, in which phantom dark energy plays a crucial role.
In this peculiar cyclic model, the mechanism of solving the black hole problem
is through tearing up black holes by phantom. However, using the theory of
fluid accretion onto black holes, we show in this paper that there exists
another possibility: that black holes cannot be torn up by phantom in this
cyclic model. We discussed this possibility and showed that the masses of black
holes might first decrease and then increase, through phantom accretion onto
black holes in the expanding stage of the cyclic universe.Comment: 6 pages, 2 figures; discussions adde
Hawking radiation and thermodynamics of dynamical black holes in phantom dominated universe
The thermodynamic properties of dark energy-dominated universe in the
presence of a black hole are investigated in the general case of a varying
equation-of-state-parameter . We show that all the thermodynamics
quantities are regular at the phantom divide crossing, and particularly the
temperature and the entropy of the dark fluid are always positive definite. We
also study the accretion process of a phantom fluid by black holes and the
conditions required for the validity of the generalized second law of
thermodynamics. As a results we obtain a strictly negative chemical potential
and an equation-of-state parameter Comment: 22 pages,3 figure
Linearized stability analysis of gravastars in noncommutative geometry
In this work, we find exact gravastar solutions in the context of
noncommutative geometry, and explore their physical properties and
characteristics. The energy density of these geometries is a smeared and
particle-like gravitational source, where the mass is diffused throughout a
region of linear dimension due to the intrinsic uncertainty
encoded in the coordinate commutator. These solutions are then matched to an
exterior Schwarzschild spacetime. We further explore the dynamical stability of
the transition layer of these gravastars, for the specific case of
, where M is the black hole mass, to linearized
spherically symmetric radial perturbations about static equilibrium solutions.
It is found that large stability regions exist and, in particular, located
sufficiently close to where the event horizon is expected to form.Comment: 6 pages, 3 figure
Dark energy and dust matter phases from an exact -cosmology model
We show that dust matter-dark energy combined phases can be achieved by the
exact solution derived from a power law cosmological model. This example
answers the query by which a dust-dominated decelerated phase, before
dark-energy accelerated phase, is needed in order to form large scale
structures.Comment: 6 pages, 5 figures. to appear in Phys. Lett.
Generic thin-shell gravastars
We construct generic spherically symmetric thin-shell gravastars by using the
cut-and-paste procedure. We take considerable effort to make the analysis as
general and unified as practicable; investigating both the internal physics of
the transition layer and its interaction with "external forces" arising due to
interactions between the transition layer and the bulk spacetime. Furthermore,
we discuss both the dynamic and static situations. In particular, we consider
"bounded excursion" dynamical configurations, and probe the stability of static
configurations. For gravastars there is always a particularly compelling
configuration in which the surface energy density is zero, while surface
tension is nonzero.Comment: V1: 39 pages, 9 figures; V2: 40 pages, 9 figures. References added,
some discussion added, some typos fixed. Identical to published version.
arXiv admin note: text overlap with arXiv:1112.205
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