50 research outputs found
Cylindrical wormholes with positive cosmological constant
We construct cylindrical, traversable wormholes with finite radii by taking
into account the cut-and-paste procedure for the case of cosmic string
manifolds with a positive cosmological constant. Under reasonable assumptions
about the equation of state of the matter located at the shell, we find that
the wormhole throat undergoes a monotonous evolution provided it moves at a
constant velocity. In order to explore the dynamical nonlinear behaviour of the
wormhole throat, we consider that the matter at the shell is supported by
anisotropic Chaplygin gas, anti-Chaplygin gas, or a mixed of Chaplygin and
anti-Chaplygin gases implying that wormholes could suffer an accelerated
expansion or contraction but the oscillatory behavior seems to be forbidden.Comment: 5 pages, no figures. Accepted for publication in PRD.
(http://link.aps.org/doi/10.1103/PhysRevD.88.027507
Black holes in Einstein-Gauss-Bonnet gravity with a string cloud background
We obtain a black hole solution in the Einstein-Gauss-Bonnet theory for the
string cloud model in a five dimensional spacetime. We analyze the event
horizons and naked singularities. Later, we compute the Hawking temperature
, the specific heat , the entropy , and the Helmholtz
free energy of the black hole. The entropy was computed using the Wald
formulation. In addition, the quantum correction to the Wald's entropy is
considered for the string cloud source.
We mainly explore the thermodynamical global and local stability of the
system with vanishing or non-vanishing cosmological constant. The global
thermodynamic phase structure indicates that the Hawking-Page transition is
achieved for this model. Further, we observe that there exist stable black
holes with small radii and that these regions are enlarged when choosing small
values of the string cloud density and of the Gauss-Bonnet parameter. Besides,
the rate of evaporation for these black holes are studied, determining whether
the evaporation time is finite or not. Then, we concentrate on the dynamical
stability of the system, studying the effective potential for s-waves
propagating on the string cloud background.Comment: 13 pages, 5 figures. References adde
Big brake singularity is accommodated as an exotic quintessence field
We describe a big brake singularity in terms of a modified Chaplygin gas
equation of state p=(\ga_{m}-1)\rho+\al\ga_{m}\rho^{-n}, accommodate this
late-time event as an exotic quintessence model obtained from an
energy-momentum tensor, and focus on the cosmological behavior of the exotic
field, its kinetic energy and the potential energy. At the background level,
the exotic field does not blow up whereas its kinetic energy and potential both
grow without limit near the future singularity. We evaluate the classical
stability of this background solution by examining the scalar perturbations of
the metric along with the inclusion of entropy perturbation in the perturbed
pressure. Within the Newtonian gauge, the gravitational field approaches a
constant near the singularity plus additional regular terms. When the perturbed
exotic field is associated with \al>0 the perturbed pressure and contrast
density both diverge, whereas the perturbed exotic field and the divergence of
the exotic field's velocity go to zero exponentially. When the perturbed exotic
field is associated with \al<0 the contrast density always blows up, but the
perturbed pressure can remain bounded. In addition, the perturbed exotic field
and the divergence of the exotic field's velocity vanish near the big brake
singularity. We also briefly look at the behavior of the intrinsic entropy
perturbation near the singular event.Comment: 11 pages, no figures. Accepted for its publication in PR
Dark radiation and dark matter coupled to holographic Ricci dark energy
We investigate a universe filled with interacting dark matter, holographic
dark energy, and dark radiation for the spatially flat
Friedmann-Robertson-Walker (FRW) spacetime. We use a linear interaction to
reconstruct all the component energy densities in terms of the scale factor by
directly solving the balance's equations along with the source equation. We
apply the method to the observational Hubble data for constraining
the cosmic parameters, contrast with the Union 2 sample of supernovae, and
analyze the amount of dark energy in the radiation era. It turns out that our
model exhibits an excess of dark energy in the recombination era whereas the
stringent bound at big-bang
nucleosynthesis is fulfilled. We find that the interaction provides a physical
mechanism for alleviating the triple cosmic coincidence and this leads to
.Comment: 8 pages, 5 figures, 2 tables. Accepted for its publication in The
European Physical Journal C (2013).
http://link.springer.com/article/10.1140/epjc/s10052-013-2352-7 arXiv admin
note: substantial text overlap with arXiv:1210.550
Dark matter, dark energy, and dark radiation coupled with a transversal interaction
We investigate a cosmological scenario with three interacting components that
includes dark matter, dark energy, and radiation in the spatially flat
Friedmann-Robertson-Walker universe. We introduce a 3-dimensional internal
space, the interaction vector satisfying the
constraint plane , the barotropic index vector
\boldmath {\gamma}=(\ga_x,\ga_m,\ga_r) and select a transversal interaction
vector in a sense that \mathbf{Q_t}\cdot \boldmath
{\gamma}. We exactly solve the source equation for a linear
, that depends on the total energy density and its derivatives up
to third order, and find all the component energy densities. We obtain a large
set of interactions for which the source equation admits a power law solution
and show its asymptotic stability by constructing the Lyapunov function. We
apply the method to the observational Hubble data for constraining
the cosmic parameters, and analyze the amount of dark energy in the radiation
era for the above linear . It turns to be that our model fulfills
the severe bound of and is consistent with the
future constraints achievable by Planck and CMBPol experiments.Comment: 9 pages, 4 figures. Accepted for publication in Physical Review