130 research outputs found
Form invariance symmetry generates a large set of FRW cosmologies
We show that Einstein's field equations for spatially flat
Friedmann-Robertson-Walker (FRW) space times have a form invariance symmetry
(FIS) realized by the form invariance transformations (FIT) which are indeed
generated by an invertible function of the source energy density. These
transformations act on the Hubble expansion rate, the energy density, and
pressure of the cosmic fluid; likewise such transformations are endowed with a
Lie group structure. Each representation of this group is associated with a
particular fluid and consequently a determined cosmology, so that, the FIS
defines a set of equivalent cosmological models. We focus our seek in the FIT
generated by a linear function because it provides a natural framework to
express the duality and also produces a large sets of cosmologies, starting
from a seed one, in several contexts as for instance in the cases of a perfect
fluid source and a scalar field driven by a potential depending linearly on the
scalar field kinetic energy density.Comment: 11 pages, 3 figures. Accepted for publication in Modern Physics
Letters A (2012
Traversable wormholes in a string cloud
We study spherically symmetric thin-shell wormholes in a string cloud
background in (3+1)-dimensional spacetime. The amount of exotic matter required
for the construction, the traversability and the stability under radial
perturbations, are analyzed as functions of the parameters of the model.
Besides, in the Appendices a non perturbative approach to the dynamics and a
possible extension of the analysis to a related model are briefly discussed.Comment: 21 pages, 10 figures; accepted for publication in Int. J. Mod. Phys.
d-dimensional non-asymptotically flat thin-shell wormholes in Einstein-Yang-Mills-Dilaton gravity
Thin-shell wormholes in Einstein-Yang-Mills-dilaton (EYMD) gravity are
considered. We show that a non-asymptotically flat (NAF) black hole solution of
the d-dimensional EYMD theory provides stable thin-shell wormholes which are
supported entirely by exotic matter. The presence of dilaton makes the
spacetime naturally NAF, and with our conclusion it remains still open to
construct wormholes supported by normal matter between two such spacetimes.Comment: 6 pages, 5 figures. Title changed, the first version has been
generalized to d-dimensions. The EYMGB part was removed and was considered in
a more general form separately in arXiv:1007.462
More about thin-shell wormholes associated to cosmic strings
Previous analysis about thin-shell wormholes associated to cosmic strings are
extended. More evidence is found supporting the conjecture that, under
reasonable assumptions about the equations of state of matter on the shell, the
configurations are not stable under radial velocity perturbations.Comment: 8 pages; accepted for publication in Physical Review
Black hole and thin-shell wormhole solutions in Einstein-Hoffman-Born-Infeld theory
We employ an old field theory model, formulated and discussed by Born,
Infeld, Hoffman and Rosen during 1930s. Our method of cutting-gluing of
spacetimes resolves the double-valuedness in the displacement vector D(E),
pointed out by these authors. A characteristic feature of their model is to
contain a logarithmic term, and by bringing forth such a Lagrangian anew, we
aim to attract the interest of field theorists to such a Lagrangian. We adopt
the Hoffman-Born-Infeld (HBI) Lagrangian in general relativity to construct
black holes and investigate the possibility of viable thin-shell wormholes. In
particular, the stability of thin-shell wormholes supported by normal matter in
5-dimensional Einstein-HBI-Gauss-Bonnet gravity is highlighted.Comment: 17 pages, one figure, some constructive changes has been don
Internal space structure generalization of the quintom cosmological scenario
We introduce the Lagrangian for a multi-scalar field configuration in a
-dimensional internal space endowed with a constant metric and
generalize the quintom cosmological scenario. We find the energy momentum
tensor of the model and show that the set of dual transformations, that
preserve the form of the Einstein equations in the Friedmann-Robertson-Walker
(FRW) cosmology, is enlarged. We show that the stability of the power law
solutions leads to an exponential potential which is invariant under linear
transformations in the internal space. Moreover, we obtain the general exact
solution of the Einstein-Klein-Gordon equations for that potential. There exist
solutions that cross the phantom divide and solutions that blow up at a finite
time, exhibiting a superaccelerated behavior and ending in a big rip. We show
that the quintom model with a separable potential can be identified with a
mixture of several fluids. This framework includes the CDM model, a
bouncing model, and a setting sourced by a cosmic string network plus a
cosmological constant. The we concentrate on the case where the dimension of
the internal quintessence sector exceeds the dimension of the internal
phantom sector . For the dark energy density
derived from the 3-scalar field crosses the phantom divide and its negative
component plays the role of the negative part of a classical Dirac Field.Comment: 10 pages, 2 figure
Crossing the phantom divide with k-essence in brane-worlds
We study a flat 3-brane in presence of a linear field with nonzero
cosmological constant . In this model the crossing of the phantom
divide (PD) occurs when the -essence energy density becomes negative. We
show that in the high energy regime the effective equation of state has a
resemblance of a modified Chaplygin gas while in the low energy regime it
becomes linear. We find a scale factor that begins from a singularity and
evolves to a de Sitter stable stage while other solutions have a
super-accelerated regime and end with a big rip. We use the energy conditions
to show when the effective equation of state of the brane-universe crosses the
PD.Comment: 8 pages, 5 figures. The article was fully rewritten. References
added. Accepted for publication in MPLA (2010
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