7,624 research outputs found
Exact solution and exotic fluid in cosmology
We investigate cosmological consequences of nonlinear sigma model coupled
with a cosmological fluid which satisfies the continuity equation. The target
space action is of de Sitter type and is composed of four scalar fields. The
potential which is a function of only one of the scalar fields is also
introduced. We perform a general analysis of the ensuing cosmological equations
and give various critical points and their properties. Then, we show that the
model exhibits exact cosmological solution which yields a transition from
matter domination into dark energy and compare it with the CDM
behavior. Especially, we calculate the age of the Universe and show that it is
consistent with the observational value if the equation of the state
for the cosmological fluid is within the range of
Some implication of this result is also discussed.Comment: 13 pages, 1 table, 1 figure, published versio
Bouncing cosmological solutions from f(R,T) gravity
In this work we study classical bouncing solutions in the context of gravity in a flat {\sf FLRW} background using a
perfect fluid as the only matter content. Our investigation is based on
introducing an effective fluid through defining effective energy density and
pressure; we call this reformulation as the "effective picture". These
definitions have been already introduced to study the energy conditions in
gravity. We examine various models to which different
effective equations of state, corresponding to different
functions, can be attributed. It is also discussed that one can link between an
assumed model in the effective picture and the theories
with generalized equation of state ({\sf EoS}). We obtain cosmological
scenarios exhibiting a nonsingular bounce before and after which the Universe
lives within a de-Sitter phase. We then proceed to find general solutions for
matter bounce and investigate their properties. We show that the properties of
bouncing solution in the effective picture of gravity are
as follows: for a specific form of the function, these solutions
are without any future singularities. Moreover, stability analysis of the
nonsingular solutions through matter density perturbations revealed that except
two of the models, the parameters of scalar-type perturbations for the other
ones have a slight transient fluctuation around the bounce point and damp to
zero or a finite value at late times. Hence these bouncing solutions are stable
against scalar-type perturbations. It is possible that all energy conditions be
respected by the real perfect fluid, however, the null and the strong energy
conditions can be violated by the effective fluid near the bounce event.Comment: 49 pages, 11 figures, one tabl
Tunneling in Decaying Cosmologies and the Cosmological Constant Problem
The tunneling rate, with exact prefactor, is calculated to first order in
for an empty closed Friedmann-Robertson-Walker (FRW) universe with
decaying cosmological term ( is the scale factor and
is a parameter ). This model is equivalent to a cosmology
with the equation of state . The calculations are
performed by applying the dilute-instanton approximation on the corresponding
Duru-Kleinert path integral.
It is shown that the highest tunneling rate occurs for corresponding to
the cosmic string matter universe. The obtained most probable cosmological
term, like one obtained by Strominger, accounts for a possible solution to the
cosmological constant problem.Comment: 21 pages, REVTEX, The section 3 is considerably completed including
some physical mechanisms supporting the time variation of the cosmological
constant, added references for the section 3. Accepted to be published in
Phys. Rev.
A Solution to the Graceful Exit Problem in Pre-Big Bang Cosmology
We examine the string cosmology equations with a dilaton potential in the
context of the Pre-Big Bang Scenario with the desired scale factor duality, and
give a generic algorithm for obtaining solutions with appropriate evolutionary
properties. This enables us to find pre-big bang type solutions with suitable
dilaton behaviour that are regular at , thereby solving the graceful exit
problem. However to avoid fine tuning of initial data, an `exotic' equation of
state is needed that relates the fluid properties to the dilaton field. We
discuss why such an equation of state should be required for reliable dilaton
behaviour at late times.Comment: 16 pages LaTeX, 5 figures. To appear in Physical Review
Cosmological Bianchi Class A models in S\'aez-Ballester theory
We use the S\'aez-Ballester (SB) theory on anisotropic Bianchi Class A
cosmological model, with barotropic fluid and cosmological constant, using the
Hamilton or Hamilton-Jacobi approach. Contrary to claims in the specialized
literature, it is shown that the S\'aez-Ballester theory cannot provide a
realistic solution to the dark matter problem of Cosmology for the dust epoch,
without a fine tunning because the contribution of the scalar field in this
theory is equivalent to a stiff fluid (as can be seen from the energy--momentum
tensor for the scalar field), that evolves in a different way as the dust
component. To have similar contributions of the scalar component and the dust
component implies that their past values were fine tunned. So, we
reinterpreting this null result as an indication that dark matter plays a
central role in the formation of structures and galaxy evolution, having
measureable effects in the cosmic microwave bound radiation, and than this
formalism yield to this epoch as primigenius results. We do the mention that
this formalism was used recently in the so called K-essence theory applied to
dark energy problem, in place to the dark matter problem. Also, we include a
quantization procedure of the theory which can be simplified by reinterpreting
the theory in the Einstein frame, where the scalar field can be interpreted as
part of the matter content of the theory, and exact solutions to the
Wheeler-DeWitt equation are found, employing the Bianchi Class A cosmological
models.Comment: 24 pages; ISBN: 978-953-307-626-3, InTec
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