476 research outputs found
Phantom cosmologies and fermions
Form invariance transformations can be used for constructing phantom
cosmologies starting with conventional cosmological models. In this work we
reconsider the scalar field case and extend the discussion to fermionic fields,
where the "phantomization" process exhibits a new class of possible accelerated
regimes. As an application we analyze the cosmological constant group for a
fermionic seed fluid.Comment: 5 pages, version which was accepted for publication in CQ
Form Invariance of Differential Equations in General Relativity
Einstein equations for several matter sources in Robertson-Walker and Bianchi
I type metrics, are shown to reduce to a kind of second order nonlinear
ordinary differential equation . Also, it appears in the generalized statistical mechanics
for the most interesting value q=-1. The invariant form of this equation is
imposed and the corresponding nonlocal transformation is obtained. The
linearization of that equation for any and is
presented and for the important case with its explicit general solution is found. Moreover, the form
invariance is applied to yield exact solutions of same other differential
equations.Comment: 22 pages, RevTeX; to appear in J. Math. Phy
Exactly solved models of interacting dark matter and dark energy
We introduce an effective one-fluid description of the interacting dark
sector in a spatially flat Friedmann-Robertson-Walker space-time and
investigate the stability of the power-law solutions. We find the "source
equation" for the total energy density and determine the energy density of each
dark component. We study linear and nonlinear interactions which depend on the
dark matter and dark energy densities, their first derivatives, the total
energy density with its derivatives up to second order and the scale factor. We
solve the evolution equations of the dark components for both interactions,
examine exhaustively several examples and show cases where the problem of the
coincidence is alleviated. We show that a generic nonlinear interaction gives
rise to the "relaxed Chaplygin gas model" whose effective equation of state
includes the variable modified Chaplygin gas model while some others nonlinear
interactions yield de Sitter and power-law scenarios.Comment: To appear in the proceedings of the CosmoSul conference, held in Rio
de Janeiro, Brazil, 01-05 august of 2011. References adde
k-essence and extended tachyons in brane-worlds
We study a k-essence field evolving linearly with the cosmic time and the
atypical k-essence model on a homogeneous and isotropic flat 3-brane. We show
that the k-field is driven by an inverse quadratic polynomial potential. The
solutions represent expanding, contracting or bouncing universes with a finite
time span and some of them end in a big crunch or a big rip. Besides, by
selecting the extended tachyonic kinetic functions we analyze the high and low
energy limits of our model, obtaining the nearly power law solution. We
introduce a tachyon field with negative energy density and show that the
universe evolves between two singularities.Comment: 4 pages, jpconf.cls, to appear in Proceedings of Spanish Relativity
Meeting 2009 (ERE 09
Comments on Unified dark energy and dark matter from a scalar field different from quintessence
In a recent paper by C. Gao, M. Kunz, A. Liddle and D. Parkinson
[arXiv:0912.0949], the unification of dark matter and dark energy was explored
within a theory containing a scalar field of non-Lagrangian type. This scalar
field, different from the classic quintessence, can be obtained from the scalar
field representation of an interacting two-fluid mixture described in the paper
by L.P. Chimento and M. Forte [arXiv:0706.4142
Scalar Field Cosmologies with Viscous Fluid
We investigate cosmological models with a free scalar field and a viscous
fluid. We find exact solutions for a linear and nonlinear viscosity pressure.
Both yield singular and bouncing solutions. In the first regime, a de Sitter
stage is asymptotically stable, while in the second case we find power-law
evolutions for vanishing cosmological constant.Comment: 8 pages, LaTeX. To be published in International Journal of Modern
Physics
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