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

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

General behaviour of Bianchi VI_0 solutions with an exponential-potential scalar field

The solutions to the Einstein-Klein-Gordon equations without a cosmological constant are investigated for an exponential potential in a Bianchi VI_0 metric. There exists a two-parameter family of solutions which have a power-law inflationary behaviour when the exponent of the potential, k, satisfies k^2<2. In addition, there exists a two-parameter family of singular solutions for all k^2 values. A simple anisotropic exact solution is found to be stable when 2<k^2.Comment: 10 pages, no figures. To be published in General Relativity and Gravitatio

Naudts-like duality and the extreme Fisher information principle

We show that using the most parsimonious version of Frieden and Soffer's extreme information principle (EPI) with a Fisher measure constructed with escort probabilities, the concomitant solutions obey a type of Naudts' duality for nonextensive ensembles. We also determine in closed form the general (normalized) probability distribution that minimizes Fisher's information.Comment: 4 pages, no figure

Dissipative cosmological solutions

The exact general solution to the Einstein equations in a homogeneous Universe with a full causal viscous fluid source for the bulk viscosity index $m=1/2$ is found. We have investigated the asymptotic stability of Friedmann and de Sitter solutions, the former is stable for $m\ge 1/2$ and the latter for $m\le 1/2$. The comparison with results of the truncated theory is made. For $m=1/2$, it was found that families of solutions with extrema no longer remain in the full case, and they are replaced by asymptotically Minkowski evolutions. These solutions are monotonic.Comment: 17 pages, LaTeX 2.09, 1 figure. To be published in Classical and Quantum Gravit

Cosmological solutions with nonlinear bulk viscosity

A recently proposed nonlinear transport equation is used to model bulk viscous cosmologies that may be far from equilibrium, as happens during viscous fluid inflation or during reheating. The asymptotic stability of the de Sitter and Friedmann solutions is investigated. The former is stable for bulk viscosity index $q1$. New solutions are obtained in the weakly nonlinear regime for $q=1$. These solutions are singular and some of them represent a late-time inflationary era.Comment: 16 pages Latex (IOP style); to appear Class. Quantum Gra

Interacting quintessence and the coincidence problem

We investigate the role of a possible coupling of dark matter and dark energy. In particular, we explore the consequences of such an interaction for the coincidence problem, i.e., for the question, why the energy densities of dark matter and dark energy are of the same order just at the present epoch. We demonstrate, that, with the help of a suitable coupling, it is possible to reproduce any scaling solution $\rho_X \propto \rho_M a^\xi$, where $a$ is the scale factor of the Robertson-Walker metric and $\xi$ is a constant parameter. $\rho_X$ and $\rho_M$ are the densities of dark energy and dark matter, respectively. Furthermore, we show that an interaction between dark matter and dark energy can drive the transition from an early matter dominated era to a phase of accelerated expansion with a stable, stationary ratio of the energy densities of both components.Comment: 3 pages, contribution to the Tenth Marcel Grossmann Meeting, Rio de Janeiro, 20-26 July 200

Stability of inflationary solutions driven by a changing dissipative fluid

In this paper the second Lyapunov method is used to study the stability of the de Sitter phase of cosmic expansion when the source of the gravitational field is a viscous fluid. Different inflationary scenarios related with reheating and decay of mini-blackholes into radiation are investigated using an effective fluid described by time--varying thermodynamical quantities.Comment: 17 pages, LaTeX 2.09, 2 figures. To be published in Classical and Quantum Gravit

Dark matter to dark energy transition in k-essence cosmologies

We implement the transition from dark matter to dark energy in k-essence cosmologies for a very large set of kinetic functions $F$, in a way alternative to recent proposals which use generalized Chaplygin gas and transient models. Here we require that the pressure admits a power-law expansion around some value of the kinetic energy where the pressure vanishes. In addition, for suitable values of the parameters of the model, the speed of sound of the dark matter will be low. We first present the discussion in fairly general terms, and later consider for illustration two examples.Comment: 5 pages, revte