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

Perfect fluid cosmologies with varying light speed

We have found exact constant solutions for the cosmological density parameter using a generalization of general relativity that incorporates a cosmic time-variation of the velocity of light in vacuum and the Newtonian gravitation constant. We have determined the conditions when these solutions are attractors for an expanding universe and solved the problems of the Standard Big Bang model for perfect fluids.Comment: 10 pages, LaTeX 2.09. To be published in International Journal of Modern Physics

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

Symmetries leading to inflation

We present here the general transformation that leaves unchanged the form of the field equations for perfect fluid Friedmann--Robertson--Walker and Bianchi V cosmologies. The symmetries found can be used as algorithms for generating new cosmological models from existing ones. A particular case of the general transformation is used to illustrate the crucial role played by the number of scalar fields in the occurrence of inflation. Related to this, we also study the existence and stability of Bianchi V power law solutions.Comment: 16 pages, uses RevTeX 4, to appear in Phys. Rev.