Alejandro S. Jakubi

Interacting quintessence solution to the coincidence problem

We show that a suitable interaction between a scalar field and a matter fluid in a spatially homogeneous and isotropic spacetime can drive the transition from a matter dominated era to an accelerated expansion phase and simultaneously solve the coincidence problem of our present Universe. For this purpose we study the evolution of the energy density ratio of these two components. We demonstrate that a stationary attractor solution is compatible with an accelerated expansion of the Universe. We extend this study to account for dissipation effects due to interactions in the dark matter fluid. Finally, type Ia supernovae and primordial nucleosynthesis data are used to constrain the parameters of the model.

Enlarged quintessence cosmology

Fil: Chimento, Luis Pascual. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Fisica de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisica de Buenos Aires; Argentina

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 q 1. 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.

Dark energy, dissipation, and the coincidence problem

In a recent paper we showed that a quintessence scalar field plus a dissipative matter fluid can drive late cosmic accelerated expansion and simultaneously solve the coincidence problem. In this Brief Report we extend this result to the cases when the scalar field is replaced either by a Chaplygin gas or a tachyonic fluid.

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 is found. We have investigated the asymptotic stability of Friedmann and de Sitter solutions, the former is stable for and the latter for . The comparison with results of the truncated theory is made. For , 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.

SCALAR FIELD COSMOLOGIES WITH PERFECT FLUID IN ROBERTSON-WALKER METRIC

Several isotropic, homogeneous cosmological models containing a self-interacting minimally coupled scalar field, a perfect fluid source and cosmological constant are solved. New exact, asymptotically stable solutions with an inflationary regime or a final Friedmann stage are found for some simple, interesting potentials. It is shown that the fluid and the curvature may determine how these models evolve for large times.

Relaxation dominated cosmological expansion

Abstract The behavior near the singularity of an isotropic, homogeneous cosmological model with a viscous fluid source is investigated. This turns out to be a relaxation dominated regime. Full extended irreversible thermodynamics is used, and a comparison with results of the truncated theory is made. New singular behaviors are found and it is shown that a relaxation dominated inflationary epoch may exist for fluids with a small heat capacity.

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.

NEW COSMOLOGICAL SOLUTIONS AND STABILITY ANALYSIS IN FULL EXTENDED THERMODYNAMICS

The Einsteins field equations of FRW universes filled with a dissipative fluid described by full theory of causal transport equations are analyzed. New exact solutions are found using a nonlocal transformation on the nonlinear differential equation for the Hubble factor. The stability of the de Sitter and asymptotically Friedmannian solutions are analyzed using Lyapunov function method.

Synergistic warm inflation

We consider an alternative warm inflationary scenario in which n scalar fields coupled to a dissipative matter fluid cooperate to produce power-law inflation. The scalar fields are driven by an exponential potential and the bulk dissipative pressure coefficient is linear in the expansion rate. We find that the entropy of the fluid attains its asymptotic value in a characteristic time proportional to the square of the number of fields. This scenario remains nearly isothermal along the inflationary stage. The perturbations in energy density and entropy are studied in the long-wavelength regime and seen to grow roughly as the square of the scale factor. They are shown to be compatible with COBE measurements of the fluctuations in temperature of the CMB.