Aspectos termodinâmicos em cosmologia

Abstract

Astronomical observations performed in the last 20 years involving type Ia Supernovae, cosmic microwave background and large scales structures, indicate that the Universe is described by a spatially flat cosmological model and is currently in an accelerated expansion state. The cause of the acceleration is unknown, and this is one of the greatest challenges in modern physics. The model that better describes the data, the ACDM model, presents technical inconsistencies, and because of these problems, the scientific community has been proposing alternative scenarios to ACDM model, which can simultaneously fits the observational data and have some consistent theoretical framework. Among the alternatives to a cosmological constant is common to assign additional degrees of freedom for new and exotic forms of matter with negative pressure called dark energy. In this thesis, we investigate the possible connections between cosmology, gravitation, and thermodynamics. We first investigate how thermodynamics beyond the Boltzmann-Gibbs statistics (non-Gaussian statistics) in combination with an entropic gravitational scenario can affect the dynamics of the Universe. We find that these scenarios lead to a change in the gravitational constant, which can make the gravitational field weaker or stronger at large scales. We compare our model of Universe with data from type Ia Supernovae, baryon acoustic oscillations (BAO), the rate of expansion of the University, and the growth function of the linear perturbations of matter. In the second part, we investigate how the limits imposed by the conditions of thermodynamic stability, can constrain theoretically dark energy models. We show that, if dark fluids with ! < 0 satisfy these conditions of stability, such requirement poses difficulties on the existence of such models. Comparing our thermodynamic limits with the observational constraints, we note that a very significant portion of the parametric space for a dynamical dark energy model is excluded when compared to the thermodynamic constraints. We believe this result can be an important step in modeling phenomenological of dark energy.