Brane Models and Preheating of Fermions: New Scenarios and New Phenomenology for the Early Universe

Astroparticle physics and cosmology are experiencing an impressive development. The acquisition of more and more accurate observational data has recalled a great interest on these subjects, that provide a unique source of hints about the physics that lies far beyond the Standard Model. Parallely, the theoretical aspects of cosmology are also in rapid evolution along two main directions. The first one is related to the rise of radically new paradigms in model building, that has opened a wide range of new perspectives. The second is aimed at the achievement of a deeper and more detailed understanding of the "orthodox" picture that we have of the Early Universe. This thesis is divided into two parts. In the first part, cosmological aspects of brane models are discussed. This class of models, in which gravity can propagate in a supplementary space while matter is bound to the usual three dimensional space, has been recently proposed as a solution to the hierarchy problem. However, it has been soon realized that new scenarios arise for cosmology in this framework. In particular, possible modifications of the standard baryogenesis schemes and of the cosmological evolution of the Universe will be discussed. In the second part, a detailed analysis of fermion preheating will be carried out. The stage between the inflationary and the radiation dominated epochs of the early Universe is currently one of the most active areas of research in cosmology. In particular, nonthermal production of matter has been proven to be a substantial ingredient in the analysis of this stage. The phenomenologically most relevant effects related to the production of gravitinos at preheating will be especially highlighted.