Non-Gaussianities and Preheating from N-flation and Magnetogenesis via rotating cosmic string loops

In this thesis we examine multi-field inflationary models of the early Universe. Since non-Gaussianities may allow for the possibility to discriminate between models of inflation, we compute deviations from a Gaussian spectrum of primordial perturbations by extending the delta-N formalism. We use N-flation as a concrete model; our findings show that these models are generically indistinguishable as long as the slow roll approximation is still valid. Besides computing non-Guassinities, we also investigate Preheating after multi-field inflation. Within the framework of N-flation, we find that preheating via parametric resonance is suppressed, an indication that it is the old theory of preheating that is applicable. In addition to studying non-Gaussianities and preheatng in multi-field inflationary models, we study magnetogenesis in the early universe. To this aim, we propose a mechanism to generate primordial magnetic fields via rotating cosmic string loops. Magnetic fields in the micro-Gauss range have been observed in galaxies and clusters, but their origin has remained elusive. We consider a network of strings and find that rotating cosmic string loops, which are continuously produced in such networks, are viable candidates for magnetogenesis with relevant strength and length scales, provided we use a high string tension and an efficient dynamo.

In this thesis we cover two distinct major topics, non-Gaussianities and Preheating in N-flation, as well as primordial magnetogenesis from rotating cosmic string loops. N-flation is a model of Inflation (the rapid expansion of the early Universe) whereby many fields cooperate with each other to drive the inflationary phase as opposed to having only a single field. This model originates from string theory (the quantum theory of gravity), which contains the necessary large number of fields. We investigate the statistical properties of fluctuations in this model with the hope of finding additional contributions which go beyond the "Gaussian" ones of single field models. However, we find that these non-Guassianities are heavily suppressed, and thus the model is indistinguishable. We address preheating (the stage right after inflation during which the Universe becomes filled with ordinary hot matter) within the same framework. We find that it is the old theory of preheating (energy is slowly transfered from the fields to matter) and not parametric resonance (explosive particle production relying on resonance effects) that is applicable in this scenario. In the second part of this thesis we consider a possible origin for magnetic fields which are observed in galaxies. We propose that cosmic string loops, which are produced in many models of the early universe, can cause magnetic fields a few hundred thousand years after the Big Bang. The loops stir up the plasma, creating vortices, which in turn give rise to currents and hence to magnetic fields. We find that these fields are strong enough and have large enough length scales to account for the presently observed fields in galaxies; this holds true if strings are quite "massive" and that the seed fields are strongly enhanced during galactic rotations (also called the dynamo).