Perturbations and Preheating in Inflation and NEC-Violating Alternative Models.

摘要

We apply the theory of cosmological perturbations and preheating to hybrid-type class of models of inflation and to two specific alternative models based on the violation of the null energy condition (NEC) in order to verify whether it is possible to resolve or avoid specific problems of the inflationary paradigm in its traditional formulation. After introducing the framework of the potential-driven inflationary paradigm, we review its two pillars which allow inflationary models to be predictive: the theory of cosmological perturbations and the theory of preheating/reheating, with particular focus on parametric resonance. We then apply the theory of resonant instability to entropy perturbations at the time of preheating in the context of multi-field hybrid inflation. We find that the back-reaction of small-scale entropy modes on large-scale linear adiabatic modes alone is not fast enough to truncate the parametric instability of the long wavelength modes before the latter has sourced a sizeable modification of the curvature perturbations on observational scales. Therefore, unless there are other mechanisms which truncate the instability, this effect might rule out this class of models.;This problem together with other, more fundamental problems which stain the inflationary paradigm- such as the cosmological singularity problem and the trans-Planckian problem - motivate the search for new avenues to model the physics of the early Universe. In that perspective, we study the production of cosmological perturbations and reheating in the context of two non-singular models based on the introduction of NEC-violating matter. The first of these models is based on the use of a ghost condensate to realize a stable bouncing scenario. We find that the produced spectrum of perturbations is scale invariant. The latter of the models relies on the use of Galileons to realize an emergent Universe scenario. In the context of this model, we compute that the produced spectrum of matter field perturbations is scale invariant and verify that the model does not suffer from a graceful exit problem. However, a more careful analysis shows that the curvature perturbations are quadratic in the field perturbations, which spoils the flatness of the curvature power spectrum and might rule out the model in its current form.