Consequences of dissipative dynamics in the early universe

Abstract

Warm inflation presents an exceptional description of the early universe cosmology. It is a scenario of an inflationary dynamics in which the state of the universe during inflation is not the vacuum state, but rather an excited statistical thermal state. It introduces dissipation into the inflationary dynamics which can be well explained by first principles of a quantum multi-field theory. Besides, this approach has several attractive features. For instance, the additional friction may ease the required flatness of the inflaton potential. Besides, even if radiation is subdominant during inflation, may smoothly become the leading component if the ratio of dissipation Q ≳ 1 at the end of inflation (ϵeff ~ 1 + Q), with no need for a separate reheating period. It also may explain the nature of the classical inhomogeneities observed in the CMB, since for WI the fluctuations of the inflaton are thermally induced; hence there is no need to explain the troublesome quantum-to-classical transition problem of the standard inflation picture, cold inflation, due to the purely quantum origin of the density perturbations. Furthermore, one well established key aspect is the prediction for a low tensor-to-scalar ratio, which now we see is consistent with Planck legacy. Taking into account above encouraging warm inflation characteristics, in this thesis we will describe both warm inflation model building and the confrontation of theory with observation. We will examine two basic models: The Warm Little Inflaton scenario and the distributed mass model. In each case, we determine the parametric regimes in which the dynamical evolution is consistent for 50-60 e-folds of inflation, taking into account thermal corrections to the scalar potential (if necessary). In the first model we consider three distinct types of scalar potentials for the inflaton, namely chaotic inflation with a quartic monomial potential, a Higgs-like symmetry breaking potential and a non-renormalizable plateau-like potential. On the other hand, the distributed mass model is examined for various mass distributions considering a chaotic quartic potential. Both scenarios are theoretically and observationally successful for a broad range of parameter values. Indeed, they agree remarkably with the Planck legacy data. The Warm Little Inflaton is undoubtedly the simplest realisation of warm inflation within a concrete quantum field theory construction, since it requires only a small number of fields; in particular, the inflation is directly coupled to just two light fields. Distributed mass models can be viewed as realisations of the landscape property of string theory, with the mass distributions coming from the underlying spectra of the theory, which themselves would be affected by the vacuum of the theory.