Generating curvature perturbations in a contracting universe

Abstract

This thesis studies bouncing cosmologies in which the present-day expansion of the universe was preceded not by a “big bang”-- before which time and space ceased to have meaning-- but by a contracting phase that then bounced. We discuss two competing paradigms for generating the observed, scale-invariant spectrum of primordial density perturbations during the contracting phase: "the matter bounce scenario” and "ekpyrosis.” First, we discuss the matter bounce scenario, and in particular, its fine-tuning instability to the growth of anisotropic stress. Then, we examine ekpyrosis. In the best-understood ekpyrotic models, one scalar field drives the background evolution of the universe while another (entropic) scalar field generates the density perturbations. We study the stability of these models, showing that in contrast to previous theorems, the simplest (as measured by parameters and degrees of freedom), observationally viable realizations are dynamical attractors. Finally, we present a new mechanism called “warm ekpyrosis,” which eliminates altogether the need for the second (entropic) scalar field. Rather, a single field falls down its ekpyrotic potential, smoothing and flattening the universe, while simultaneously, through couplings to lighter degrees of freedom, decaying into hot, ultrarelativistic matter. This decay allows both for the production of a scale-invariant density perturbation and for a possible mechanism of reheating.