Nonsingular Bouncing Cosmology

Abstract

This thesis studies the cosmological theory in which the universe transitions from a contraction phase into an expansion phase through a big bounce. Primordial fluctuations that seed structure formation in the expansion phase arise from adiabatic perturbations in the preceding contraction phase. The purpose of this study is to understand how the properties of the adiabatic perturbations are affected by the bounce. In particular, a nonsingular type of bounce is considered in which the universe ceases contraction and reverses to expansion at a finite size, fully described by known theories of classical gravity and effective field theory. Two major aspects of such a nonsingular bounce are studied -- the stability of the bounce against inhomogeneities, and the power spectrum of adiabatic perturbations after the bounce. Results show that a class of bouncing models based on ghost condensation are subject to unstable growth of curvature and anisotropy, which alters the adiabatic perturbations and disrupts the nonsingular bounce. Another class of models with a ghost field are shown to have limited instability, though the contraction phase requires fine-tuning; sufficiently small perturbations can pass through the bounce and maintain a nearly scale-invariant power spectrum, consistent with observational constraints. Incorporating features of both models and resolving their problems, an ekpyrotic nonsingular bounce is proposed to support stable contraction and bouncing phases yet produce scale-invariant perturbations. Thus the nonsingular bouncing cosmology provides a possible explanation for the early universe.