### Abstract

The very early universe is where we expect the observed primordial perturbations in the cosmic microwave background to have originated. In this thesis we study isocurvature field fluctuations during inflation and ekpyrotic contraction as sources of the primordial curvature perturbations.We start by introducing concepts of modern cosmology followed by an overview of early universe cosmology. After, we introduce perturbation theory and how to compute perturbations from early universe models.

After reviewing all fundamental concepts necessary for this thesis, we estimate largescale curvature perturbations from isocurvature fluctuations in the waterfall field during hybrid inflation, in addition to the usual inflaton field perturbations. The tachyonic instability at the end of this inflation model leads to an explosive growth of super-Hubble scale perturbations, but they retain the steep blue spectrum characteristic of vacuum fluctuations in a massive field during inflation. We extend the usual δN formalism to include the essential role of small fluctuations when estimating the large-scale curvature perturbation.

The following two chapters study perturbations within the curvaton proposal. Firstly, we consider how non-Gaussianity of the primordial density perturbation and the amplitude of gravitational waves from inflation can be used to determine parameters of the curvaton scenario for the origin of structure. We show that in the simplest quadratic model, where the curvaton evolves as a free scalar field, measurement of the bispectrum relative to the power spectrum, fNL, and the tensor-to-scalar ratio can determine both the expectation value of the curvaton field during inflation and its dimensionless decay rate relative to the curvaton mass. We show how these predictions are altered by the introduction of self-interactions. In the following chapter, we then characterise the primordial perturbations produced due to both inflaton and curvaton fluctuations. We show how observational bounds on non-linearity parameters and the tensor-scalar ratio can be used to constrain curvaton and inflaton parameters.

The final research presented in this thesis, considers a simple model of cosmological collapse driven by canonical fields with exponential potentials. We generalise the two-field ekpyrotic collapse to consider non-orthogonal potentials and give the general condition for isocurvature field fluctuations to have a slightly red spectrum of perturbations as required by current observations. However a red spectrum of fluctuations implies that the two-field ekpyrotic phase must have a finite duration and requires a preceding phase which sets the initial conditions for what otherwise appears to be a fine-tuned trajectory in the phase space.

We end this thesis with some concluding remarks and comments on possible future work.

Date of Award | Oct 2012 |
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Original language | English |

Supervisor | David Wands (Supervisor) |