Models of the very early universe with multiple scalar fields

There is a reasonable amount of observational evidence that suggests space was expanding exponentially in the very early universe --- an expansion that has become known as inflation. The mechanism by which this happens remains up for debate, however, and this thesis looks at a number of potential scenarios using multiple scalar fields to drive the expansion. There are two studies that look at how additional couplings either between the fields themselves or to gravity can influence the observable consequences of inflation on the Cosmic Microwave Background and one which tries to extend a gravitational coupling to explain the current expansionary epoch caused by dark energy. The importance of reheating in such scenarios is also investigated. In the case of a non-canonical kinetic coupling, an approximation is used to show how the curvature perturbation can evolve on super-horizon scales to a much improved accuracy over previous work. The gravitational coupling results in a vast increase in the amplitude of the curvature power spectrum via the non-adiabatic pressure perturbation and, finally, the attempts to link this to dark energy are demonstrated to be much more difficult than one might initially assume.