Studies of inflation and dark energy with coupled scalar fields

Abstract: Currently there is no definitive description for the accelerated expansion of the Universe at both early and late times; we know these two periods as the epochs of inflation and dark energy. Contained within this Thesis are two studies of inflation and one in the context of dark energy. The first study involves two noncanonical kinetic terms each in a two-field scenario, and their effects on the generation of isocurvature modes. As a result, these terms affect the isocurvature perturbations produced, and consequently the Cosmic Microwave Background. In the following study, the impact of a sharp transition upon the effective Planck mass is considered in both a single-field and two-field model. A feature in the primordial power spectrum arising from these transitions is found in single-field models, but not for two-field models. The final model discussed is on the subject of dark energy. A type of nonconformal coupling is examined namely the "disformal" coupling; in this scenario a scalar field is disformally coupled to matter species. Two consistency checks are undertaken, the first to provide a fluid description and the second, a kinetic theory. From this, observables are constructed and used to create constraints on the individual coupling strengths.