Studies of inflation and dark energy with coupled scalar fields
  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.
