| Summary: | Inflation is currently the most promising paradigm of the Early Universe. The simple paradigm involves a single canonical scalar field minimally coupled to gravity slowly rolling down a potential. In this thesis, we discuss an extension to the simple paradigm, multifield inflation, in which inflation is driven by more than one scalar field. Unlike in the single field paradigm, isocurvature perturbations could be non-vanishing and source curvature perturbation on superhorizon scales. Analytic model predictions during the slow-roll regime in some classes of multifield inflation models have been worked out in the literature. However, curvature perturbation may continue to evolve after slow-roll as isocurvature perturbations are not necessarily exhausted when inflation ends. In this thesis, by using the δ N formalism, we investigate the effects of perturbative reheating on the curvature perturbation and related observables in multifield models. By considering various two-field models, we demonstrate that the subsequent (p)reheating evolution is significant and must be taken into account even for perturbative reheating. How the model predictions evolve during reheating is a model dependent question, implying that models of multifield inflation cannot be compared to observations directly without specifying how reheating takes place. We also discuss a different class of two-field models, conformal inflation, which is locally scale invariant. Universal behaviour emerges as a critical phenomenon near the enhanced SO(1,1) or shift symmetry point, leading to model independent predictions. Going beyond the original model proposed by Kallosh and Linde, we show that this universal behaviour extends to more generalised models involving higher order derivatives for slow-roll potential driven inflation.
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