Dynamics and observational signatures from multi-field inflation

In this thesis, we focus on the dynamics and predictions of multi-field models of inflation. In the first three chapters, we introduce the basic elements of modern cosmology with emphasis on the inflationary scenario. In chapter 4 we examine in detail the multi-field dynamics for generic models and briefly discuss observables for models following the gradient flow. We continue in chapter 5 with an investigation of the observational viability of a two-field generalization of the usual α-attractor model. We discover a novel attractor solution that characterizes the late-time dynamics for certain parameter values and which can substantially alter the universal predictions of the single-field model. To better understand the existence of the latter attractor solution, in chapter 6 we study two-field models that admit scaling solutions. We provide an extensive classification of stable late-time solutions and we calculate their stability criteria, which, as we demonstrate, may differ from the standard ones commonly used in the literature. In chapter 7 we show that the late-time dynamics for a plethora of viable models is characterized by the critical points of an effective potential, constructed from the field metric and the potential gradients. These critical points can undergo pitchfork bifurcations explaining previously found phenomena such as geometrical destabilization. Finally, in chapter 8 we investigate predictions at the many-field limit using N-flation as a toy model. We demonstrate that multi-field inflation is prior dependent, and hence non-predictive, even when a notion of universality is present