Axion-like particles and inflation from String Theory

This thesis is focused on cosmological applications of the 4D Effective Field Theory (EFT) coming from type IIB string theory. We focus in particular on model building in inflation, dark matter and dark radiation using Kaehler moduli and axion-like fields which are ubiquitous features of type IIB flux compactifications. These fields enjoy effective approximate symmetries which can protect their potential against quantum corrections. This property makes both of them good inflaton candidates and implies that axion-like particles from string theory tend naturally to be very light with intriguing applications to dark radiation and dark matter. We first consider a class of type IIB inflationary models called "Fibre Inflation" where the inflaton is a Kaehler modulus. We provide a consistent global embedding of these models into Calabi-Yau orientifolds with D-branes, fluxes and a chiral visible sector. We also analyse the multi-field dynamics of this class of models, including both Kaehler moduli and axion-like particles which give rise to isocurvature perturbations. We then focus on different applications of axion-like particles coming from string theory. Depending on the value of their mass and decay constant, together with their production mechanism, these particles can drive inflation or can represent a non-negligible component of dark matter and dark radiation. We provide a string embedding of a model that explains the 3.5 keV line recently detected from galaxy clusters by exploting axion-photon conversion in astrophysical magnetic fields. Finally, we analyse the mechanisms of electro-magnetic dissipation in models where the inflaton is an axion, finding a new resonant behaviour in the gauge field production that affects the shape of the cosmological parameters.