On the cosmology of string axions

Abstract

In this thesis, we discuss various aspects of the phenomenology of axion fields originating from the compactification of type IIB superstring theory. First, we give an overview of moduli stabilisation and string axions. Starting from their origin as p-form gauge potentials integrated over p-cycles of the internal manifold, we discuss how they behave as four-dimensional effective fields and how they relate with certain swampland conjectures, namely the axionic version of the Weak Gravity Conjecture and the Festina Lente bound. Moreover, we study the effective theory of thraxions, i.e. ultralight axionic modes living in warped throats of Calabi-Yau manifolds. Then, we move to cosmology and in particular we focus on inflation and dark matter, two open issues which could find a solution once axions are taken into account. After reviewing the inflationary dynamics and some of the main proposals of axion inflation, we proceed to discuss how to realise hybrid inflation with axions supplying also a way to UV-complete the model. Moreover, and we provide a new approach to winding inflation, where the inflationary sector and dynamics arise from the complex structure moduli and are governed by certain topological invariants of the Calabi-Yau. Finally, we give a string-theoretical explanation of fuzzy dark matter as made of ultralight axions coming from type IIB compactifications. We show how it is possible to obtain such light axions in the type IIB axiverse including also thraxions, and we relate them with the present observational constraints. This allows us to both restrict the parameter space where future experiments could possibly detect string axions, and to use the observational bounds to put constraints on the UV theory and the compact dimensions.