Moduli stabilization for the axiverse

This Master’s thesis focuses on the study of phenomenological and cosmological applications of string theory. The low-energy limit of string compactifications has been argued to give rise to a so-called ”axiverse,” which comprises a plethora of axion-like particles with a mass spectrum that is logarithmically distributed over a wide range of mass scales. However, this scenario is currently unsupported by any explicit computations based on moduli stabilization. In this thesis, we aim to fill in this gap by analyzing the mass spectrum of type IIB Calabi-Yau orientifold compactifications with a large number of moduli and axions. We will focus on the regime where α′-corrections are under control, which implies a very large overall volume. Moreover, we will consider various types of perturbative and higher derivative corrections to the four-dimensional effective action to achieve full moduli stabilization analytically, considering two types of Calabi-Yau manifolds: Swiss-Cheese and Fibred Calabi-Yau threefolds. The final goal is to determine the axionic mass spectrum while taking into account constraints imposed by consistency with standard cosmology. Consequently, we can explicitly investigate the conditions under which an actual axiverse exists, instead of a plethora of effective massless axions. Furthermore, the existence of the axiverse may give rise to several potential observable effects, which are governed by the mass spectrum. By identifying the spectra, we can establish a concrete connection between cosmological observations and the topology of Calabi-Yau manifolds.