Spectator String Axions: Messages from the Early Universe

Abstract

String theory compactifications generically predict the existence of a rich spectrum of axion- like fields, the so-called Axiverse, whose masses and couplings are determined by the geometry and flux structure of the compactification. In this thesis, we explore the cosmological and phenomenological implications of such axions, combining insights from both Type IIB orientifold and heterotic E8 × E8 Calabi-Yau compactifications. We initiate the characterization of the generic structure of the heterotic axiverse, mapping the axion mass spectrum and the effective couplings of axions to gauge fields. This analysis reveals model-independent constraints on their role in solving the strong CP problem, contributing to dark matter, and mediating hidden-visible sector interactions. We further investigate inflationary scenarios where spectator axions couple to hidden gauge sec- tors via Chern-Simons interactions, leading to distinctive multi-peak features in the scalar and tensor spectra. We study axion-gauge field models with transient fast-roll phases, showing how they can enhance spectral distortions while satisfying large-scale CMB bounds, and identifying the challenges of controlling such dynamics in non-Abelian scenarios. These models can pro- duce a “gravitational-wave forest” spanning several decades in frequency, with the nHz band potentially explaining recent pulsar timing array signals. We analyze the complementary role of CMB spectral distortions in constraining the scalar sector, and identify the details of the string theory compactification that naturally realize the required couplings in the string landscape. Finally, we examine non-perturbative dynamics in the early universe, focusing on axion produc- tion via parametric resonance during preheating in string inflation models. Kinetic couplings and moduli-dependent masses lead to generalized Mathieu equations, enabling efficient produc- tion of heavy axions that can overclose the universe unless inflationary parameters are suitably constrained. Taken together, these results demonstrate how string theory provides a natural framework for embedding rich axion sectors, and how their cosmological dynamics offer unique observational windows, from gravitational waves to spectral distortions, into the high-energy physics of the early universe.