PhD Thesis: String theory in the early universe

TL;DR

This thesis explores the application of string theory to early universe cosmology, addressing axion realization, cosmic string network properties, and their potential role in generating galactic magnetic fields.

Contribution

It provides new insights into string theory realizations of axions, cosmic superstring network dynamics in warped backgrounds, and their cosmological implications.

Findings

Warped compactifications can lower axion decay constants.

Cosmic superstring networks differ quantitatively from GUT strings.

Heterotic cosmic strings could generate galactic magnetic fields.

Abstract

The intersection of string theory with cosmology is unavoidable in the early universe, and its exploration may shine light on both fields. In this thesis, three papers at this intersection are presented and reviewed, with the aim of providing a thorough and pedagogical guide to their results. First, we address the longstanding problem of finding a string theory realisation of the axion. Using warped compactifications in heterotic string theory, we show that the axion decay constant can be lowered to acceptable values by the warp factor. Next, we move to the subject of cosmic strings, whose network evolution could have important consequences for astrophysics and cosmology. In particular, there are quantitative differences between cosmic superstring networks and GUT cosmic string networks. We investigate the properties of cosmic superstring networks in warped backgrounds, giving the tension and properties of three-string junctions in these backgrounds. Finally, we examine the possibility that cosmic strings in heterotic string theory could be responsible for generating the galactic magnetic fields that seeded those observed today.