String Cosmology - Large-Field Inflation in String Theory
Alexander Westphal
TL;DR
This review analyzes how string theory can realize large-field inflation, with a focus on axion monodromy as a robust mechanism to achieve trans-Planckian field excursions while maintaining radiative stability. It emphasizes the necessity of full moduli stabilization before inflationary dynamics, and surveys how axion monodromy, multi-axion alignments, and backreaction from moduli and branes influence observables such as $n_s$ and $r$ within a UV-complete framework. The work surveys stabilization schemes like KKLT and LVS, discusses the general structure of inflaton potentials in string compactifications, and highlights the key open questions and future directions for constructing explicit, calculable models. Overall, it positions large-field inflation in string theory as a promising conduit to probe string-scale physics through cosmological observations.
Abstract
This is a short review of string cosmology. We wish to connect string-scale physics as closely as possible to observables accessible to current or near-future experiments. Our possible best hope to do so is a description of inflation in string theory. The energy scale of inflation can be as high as that of Grand Unification (GUT). If this is the case, this is the closest we can possibly get in energy scales to string-scale physics. Hence, GUT-scale inflation may be our best candidate phenomenon to preserve traces of string-scale dynamics. Our chance to look for such traces is the primordial gravitational wave, or tensor mode signal produced during inflation. For GUT-scale inflation this is strong enough to be potentially visible as a B-mode polarization of the cosmic microwave background (CMB). Moreover, a GUT-scale inflation model has a trans-Planckian excursion of the inflaton scalar field during the observable amount of inflation. Such large-field models of inflation have a clear need for symmetry protection against quantum corrections. This makes them ideal candidates for a description in a candidate fundamental theory like string theory. At the same time the need of large-field inflation models for UV completion makes them particularly susceptible to preserve imprints of their string-scale dynamics in the inflationary observables, the spectral index $n_s$ and the fractional tensor mode power $r$. Hence, we will focus this review on axion monodromy inflation as a mechanism of large-field inflation in string theory.