String Cosmology - Large-Field Inflation in String Theory

TL;DR

This paper reviews how string theory can potentially connect high-energy string-scale physics to observable cosmological signals, focusing on large-field inflation models like axion monodromy inflation that could leave detectable imprints in the CMB.

Contribution

It highlights the role of GUT-scale large-field inflation in string theory and discusses axion monodromy as a promising mechanism for observable signatures of string-scale physics.

Findings

GUT-scale inflation can produce observable gravitational waves.

Large-field inflation models require symmetry protection against quantum corrections.

Axion monodromy inflation offers a viable string-theoretic realization of large-field inflation.

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.