String moduli inflation: an overview

TL;DR

This paper reviews string theory-based inflation models focusing on closed string moduli, discussing challenges like the eta-problem, moduli stabilization, and the implications for observable gravitational waves, highlighting the current state and open issues.

Contribution

It provides a comprehensive overview of string moduli inflation, analyzing various models, stabilization methods, and their observational predictions, especially in relation to gravitational waves.

Findings

Most models predict negligible gravitational waves.

Addressing the eta-problem remains a key challenge.

String inflation scenarios face open issues beyond inflation itself.

Abstract

We present an overview of inflationary models derived from string theory focusing mostly on closed string moduli as inflatons. After a detailed discussion of the eta-problem and different approaches to address it, we describe possible ways to obtain a de Sitter vacuum with all closed string moduli stabilised. We then look for inflationary directions and present some of the most promising scenarios where the inflatons are either the real or the imaginary part of Kaehler moduli. We pay particular attention on extracting potential observable implications, showing how most of the scenarios predict negligible gravitational waves and could therefore be ruled out by the Planck satellite. We conclude by briefly mentioning some open challenges in string cosmology beyond deriving just inflation.