Axion Inflation and Gravity Waves in String Theory

TL;DR

This paper explores string theory models of axion inflation that could produce detectable gravitational waves, focusing on specific Kahler potentials and their viability within supergravity frameworks.

Contribution

It analyzes the conditions under which axion inflation can occur in string theory, highlighting the importance of Kahler potentials and identifying promising model classes.

Findings

Axion inflation is challenging with logarithmic Kahler potentials in Calabi-Yau compactifications.

Quadratic shift-symmetric Kahler potentials may support viable axion inflation.

More research needed to confirm the existence of workable models.

Abstract

The majority of models of inflation in string theory predict an absence of measurable gravitational waves, r << 10^{-3}. The most promising proposals for making string theoretic models that yield measurable tensor fluctuations involve axion fields with slightly broken shift symmetry. We consider such models in detail, with a particular focus on the N-flation scenario and on axion valley/natural inflation models. We find that in Calabi-Yau threefold compactifications with logarithmic Kahler potentials K it appears to be difficult to meet the conditions required for axion inflation in the supergravity regime. However, in supergravities with an (approximately) quadratic shift-symmetric K, axion inflation may be viable. Such Kahler potentials do arise in some string models, in specific limits of the moduli space. We describe the most promising classes of models; more detailed study will be required before one can conclude that working models exist.