Constraints on small-field axion inflation

TL;DR

This paper investigates small-field axion inflation models, identifying constraints on axion decay constants to avoid cosmological issues, and finds that viable decay constants are typically near the GUT or string scale.

Contribution

It provides a comprehensive analysis of constraints on axion decay constants in small-field inflation, linking them to string theory predictions and cosmological observations.

Findings

Axion decay constants must be carefully constrained to avoid disrupting nucleosynthesis.

Viable decay constants are generally around the GUT or string scale.

The study connects axion inflation models with string theory expectations.

Abstract

We study general class of small-field axion inflations which are the mixture of polynomial and sinusoidal functions suggested by the natural and axion monodromy inflations. The axion decay constants leading to the successful axion inflations are severely constrained in order not to spoil the Big-Bang nucleosynthesis and overproduce the isocurvature perturbation originating from the QCD axion. We in turn find that the cosmologically favorable axion decay constants are typically of order the grand unification scale or the string scale which is consistent with the prediction of closed string axions.