Constraints on Axion Inflation from the Weak Gravity Conjecture

TL;DR

This paper explores constraints on axion inflation models from string theory and quantum gravity, focusing on decay constant alignment and the weak gravity conjecture, revealing strict no-go results and geometric limitations.

Contribution

It provides a string-theoretic analysis of axion decay constants, establishes no-go theorems for anti-alignment, and connects these constraints to the weak gravity conjecture.

Findings

Anti-alignment of axion decay constants is strictly forbidden in certain string models.

The weak gravity conjecture imposes significant constraints on axion inflation scenarios.

A specific decay constant alignment model evades some constraints but faces geometric limitations.

Abstract

We derive constraints facing models of axion inflation based on decay constant alignment from a string-theoretic and quantum gravitational perspective. In particular, we investigate the prospects for alignment and `anti-alignment' of $C_4$ axion decay constants in type IIB string theory, deriving a strict no-go result in the latter case. We discuss the relationship of axion decay constants to the weak gravity conjecture and demonstrate agreement between our string-theoretic constraints and those coming from the `generalized' weak gravity conjecture. Finally, we consider a particular model of decay constant alignment in which the potential of $C_4$ axions in type IIB compactifications on a Calabi-Yau three-fold is dominated by contributions from $D7$-branes, pointing out that this model evades some of the challenges derived earlier in our paper but is highly constrained by other geometric considerations.