Weak Gravity Strongly Constrains Large-Field Axion Inflation

TL;DR

This paper argues that the Weak Gravity Conjecture imposes strong constraints on large-field axion inflation models, likely preventing super-Planckian field ranges and challenging their viability.

Contribution

It extends the Weak Gravity Conjecture constraints to multiple axion models, including N-flation and alignment models, revealing fundamental limitations.

Findings

Field ranges cannot be parametrically larger than the Planck scale.

Multiple axion models are constrained by the Weak Gravity Conjecture.

Large-field inflation models based on compact axions face fundamental inconsistencies.

Abstract

Models of large-field inflation based on axion-like fields with shift symmetries can be simple and natural, and make a promising prediction of detectable primordial gravitational waves. The Weak Gravity Conjecture is known to constrain the simplest case in which a single compact axion descends from a gauge field in an extra dimension. We argue that the Weak Gravity Conjecture also constrains a variety of theories of multiple compact axions including N-flation and some alignment models. We show that other alignment models entail surprising consequences for how the mass spectrum of the theory varies across the axion moduli space, and hence can be excluded if further conjectures hold. In every case that we consider, plausible assumptions lead to field ranges that cannot be parametrically larger than the Planck scale. Our results are strongly suggestive of a general inconsistency in models of large-field inflation based on compact axions, and possibly of a more general principle forbidding super-Planckian field ranges.