Constraints on Early Dark Energy from the Axion Weak Gravity Conjecture

TL;DR

This paper shows that early dark energy models involving axions face strong constraints from the axion weak gravity conjecture, making their realization in quantum gravity frameworks challenging.

Contribution

It provides a novel constraint on early dark energy models using the axion weak gravity conjecture, limiting the axion decay constant to values below 0.008 times the Planck mass.

Findings

Axion decay constant must be less than 0.008 M_{Pl} for typical models.

Loopholes to the constraint are small and difficult to realize.

Constraints challenge the UV completion of early dark energy models.

Abstract

A popular proposal for resolving the Hubble tension involves an early phase of dark energy, driven by an axion field with a periodic potential. In this paper, we argue that these models are tightly constrained by the axion weak gravity conjecture: for typical parameter values, the axion decay constant must satisfy $f < 0.008 M_{\textrm{Pl}}$, which is smaller than the axion decay constants appearing in the vast majority of early dark energy models to date. We discuss possible ways to evade or loosen this constraint, arguing that its loopholes are small and difficult to thread. This suggests that it may prove challenging to realize early dark energy models in a UV complete theory of quantum gravity.