Constraints on Early Dark Energy from the Axion Weak Gravity Conjecture

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 < 0.008M 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.

String Cosmology: from the Early Universe to Today

“…For an axionic potential, the periodic terms in (309) can be seen as the leading terms in an instanton expansion. Therefore, the value n = 3 would correspond to a delicate balance of the terms in an instanton expansion [989]. Although most of the work on this proposal has been phenomenological, a proposal to realise the EDE potential (309) in supergravity and string theory has been put forward in [990], using racetrack-like type of superpotentials with suitable coefficients to ensure the required behaviour.…”

Some open questions in axion theory

“…In addition to its application to large-field inflation, the axion Weak Gravity Conjecture also constrains the QCD axion decay constant [169] and models of early dark energy [170]. Yet, the conjecture itself is quite mysterious: while the ordinary Weak Gravity Conjecture is motivated by demanding consistent black hole decay, the physical motivation underlying the axion Weak Gravity Conjecture is unclear, and the primary evidence for the conjecture comes simply from examples in string theory and from connections to the ordinary Weak Gravity Conjecture.…”