Relaxing Axions

TL;DR

This paper proposes a mechanism to relax the cosmological upper bound on the axion decay constant by transferring energy from axion oscillations to a nearly massless radial mode, which then redshifts away.

Contribution

It introduces a novel approach to lift the axion decay constant bound by coupling axion oscillations to a nearly massless radial mode.

Findings

The mechanism effectively reduces the energy in axion oscillations.

It allows for higher axion decay constants without conflicting with cosmological constraints.

The radial mode's dynamics are crucial for energy transfer and redshifting.

Abstract

A mechanism for lifting the cosmological upper bound on the axion decay constant, $f_a$, is proposed. It entails the near masslessness of the radial mode whose vacuum expectation value is $f_a$. Energy in the coherent oscillations of the axion field in the early universe gets fed into the motion of the radial mode, from which it is then redshifted away.