Removing the Cosmological Bound on the Axion Scale via Confinement During Inflation

TL;DR

This paper proposes a mechanism where early confinement during inflation relaxes the axion potential, removing the cosmological bound on its decay constant, applicable across various axion models.

Contribution

It introduces a novel inflationary confinement scenario in SU(5) GUT that enables large axion decay constants without cosmological constraints.

Findings

Early confinement dilutes axion energy density.

The mechanism works for PQ and gauge axion formulations.

Axion can be a dark matter candidate with arbitrarily large decay constant.

Abstract

We implement the scenario of early relaxation of the axion via a high scale confinement within $SU(5)$ grand unified theory and study an epoch of strong QCD in inflationary cosmology. We consider scenarios in which, during inflation, the $SU(5)$ is either entirely or partially in the confining phase. This generates an early potential for the axion and dilutes its energy density removing any cosmological upper bound on the decay constant. We show that a phase of strong QCD can be realized by at least two mechanisms: 1) A direct coupling between the inflaton and the gauge fields and/or 2) by restoration of the $SU(5)$ symmetry during the inflationary epoch. In the latter case, strong coupling is already achieved via the RG running of the $SU(5)$ gauge coupling. We show that the mechanism works for all known realizations of the invisible axion idea: Peccei-Quinn (PQ) type formulations in which the anomalous global symmetry is realized via additional scalars (DFSZ) or heavy fermions (KSVZ) as well as the two-form gauge axion formulation based entirely on the QCD gauge redundancy without any anomalous global symmetry. Even if the expectation value of the PQ scalar vanishes during inflation, the axion is a well defined degree of freedom represented by the phase of the fermion 't Hooft determinant. For the DFSZ case, this phase is composed out of a condensate of the ordinary quarks, amounting to an early universe version of the $\eta'$-meson. In all considered scenarios, the present day axion can be a viable dark matter candidate for an arbitrarily large value of the decay constant.