REMOVING THE COSMOLOGICAL BOUND ON THE AXION SCALE

TL;DR

This paper proposes a mechanism in supergravity theories that can evade the cosmological bounds on the axion scale by altering the early universe dynamics of the axion field, potentially expanding the viable axion parameter space.

Contribution

It introduces a scenario where the initial conditions of scalar fields in supergravity modify axion evolution, removing the cosmological bound on the axion scale.

Findings

Early universe color confinement fixes the axion to its minimum

Scalar field adjustment suppresses axion oscillations

Standard axion window may not apply in supergravity models

Abstract

The current cosmological bound on the invisible axion scale may be avoided in the class of theories in which the gauge coupling constant is determined through the expectation value of some scalar field (e.g. moduli in supergravity and string theories). This leads to the cosmological scenario different from that of the standard invisible axion, since the initial values of the fields are usually far away from their true minima, allowing for the color group becoming strong in the very early universe and fixing the axion field to its minimum. The effect disappears as soon as scalar field adjusts to its present value, but the above is enough to ensure that the deviation of the axion expectation value from the minimum is negligible at the moment of the QCD phase transition and thus to eliminate the troublesome coherent oscillations. This may imply that the standard axion window does not necessarily hold in generic supergravity theories. The above observation may open a natural possibility for the existence of the axion resulting from the GUT or R-symmetry breaking.