Suppressing Isocurvature Perturbations of QCD Axion Dark Matter

TL;DR

This paper introduces a new mechanism to suppress isocurvature perturbations of QCD axion dark matter by leveraging strong QCD interactions at high energies, allowing for higher inflation scales.

Contribution

The paper presents a novel approach to reduce axion isocurvature perturbations by modifying QCD dynamics during the early Universe, enabling relaxed inflation constraints.

Findings

QCD interactions can be strengthened at high energies with a large Higgs expectation value.

The axion becomes heavy during inflation, suppressing isocurvature perturbations.

This mechanism relaxes the constraints on inflation scale imposed by axion dark matter models.

Abstract

We propose a novel mechanism to suppress the isocurvature perturbations of the QCD axion. The point is that the QCD interactions become strong at an intermediate or high energy scale in the very early Universe, if the Higgs field has a sufficiently large expectation value. The effective QCD scale can be even higher in the presence of extra colored particles. We show that the QCD axion then becomes so heavy during inflation that its isocurvature perturbations are significantly suppressed, thereby relaxing the constraint on the inflation scale.