High-Scale Axions without Isocurvature from Inflationary Dynamics

TL;DR

This paper explores how inflationary modifications to the axion potential can relax isocurvature constraints, enabling high-scale axions to be consistent with observable primordial tensor modes in the CMB.

Contribution

It introduces models where inflationary dynamics alter the axion potential, relaxing isocurvature bounds and allowing high-scale axions without disrupting inflation.

Findings

Models with larger PQ-breaking scale during inflation require late-time dilution or super-Planckian scales.

Explicit PQ breaking during inflation can permit Planck-scale axions.

Inflationary dynamics impose limits on axion parameter space.

Abstract

Observable primordial tensor modes in the cosmic microwave background (CMB) would point to a high scale of inflation $H_{I}$. If the scale of Peccei-Quinn (PQ) breaking $f_a$ is greater than $\frac{H_{I}}{2\pi}$, CMB constraints on isocurvature naively rule out QCD axion dark matter. This assumes the potential of the axion is unmodified during inflation. We revisit models where inflationary dynamics modify the axion potential and discuss how isocurvature bounds can be relaxed. We find that models that rely solely on a larger PQ-breaking scale during inflation $f_I$ require either late-time dilution of the axion abundance or highly super-Planckian $f_I$ that somehow does not dominate the inflationary energy density. Models that have enhanced explicit breaking of the PQ symmetry during inflation may allow $f_a$ close to the Planck scale. Avoiding disruption of inflationary dynamics provides important limits on the parameter space.