Relaxing Isocurvature Bounds on String Axion Dark Matter

TL;DR

This paper demonstrates that thermal inflation can relax isocurvature constraints on string axion dark matter, allowing for viable models even with high inflation scales and exploring implications for X-ray line signals.

Contribution

It introduces a mechanism where thermal inflation dilutes axion isocurvature perturbations without inducing secondary oscillations, expanding the parameter space for axion dark matter.

Findings

Thermal inflation effectively suppresses axion isocurvature perturbations.

No secondary coherent oscillations are induced at the end of thermal inflation.

Viability of 7 keV axion dark matter explaining the 3.5 keV X-ray line is shown.

Abstract

If inflation scale is high, light scalars acquire large quantum fluctuations during inflation.If sufficiently long-lived, they will give rise to CDM isocurvature perturbations, which are highly constrained by the Planck data. Focusing on string axions as such light scalars, we show that thermal inflation can provide a sufficiently large entropy production to dilute the CDM isocurvature perturbations. Importantly, efficient dilution is possible for the string axions, because effectively no secondary coherent oscillations are induced at the end of thermal inflation, in contrast to the moduli fields. We also study the viability of the axion dark matter with mass of about 7 keV as the origin of the 3.5 keV X-ray line excess, in the presence of large entropy production.