Robustness of cosmological axion mass limits

TL;DR

This paper investigates how allowing for a flexible primordial power spectrum affects cosmological bounds on the thermal axion mass, finding that constraints are only slightly weakened and that a mild preference for around 1 eV axion mass can emerge.

Contribution

It introduces a model with a piecewise cubic Hermite interpolating polynomial for the primordial power spectrum to assess its impact on axion mass limits.

Findings

Constraints on axion mass are only slightly degraded with a flexible power spectrum.

Adding galaxy count data relaxes axion mass bounds and mildly favors ~1 eV axion mass.

Considering neutrino masses as free parameters weakens the preference for a specific axion mass.

Abstract

We present cosmological bounds on the thermal axion mass in an extended cosmological scenario in which the primordial power spectrum of scalar perturbations differs from the usual power-law shape predicted by the simplest inflationary models. The power spectrum is instead modeled by means of a "piecewise cubic Hermite interpolating polynomial" (PCHIP). When using Cosmic Microwave Background measurements combined with other cosmological data sets, the thermal axion mass constraints are degraded only slightly. The addition of the measurements of $\sigma_8$ and $\Omega_m$ from the 2013 Planck cluster catalogue on galaxy number counts relaxes the bounds on the thermal axion mass, mildly favouring a $\sim 1$~eV axion mass, regardless of the model adopted for the primordial power spectrum.However, in general, such a preference disappears if the sum of the three active neutrino masses is also considered as a free parameter in our numerical analyses, due to the strong correlation between the masses of these two hot thermal relics.