Isocurvature bounds on axion-like particle dark matter in the post-inflationary scenario

TL;DR

This paper investigates how isocurvature fluctuations constrain axion-like particle dark matter in the post-inflationary universe, using CMB data to exclude certain mass ranges and discussing future observational prospects.

Contribution

It provides new bounds on post-inflationary ALP dark matter masses based on isocurvature fluctuation constraints from Planck data.

Findings

Planck data constrains ALP masses to be above 10^{-20} to 10^{-16} eV.

Isocurvature fluctuations are significant for scales near the horizon at ALP oscillation onset.

Future experiments could improve mass constraints by 1-2 orders of magnitude.

Abstract

We assume that dark matter is comprised of axion-like particles (ALPs) generated by the realignment mechanism in the post-inflationary scenario. This leads to isocurvature fluctuations with an amplitude of order one for scales comparable to the horizon at the time when the ALP field starts oscillating. The power spectrum of these fluctuations is flat for small wave numbers, extending to scales relevant for cosmological observables. Denoting the relative isocurvature amplitude at $k_*$ = 0.05 Mpc$^{-1}$ by $f_{\rm iso}$, Planck observations of the cosmic microwave background (CMB) yield $f_{\rm iso}$ < 0.31 at the 2$\sigma$-level. This excludes the hypothesis of post-inflationary ALP dark matter with masses $m_{a}$ < 10$^{-20}-$10$^{-16}$ eV, where the range is due to details of the ALP mass-temperature dependence. Future CMB stage IV and 21-cm intensity mapping experiments may improve these limits by 1$-$2 orders of magnitude in $m_{a}$.