How light can ALP dark matter be?

TL;DR

This paper investigates the minimum mass of axion-like particles (ALPs) as dark matter, considering different cosmological scenarios and observational constraints, and finds bounds ranging from 10^{-17} eV to 10^{-7} eV.

Contribution

It provides updated bounds on ALP dark matter mass by incorporating cosmic string radiation, isocurvature perturbations, and CMB tensor mode constraints in various inflationary scenarios.

Findings

ALP mass bound is approximately 10^{-17} eV for most cases.

Stronger bounds up to 5×10^{-7} eV are derived from CMB tensor mode non-observation.

Post-inflationary scenario allows for significantly lighter ALPs under certain conditions.

Abstract

We assume axion-like particles (ALPs) to provide the full dark matter abundance and derive various lower bounds on the ALP mass. We contrast the post- and pre-inflationary symmetry breaking cases and present allowed regions in the plane of ALP mass and energy scale of inflation. For the post-inflationary case, we revisit bounds from isocurvature perturbations taking into account that, as suggested by simulations, axion radiation by cosmic strings during the scaling regime provides the dominant production mechanism of dark matter, obtaining significantly weaker limits than previously. Combining isocurvature, with constraints from black hole superradiance and free streaming, we find that the bound $m_a \gtrsim 10^{-17}$ eV applies for most cases considered here. It can be potentially relaxed to $\sim 6\times 10^{-19}$ eV only in the post-inflationary case with a strongly temperature-dependent axion mass, subject to uncertainties on the axion emission spectrum. Significantly stronger bounds are obtained in the post-inflationary scenario from the non-observation of CMB tensor modes, which can be as strong as $m_a > 5\times 10^{-7}$ eV for small reheating efficiencies, $\epsilon \lesssim 5\times 10^{-4}$.