The Glow of Axion Quark Nugget Dark Matter: (IV) CMB Spectral and Anisotropy Signatures

TL;DR

This paper investigates how axion quark nugget dark matter could cause detectable spectral distortions in the cosmic microwave background without affecting anisotropies, providing a novel way to test this dark matter model.

Contribution

It introduces a detailed analysis of AQN-induced energy injection effects on CMB spectral distortions using a modified Boltzmann code, linking dark matter properties to observable signatures.

Findings

CMB anisotropies are unaffected by baryon-AQN annihilation.

Spectral distortions from AQNs are within the reach of future missions.

The AQN scenario shares features with other dark matter models but has unique spectral signatures.

Abstract

Axion quark nuggets (AQNs) are macroscopic dark-matter candidates, with masses of the order of a few grams to a kilogram and sub-micron radius, thought to form at the Quantum Chromo Dynamic era through axion-induced charge separation. This framework naturally links the dark and visible matter abundances ($\Omega_{\rm DM} \sim \Omega_{\rm visible}$) and provides a mechanism for generating the baryon-antibaryon asymmetry where dark matter is composed of matter AQNs and antimatter AQNs. Although behaving as cold dark matter on cosmological scales, baryons annihilate with antimatter AQNs, producing ionizing high-energy photons. The resulting energy injection may imprint spectral distortions on the cosmic microwave background (CMB) and modify the reionization history. Using a modified version of the \texttt{CLASS} Boltzmann code we compute the impact of this energy injection on the $\mu$ and $y$ spectral distortion parameters as well as on the optical depth. We find that (1) the CMB anisotropies remain essentially unaffected by baryon annihilation, and (2) the associated spectral distortion signatures lie within the sensitivity reach of proposed CMB spectral distortion missions. Finally, we discuss the similarities and differences between the AQN scenario and annihilating or decaying dark matter models.