Ultralight axion or axion-like particle dark matter and 21-cm absorption signals in new physics

TL;DR

The paper addresses the 21-cm absorption anomaly during the cosmic dawn and the apparent excess in radiation density by proposing ultralight axion-like dark matter with $m_a\sim10^{-22}$ eV. It develops a two-channel mechanism in which ALP DM cools baryons via coherent scattering while resonant ALP–photon conversions in primordial magnetic fields heat CMB photons, with heating and cooling potentially reaching thermal equilibrium between $z=200$ and $z=20$. This balance can mitigate the inferred $\Delta N_{\rm eff}$ while producing distinctive 21-cm signatures, contingent on the ALP-photon coupling, magnetic fields, and PBH scenarios. The framework yields testable predictions for forthcoming experiments such as IAXO, ADMX, PIXIE, PRISM, DAPPER, and FARSIDE, linking 21-cm cosmology with direct axion searches and CMB constraints to probe ULDM/ALP physics in the early universe.

Abstract

A hypothetical particle known as the axion holds the potential to resolve both the cosmic dark matter riddle and particle physics' long-standing, strong CP dilemma. An unusually strong 21-cm absorption feature associated with the initial star formation era, i.e., the dark ages, may be due to ultralight axion dark matter ($\sim$10$^{-22}$ eV) at this time. The radio wave observation's 21-cm absorption signal can be explained as either anomalous baryon cooling or anomalous cosmic microwave background photon heating. Shortly after the axions or axion-like particles (ALPs) thermalize among themselves and form a Bose--Einstein condensate, the cold dark matter ALPs make thermal contact with baryons, cooling them. ALPs are thought to be the source of some new evidence for dark matter, as the baryon temperature at cosmic dawn was lower than predicted based on presumptions. The detection of baryon acoustic oscillations is found to be consistent with baryon cooling by dark matter ALPs. Simultaneously, under the influence of the primordial black hole and/or intergalactic magnetic fields, the dark radiation composed of ALPs can resonantly transform into photons, significantly heating up the radiation in the frequency range relevant to the 21-cm tests. When examining the 21-cm cosmology at redshifts $z$ between 200 and 20, we see that, when taking into account both heating and cooling options at the same time, heating eliminated the theoretical excess number of neutrino species, $ΔN_{\rm eff}$, from the cooling effect.