Ultralight Axion or Axion-Like Particle Dark Matter and 21-cm Absorption Signals in New Physics

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, \(\Delta N_{\text{eff}}\), from the cooling effect.