Hydrogen spin oscillations in a background of axions and the 21-cm brightness temperature

TL;DR

This paper proposes that axion-induced spin oscillations in hydrogen could explain the unexpectedly strong 21-cm absorption signal observed by EDGES, linking axion physics with cosmological observations.

Contribution

It introduces a novel axion-HI interaction model using the Jaynes-Cummings analogy, explaining the 21-cm signal anomaly through coherent axion-induced spin flips.

Findings

Axion interactions can lower hydrogen spin temperature.

Coherent axion background from cosmic string decay can produce the observed effect.

Spin flip frequency depends inversely on axion decay constant f_a.

Abstract

The 21-cm line signal arising from the hyperfine interaction in hydrogen has an important role in cosmology and provides a unique method for probing of the universe prior to the star formation era. We propose that the spin flip of Hydrogen by the coherent emission/absorption of axions causes a lowering of their spin temperature and can explain the stronger than expected absorption of 21-cm light reported by the EDGES collaboration. We find the analogy of axion interaction with the two level HI with the Jaynes-Cummings model of a two level atom in a cavity and we derive the spin flip frequency in this formalism and show that the coherent oscillations frequency $\Omega \propto 1/f_a$ in contrast with the incoherent transitions between the HI hyperfine levels where the transition rates $\propto 1/f_a^2$. The axion emission and absorption rates are equal but the spin temperature is still lowered due to different selection rules for the spin flip transitions compared to the photon process. We show that the axion process goes in the right direction for explaining the EDGES observation. For this mechanism to work we require a coherent field of relativistic axions with energy $E_\nu$ peaked at the 21-cm spin-flip energy. Such a coherent background of relativistic axions can arise from the decay of cosmic strings if the decay takes place in the electroweak era.