Understand
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.