  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.
