Axion absorption and the spin temperature of primordial hydrogen
  An absorption dip in the spectrum of the cosmic microwave background observed
by the EDGES experiment suggests an unexplained reduction of the hydrogen spin
temperature at cosmic redshift z ~ 17. The mass of dark-matter axions could
correspond to the hyperfine splitting of 5.9 micro-eV, between the triplet (H1)
and singlet (H0) state. We calculate the rate for a+ H0 <-> H1 in two ways, and
find that it is orders of magnitude smaller than the CMB-mediated transition
rate, so irrelevant. As a result, this process cannot be used to rule in or out
dark matter axions of mass = hyperfine splitting. The axion rate nonetheless
has interesting features, for example, on balance it heats the spin
temperature, and the axion couplings to protons and electrons contribute on
equal footing.
