  The axions produced during the QCD phase transition by vacuum realignment,
string decay and domain wall decay thermalize as a result of their
gravitational self-interactions when the photon temperature is approximately
500 eV. They then form a Bose-Einstein condensate (BEC). Because the axion BEC
rethermalizes on time scales shorter than the age of the universe, it has
properties that distinguish it from other forms of cold dark matter. The
observational evidence for caustic rings of dark matter in galactic halos is
explained if the dark matter is axions, at least in part, but not if the dark
matter is entirely WIMPs or sterile neutrinos.
