Axion BEC Dark Matter
  Cold dark matter axions thermalize through gravitational self-interactions
and form a Bose-Einstein condensate when the photon temperature reaches
approximately 500 eV. Axion Bose-Einstein condensation provides an opportunity
to distinguish axions from the other dark matter candidates on the basis of
observation. The rethermalization of axions that are about to fall in a
galactic potential well causes them to acquire net overall rotation, whereas
ordinary cold dark matter falls in with an irrotational velocity field. The
inner caustics of galactic halos are different in the two cases.
