Cosmic Axion Bose-Einstein Condensation
  QCD axions are a well-motivated candidate for cold dark matter. Cold axions
are produced in the early universe by vacuum realignment, axion string decay
and axion domain wall decay. We show that cold axions thermalize via their
gravitational self-interactions, and form a Bose-Einstein condensate. As a
result, axion dark matter behaves differently from the other proposed forms of
dark matter. The differences are observable.
