Far from equilibrium dynamics of Bose-Einstein condensation for Axion
  Dark Matter
  Axions and similar very weakly interacting particles are increasingly
compelling candidates for the cold dark matter of the universe. Having very low
mass and being produced non-thermally in the early Universe, axions feature
extremely high occupation numbers. It has been suggested that this leads to the
formation of a Bose-Einstein condensate with potentially significant impact on
observation and direct detection experiments. In this note we aim to clarify
that if Bose-Einstein condensation occurs for light and very weakly interacting
dark matter particles, it does not happen in thermal equilibrium but is
described by a far-from-equilibrium state. In particular we point out that the
dynamics is characterized by two very different timescales, such that
condensation occurs on a much shorter timescale than full thermalization.
