Axion Dark Matter and Cosmological Parameters
  We observe that photon cooling after big bang nucleosynthesis (BBN) but
before recombination can remove the conflict between the observed and
theoretically predicted value of the primordial abundance of $^7$Li. Such
cooling is ordinarily difficult to achieve. However, the recent realization
that dark matter axions form a Bose-Einstein condensate (BEC) provides a
possible mechanism, because the much colder axions may reach thermal contact
with the photons. This proposal predicts a high effective number of neutrinos
as measured by the cosmic microwave anisotropy spectrum.
