Accurate effective fluid approximation for ultralight axions
  Ultralight axions are theoretically interesting and phenomenologically rich
dark sector candidates, but they are difficult to track across cosmological
timescales because of their fast oscillations. We resolve this problem by
developing a novel method to evolve them efficiently and accurately. We first
construct an exact effective fluid which at late times matches the axion but
which evolves in a simple way. We then approximate this evolution with a
carefully chosen equation of state and sound speed. With our scheme we find
that we can obtain subpercent accuracy for the linear theory suppression of
axion density fluctuations relative to that of cold dark matter without
tracking even a single complete oscillation of the axion field. We use our
technique to test other approximation schemes and to provide a fitting formula
for the transfer function for the matter power spectrum in linear theory in
axion models. Implementing our approach in existing cosmological axion codes is
straightforward and will help unleash the potential of high-precision
next-generation experiments.
