Future cosmological sensitivity for hot dark matter axions
  We study the potential of a future, large-volume photometric survey to
constrain the axion mass $m_a$ in the hot dark matter limit. Future surveys
such as Euclid will have significantly more constraining power than current
observations for hot dark matter. Nonetheless, the lowest accessible axion
masses are limited by the fact that axions lighter than $\sim 0.15$ eV decouple
before the QCD epoch, assumed here to occur at a temperature $T_{\rm QCD} \sim
170$ MeV; this leaves an axion population of such low density that its
late-time cosmological impact is negligible. For larger axion masses, $m_a
\gtrsim 0.15$ eV, where axions remain in equilibrium until after the QCD phase
transition, we find that a Euclid-like survey combined with Planck CMB data can
detect $m_a$ at very high significance. Our conclusions are robust against
assumptions about prior knowledge of the neutrino mass. Given that the proposed
IAXO solar axion search is sensitive to $m_a\lesssim 0.2$ eV, the axion mass
range probed by cosmology is nicely complementary.
