The phase space structure of our dark matter halo on small scales is an uncertain
and much debated topic. It has become clear over recent years that this local
structure could hold not just information about the merger and accretion history of
the Milky Way but also about the nature of dark matter particles and the mechanisms
involved in their production. In this way we can perhaps frame direct detection’s
dependence on astrophysics, not as merely an uncertainty to be marginalised over,
but instead as a central motivating question, which can only be resolved by
detecting dark matter. One can imagine a post-discovery era in which direct
detection experiments can pivot into a mode of astronomy, in the manner of neutrinos
or gravitational waves. I want to describe how exciting these prospects are for the
case of axion dark matter. Haloscope experiments designed to detect the conversion
of axions into photons would observe a signal whose shape directly provides the
speed distribution of dark matter. The excellent spectral resolution achievable in a
resonant cavity experiment for example would allow a very fine measurement of the
local halo, a feat unheard of in the context of WIMP dark matter. Furthermore, we
have some new ideas for how one would extend conventional axion haloscope designs,
using resonant cavities or dielectric disk setups, to fully directional axion
searches. This would allow a measurement of the 3-dimensional velocity distribution
and the possible discovery of a greater range of local dark matter substructure.
