Axion Dark Matter Detection with CMB Polarization
  We point out two ways to search for low-mass axion dark matter using cosmic
microwave background (CMB) polarization measurements. These appear, in
particular, to be some of the most promising ways to directly detect fuzzy dark
matter. Axion dark matter causes rotation of the polarization of light passing
through it. This gives rise to two novel phenomena in the CMB. First, the
late-time oscillations of the axion field today cause the CMB polarization to
oscillate in phase across the entire sky. Second, the early-time oscillations
of the axion field wash out the polarization produced at last-scattering,
reducing the polarized fraction (TE and EE power spectra) compared to the
standard prediction. Since the axion field is oscillating, the common (static)
`cosmic birefringence' search is not appropriate for axion dark matter. These
two phenomena can be used to search for axion dark matter at the lighter end of
the mass range, with a reach several orders of magnitude beyond current
constraints. We set a limit from the washout effect using existing Planck
results, and find significant future discovery potential for CMB detectors
searching in particular for the oscillating effect.
