Probing ultra-light axions with the 21-cm Signal during Cosmic Dawn
  Ultra-light axions (ULAs) are a promising and intriguing set of dark-matter
candidates. We study the prospects to use forthcoming measurements of 21-cm
fluctuations from cosmic dawn to probe ULAs. We focus in particular on the
velocity acoustic oscillations (VAOs) in the large-scale 21-cm power spectrum,
features imprinted by the long-wavelength ($k\sim0.1\,{\rm Mpc}^{-1}$)
modulation, by dark-matter--baryon relative velocities, of the small-scale
($k\sim 10-10^3\, {\rm Mpc}^{-1}$) power required to produce the stars that
heat the neutral hydrogen. Damping of small-scale power by ULAs reduces the
star-formation rate at cosmic dawn which then leads to a reduced VAO amplitude.
Accounting for different assumptions for feedback and foregrounds, experiments
like HERA may be sensitive to ULAs with masses up to $m_{\alpha}\approx
10^{-18}\text{eV}$, two decades of mass higher than current constraints.
