Probing ultra-light axions with the 21-cm Signal during Cosmic Dawn

Understand

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.