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

TL;DR

This paper explores how future 21-cm signal measurements during cosmic dawn can detect ultra-light axions by analyzing their impact on star formation and velocity acoustic oscillations, potentially surpassing current constraints.

Contribution

It introduces a novel method to probe ultra-light axions using 21-cm fluctuations and VAOs, extending sensitivity to lower axion masses than previous constraints.

Findings

HERA could detect ULAs with masses up to 10^{-18} eV.

ULAs damp small-scale power, reducing star formation and VAO amplitude.

The method improves constraints on ULAs compared to existing limits.

Abstract

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.