Astrophysical uncertainties challenge 21-cm forecasts: A primordial black hole case study

The 21-cm signal is a powerful probe of the early Universe's thermal history and could provide a unique avenue for constraining exotic physics. Previous studies have forecasted stringent constraints on energy injections from exotic sources that heat, excite, and ionize the background gas and thereby modify the 21-cm signal. In this work, we quantify the substantial impact that astrophysical uncertainties have on the projected sensitivity to exotic energy injection. In particular, there are significant uncertainties in the minimum star-forming dark matter halo mass, the Lyman-$\alpha$ emission, and the X-ray emission, whose values characterize the fiducial astrophysical model when projecting bounds. As a case study, we investigate the energy injection of accreting primordial black holes of mass $\sim 1~M_\odot-10^3~M_\odot$, also taking into account uncertainties in the accretion model. We show that, depending on the chosen fiducial model and accretion uncertainties, the sensitivity of future 21-cm data could constrain the abundance of primordial black holes to be either slightly stronger, or significantly weaker, than current limits from the Cosmic Microwave Background.