Influences of accreting primordial black holes on the global 21 cm signal in the dark ages

Abstract

Baryonic matter can be accreted on to primordial back holes (PBHs) formed in the early Universe. The radiation from accreting PBHs is capable of altering the evolution of the intergalactic medium (IGM), leaving marks on the global 21 cm signal in the dark ages. For accreting PBHs with mass $M_{\rm PBH}=10^{3}(10^{4})~M_{\odot}$ and mass fraction $f_{\rm PBH}=10^{-1}(10^{-3})$, the brightness temperature deviation $\Delta \delta T_{b}$ reaches $\sim 18~(26)~\rm mK$ at redshift $z\sim 90$ ($\nu \sim 16~\rm MHz$), and the gradient of the brightness temperature $d\delta T_{b}/d\nu$ reaches $ \sim 0.8~(0.5)~\rm mK~MHz^{-1}$ at frequency $\nu\sim 28~\rm MHz$ ($z\sim 50$). For larger PBHs with higher mass fraction, the brightness temperature deviation is larger in the redshift range $z\sim 30-300$ ($\nu\sim 5-46~\rm MHz$), and the gradient is lower at the frequency range $\nu \sim 20-60~\rm MHz$ ($z\sim 23-70$). It is impossible to detect these low frequency radio signals from the Earth due to the influence of the Earth's ionosphere. However, after taking care of the essential factors properly, e.g. the foreground and interference, future radio telescope in lunar orbit or on the farside surface of the Moon has a chance of detecting the global 21 cm signals impacted by accreting PBHs and distinguishing them from the standard model.