Cosmological bounds on dark matter annihilation using dark ages 21-cm signal

Abstract: We investigate the impact of dark matter (DM) annihilation on the global 21-cm signal during the dark ages and cosmic dawn eras. The 21-cm line provides a complementary probe for studying the nature of dark matter beyond standard cosmological observables. In the standard $\Lambda$CDM framework, the expected absorption amplitude of the dark ages global 21-cm signal is approximately $-42\, \rm mK$. However, energy injection from DM annihilation can significantly heat and ionize the intergalactic medium, potentially altering or even erasing this absorption feature. We evaluate the thermal and ionization history of the gas to derive an upper bound on $f_\chi^2 \langle \sigma v \rangle / M_\chi$ using the dark ages signal, which is free from astrophysical uncertainties. After incorporating observational and theoretical uncertainties arising from future lunar-based experiments and variations in cosmological parameters, respectively -- we obtain a conservative upper limit of $f_\chi^2\langle\sigma v\rangle/M_\chi \lesssim 10^{-28}~\rm cm^3\,s^{-1}\,\rm GeV^{-1}$. This constraint is nearly an order of magnitude stronger than the bounds derived from Planck 2018 data.