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 $Λ$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 $f_χ^2 \langle σv \rangle / M_χ$ 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 $f_χ^2\langleσv\rangle/M_χ\lesssim 10^{-27}~\rm cm^3\,s^{-1}\,\rm GeV^{-1}$. This constraint is stronger than the bounds derived from Planck (2018) data for mass $\lesssim 10~\rm GeV$.

Figures (2)

• Figure 1: Evolution of (a) gas temperature and (b) global 21-cm signal in the presence of DM annihilation after recombination. In panel (a), the black dashed and dotted lines represent the evolution of CMB and gas temperatures in the $\Lambda$CDM framework, respectively. The black solid line shows the IGM temperature evolution in the presence of X-ray heating during the cosmic dawn. The blue, red, and green solid lines correspond to DM annihilation with $M_{\chi} = 7.5$ GeV, $2$ GeV, and $0.75$ GeV, respectively, at a fixed $\langle\sigma v \rangle = 10^{-26}\,\mathrm{cm^3\,s^{-1}}$. The corresponding impact on the $T_{21}$ signal is shown in panel (b) with the same color coding, except for the black dashed line, which illustrates the presence of an excess radio background [see Eq. \ref{['eq:radio_flux']}] without DM annihilation.
• Figure 2: Represent upper bounds on $f_\chi^2\langle\sigma v\rangle/M_\chi$. The black dotted and dash-dotted lines represent the upper limit on the annihilation cross-section, via $\chi\chi\to\gamma\gamma$ and $\chi\chi\to e^{-}e^{+}$ respectively, above which the dark ages signal vanishes. Whereas, the red dotted and dash-dotted lines set a lower limit of $\gtrsim 6\times 10^{-28}~\rm cm^3\,s^{-1}\,GeV^{-1}$ for $\chi\chi\to\gamma\gamma$ and $\gtrsim 2.79\times 10^{-28}~\rm cm^3\,s^{-1}\,GeV^{-1}$ for $\chi\chi\to e^{-}e^{+}$ channel, respectively, required to mildly alter the standard $T_{21}$ amplitude at $z\sim 89$. The grey shaded region depicts observational uncertainty of $\Delta\, 5\,\rm mK$ in future lunar-based experiments Burns:2020gfh. The grey cross-hatched region shows theoretical uncertainties due to cosmological parameter variations. The blue solid line represents an upper limit from Planck (2018), which is $\lesssim 4.7\times 10^{-27}~\rm cm^3\,s^{-1}\,GeV^{-1}$ for $M_\chi>5~\rm GeV$Planck:2018vyg. The black dashed line presents the thermal relic cross-section of $\langle\sigma v\rangle \simeq 3\times 10^{-26}~\rm cm^3\,s^{-1}$ taken from Ref. Planck:2018vygShort:2019twc.