Exploring the Co-SIMP dark matter model using the 21-cm signal from dark ages

Abstract: The redshifted 21-cm signal from the dark ages offers a powerful probe of cosmological models and the underlying dark matter microphysics. We investigate deviations from the standard $\Lambda$CDM prediction, an absorption trough of approximately $-40.6\,\mathrm{mK}$ at redshift $z \simeq 85.6$, in the context of co-SIMP dark matter. The strength of co-SIMP interactions, quantified by the parameter $C_{\rm int}$, enhances the absorption depth and shifts the trough to higher redshifts. For example, a model with $C_{\rm int}=1.0$ produces a minimum brightness temperature of $-50.6\,\mathrm{mK}$ at $z \simeq 86.2$. The 21-cm power spectrum increases with $C_{\rm int}$ in addition to the global signal. We assess the detectability of these signatures using signal-to-noise ratio (SNR) and Fisher matrix forecasts. The maximum SNR reaches $\sim 15.7$ for $C_{\rm int}=1.0$ for the global signal. Fisher forecast for $1,000$ hours of integration time shows that this model can be distinguished from a null-signal at $4.3\sigma$ and from the $\Lambda$CDM model case at $1.6 \sigma$, with order-of-magnitude improvements for 100,000 hours of integration. For the 21-cm power spectrum, our forecasts reveal complementary trends; with a modest setup (collecting area of $5\,\mathrm{km}^2$ and 1,000 hours of integration time), the $C_{\rm int}=1.0$ model can be detected at $4.63\sigma$ and differentiated from the standard scenario at $1.78 \sigma$. These findings highlight the potential of the 21-cm cosmology to probe the properties of dark matter and demonstrate that upcoming dark ages experiments, particularly space-based and lunar observations, can offer a promising avenue to test co-SIMP models.