arXiv2026
The Dark Ages 21cm signal, observed at frequencies below 50 MHz, can serve as a powerful probe of cosmology, as the standard cosmological model predicts a well-defined 21cm spectral shape, while several non-standard scenarios produce distinctive spectral features. In this work, we present a Bayesian-evidence-based assessment of the detectability of representative Dark Ages 21cm signals and of the ability to discriminate among their spectral templates. We compare multiple cosmological signal templates within a common analysis framework, adopting physically motivated foreground models, optimistic error levels, and several observing strategies. This framework allows us to quantify how frequency coverage and sampling affect both the detectability and the ability to distinguish among different spectral shapes. Using Bayesian model comparison, we show that observations covering 1-50MHz provide evidence for a non-zero 21cm signal for all the models considered in this work. In particular, without observations below 3MHz, the standard cosmological signal cannot be detected even after 10,000h of integration, because the free-free absorption component is insufficiently constrained. Regarding template discrimination, the $Λ$CDM signal can be distinguished from other models except models with similar spectral shapes and an excess radio background model with a wide band observation. Furthermore, even with observations measured at 5 MHz intervals over the frequency range 1-50 MHz, the 21cm signal can be identified if the errors are sufficiently small. This indicates that the intrinsic 21cm spectral shape can be captured without foreground degeneracy even with a limited number of frequency channels. These results quantify, within the idealized assumptions adopted in this work, how much of the intrinsic Dark Ages 21cm spectral information can be retained under limited frequency sampling.