Probing light relics through cosmic dawn

We explore the prospects of upcoming 21-cm surveys of cosmic dawn (12 less than or similar to z less than or similar to 30) to provide cosmological information on top of upcoming cosmic microwave background (CMB) and large-scale structure surveys, such as CMB-S4, Simons Observatory (SO) and DESI. We focus on the effective number of relativistic species N-eff which is a promising observable for probing beyond the Standard Model theories. We show including upcoming 21-cm surveys such as the Square Kilometre Array (SKA) can allow probing a wide range of models for light particles at 2 sigma level achieving 2 sigma(N-eff) 1/4 0.034 with CMBS4, for example. Taking into account the degeneracy between N-eff and primordial helium fraction Y-p, one can achieve improvements in sensitivities to cosmological parameters, in particular, by more than a factor of 2 for Neff and dark matter fractional energy density omega(c).