Probing light relics through cosmic dawn

We explore the prospects of upcoming 21-cm surveys of cosmic dawn ($12\lesssim \!z\lesssim\!30$) 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 $N_{\rm eff}$ 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 $2\sigma(N_{\rm eff})=0.034$ with CMB-S4, for example. Taking into account the degeneracy between $N_{\rm eff}$ 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 $N_{\rm eff}$ and dark matter fractional energy density $\omega_c$.