The 21-cm Signal from the Cosmological Epoch of Recombination

The redshifted 21-cm emission by neutral hydrogen offers a unique tool for mapping structure formation in the early universe in three dimensions. Here we provide the first detailed calculation of the 21-cm emission signal during and after the epoch of hydrogen recombination in the redshift range of z ~ 500-1,100, corresponding to observed wavelengths of 100-230 meters. The 21-cm line deviates from thermal equilibrium with the cosmic microwave background (CMB) due to the excess Ly-alpha radiation from hydrogen and helium recombinations. The resulting 21-cm signal reaches a brightness temperature of a milli-Kelvin, orders of magnitude larger than previously estimated. Its detection by a future lunar or space-based observatory could improve dramatically the statistical constraints on the cosmological initial conditions compared to existing two-dimensional maps of the CMB anisotropies.