Abstract
The redshifted 21-cm emission by neutral hydrogen offers aunique tool for mapping structure formation in the early universe inthree dimensions. Here we provide the first detailed calculation ofthe 21-cm emission signal during and after the epoch of hydrogenrecombination in the redshift range of z ∼ 500–1,100,corresponding to observed wavelengths of 100–230 meters. The 21-cmline deviates from thermal equilibrium with the cosmic microwavebackground (CMB) due to the excess Lyα radiation from hydrogenand helium recombinations. The resulting 21-cm signal reaches a brightness temperatureof a milli-Kelvin, orders of magnitude larger than previously estimated. Itsdetection by a future lunar or space-based observatory could improvedramatically the statistical constraints on the cosmological initialconditions compared to existing two-dimensional maps ofthe CMB anisotropies.
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