Radio signatures of HI at high redshift: Mapping the end of the "dark ages"

The emission of 21 cm radiation from a neutral intergalactic medium (IGM) at high redshift is discussed in connection with the thermal and ionization history of the universe. The physical mechanisms that make such radiation detectable against the cosmic microwave background include Ly alpha coupling of the hydrogen spin temperature to the kinetic temperature of the gas and preheating of the IGM by the first generation of stars and quasars. Three different signatures are investigated in detail: (a) the fluctuations in the redshifted 21 cm emission induced by the gas density inhomogeneities that develop at early times in cold dark matter (CDM) dominated cosmologies, (b) the sharp absorption feature in the radio sky due to the rapid rise of the Ly alpha continuum background that marks the birth of the first UV sources in the universe, and (c) the 21 cm emission and absorption shells that are generated on several Mpc scales around the first bright quasars. Future radio observations with projected facilities like the Giant Meterwave Radio Telescope and the Square Kilometer Array may shed light on the power spectrum of density fluctuations at z > 5 and map the end of the "dark ages," i.e., the transition from the postrecombination universe to one populated with radiation sources.