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Published online by Cambridge University Press: 26 May 2016
Future radio surveys using facilities like the Giant Metre-wave Radio Telescope or a Square Kilometre Array may detect 21-cm radiation from the Intergalactic Medium (IGM) in either emission or absorption against the Cosmic Microwave Background (CMB) at high redshift. Prior to the reionization of the IGM, the first radiation sources would reveal their presence through their effect on the IGM. The signatures include a sharp absorption feature in the radio sky against the CMB resulting from a rapid rise of a Lyman-alpha continuum background shortly after the birth of the first UV sources in the universe, and 21-cm emission and absorption shells created on the several Mpc scale by the first bright QSOs. The detection of these signatures would map the end of the ‘dark ages', the period of transition from the post-recombination universe to one populated by radiation sources.