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Published online by Cambridge University Press: 07 February 2017
After recombination in the early Universe, gas will pass through a neutral phase on its way to condensation into galaxies and stars. Radio emission in the reshifted 21 cm line of neutral hydrogen can be used to detect condensations in the early Universe. Sunyaev and Zeldovich (1972, 1975) have suggested that primordial protoclusters may be predominantly neutral at z = 3 to 10 and may have neutral hydrogen masses ~ 1014 M⊙. Two observational approaches have been made to the search for this emission.