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We use numerical hydrodynamical simulations of early structure formation in a LCDM universe to investigate the spin temperature and 21cm brightness of the diffuse intergalactic medium (IGM) prior to the epoch of cosmic reionization, at z<20. In the absence of any radiative heating, collisions between neutral hydrogen atoms can efficiently decouple the spin temperature from the CMB only in dense minihalos and filaments. Shock heated gas shines in emission, surrounded by cooler gas visible in absorption. In the case of a warm, mostly neutral IGM, produced here by X-ray emission from an early miniquasar, the 21cm signal is strongly enhanced. Even slightly overdense filaments now shine in emission against the CMB, possibly allowing future radio arrays to probe the distribution of neutral hydrogen before reionization.
Kuhlen et al. (Tue,) studied this question.