HI 21cm absorption beyond the epoch of re-ionization

We explore the possibility of detecting HI 21cm absorption by the neutral intergalactic medium (IGM) toward very high redshift radio sources. The epoch considered is between the time when the first ionizing sources form and when the bulk of the neutral IGM becomes ionized. Due to the extreme Ly-alpha opacities of the neutral IGM, objects within this 'gray age' can only be observed at wavelengths longer than about 1micron. We use the latest simulations of the evolution of the IGM in the context of LambdaCDM structure formation models constrained by observations of the highest redshift QSOs to predict the optical depth as a function of frequency of the neutral IGM due to the HI 21cm line. We then simulate radio spectra assuming observational parameters for future large area radio telescopes. These spectra show that HI 21cm absorption studies could be a powerful probe of the rich structure of the neutral IGM prior to the epoch of reionization, including 1% absorption by the mean neutral IGM, plus deeper, narrow lines (5% and a few km/s). Most of the variations in transmissivity are due to the mild density inhomogeneities with typical values of the cosmic overdensity of about 10, precisely the structures that at later times give rise to the Ly-alpha forest. We also consider sensitivity limits and the evolution of radio source populations, and conclude that it is reasonable to hypothesize the existence of an adequate number of high-z radio sources against which such absorption studies could be performed.