Nevertheless, results from these studies have been quite successful and show that, while the IGM is highly ionized below $z\lesssim6$ (with typical $x_{HI}\lesssim10^{-5}$), a significant amount of neutral hydrogen is present above, although precisely how much remains unclear [@2001ApJ...560L...5D; @2001AJ....122.2850B; @2002AJ....123.1247F; @2003AJ....125.1649F; @2004Natur.427..815W; @2006AJ....132..117F].

The existing CMB and quasar absorption measurements are somewhat contradictory. Prior to these studies, the reionization epoch was assumed generally to be quite brief, with the transition from an IGM filled with fully neutral hydrogen to an IGM filled with highly ionized hydrogen occurring very rapidly. These results, however, open the possibility that the ionization history of the IGM may be more complicated than previously believed [@2003ApJ...595....1H; @2003ApJ...591...12C; @2003MNRAS.344..607S; @2004ApJ...604..484M].

Direct observations of the 21 cm (1420 MHz) hyperfine transition line of neutral hydrogen in the IGM during the reionization epoch would resolve the existing uncertainties and reveal the evolving properties of the IGM. The redshifted 21 cm signal should appear as a faint, diffuse background in radio frequencies below $\nu<200$ MHz for redshifts above $z>6$ (according to $\nu=1420/[1+z]$ MHz). For diffuse gas in the high-redshift ($z\approx10$) IGM, the expected unpolarized differential brightness temperature of the redshifted 21 cm line relative to the pervasive CMB is 