Neutral hydrogen at high redshift: probing structure formation

. Large inhomogeneities in neutral hydrogen in the universe can be detected at redshifts z ≤ 10 using the redshifted 21cm line emission from atomic hydrogen. This paper reviews the expected evolution of neutral hydrogen and presents estimates for future surveys of HI at z ≈ 3. We also discuss the possibility of detecting neutral hydrogen at higher redshifts.