Prospects for detecting neutral hydrogen using 21cm radiation from large   scale structure at high redshifts
  We estimate the signal from large scale structure at high redshifts in redshifted 21cm line. We focus on z ~ 3 and the LCDM cosmology. We assume that neutral hydrogen is to be found only in galaxies, and normalise the total content to the density parameter of neutral hydrogen in damped Lyman alpha absorption systems (DLAS). We find that the rms fluctuations due to the large scale distribution of galaxies is very small and cannot be observed at angular scales probed by present day telescopes. We have used the sensitivity of the Giant meter-wave Radio Telescope (GMRT) for comparison. We find that observations stretching over 1000hours will be required for a 3 sigma detection of such objects. 
J.S.Bagla and Martin White