Probing the high redshift universe with hyperfine transitions

Hyperfine transition of neutral Hydrogen with a rest frame frequency of 21cm has been used as an effective probe of galaxies at low redshifts. It allows us to probe aspects that are not accessible through observations in the optical or other wavebands. This transition has also been discussed as a probe of the epoch of reionization and the post-reionization universe. I will discuss issues related to the detection of redshifted 21cm radiation from the post-reionization universe. In particular, I will highlight the possibility of detecting emission from the large scale structure near redshift  z ~ 1.3 in the 610 MHz band of the GMRT.
The singly ionized Helium-3 isotope has a Hyperfine transition with a much shorted rest frame wavelength. I will argue that this too is a potentially useful probe of the high redshift universe and outline some possible applications.