What will anisotropies in the clustering pattern in redshifted 21 cm   maps tell us?
  The clustering pattern in high redshift HI maps is expected to be anisotropic due to two distinct reasons, the   Alcock-Paczynski effect and the peculiar velocities, both of which are sensitive to the cosmological parameters. The signal is also expected to be sensitive to the details of the HI distribution at the epoch when the radiation originated. We use simple models for the HI distribution at the epoch of reionizaation and the post-reionization era to investigate exactly what we hope to learn from future observations of the anisotropy pattern in HI maps. We find that such observations will probably tell us more about the HI distribution than about the background cosmological model. Assuming that reionization can be described by spherical, ionized bubbles all of the same size with their centers possibly being biased with respect to the dark matter, we find that the anisotropy pattern at small angles is expected to have a bump at the characteristic angular size of the individual bubbles whereas the large scale anisotropy pattern will reflect the size and the bias of the bubbles. The anisotropy also depends on the background cosmological parameters, but the dependence is much weaker. Under the assumption that the HI in the post-reionization era traces the dark matter with a possible bias, we find that changing the bias and changing the background cosmology has similar effects on the anisotropy pattern. Combining observations of the anisotropy with independent estimates of the bias, possibly from the bi-spectrum, may allow these observations to constrain cosmological parameters. 
Sk. Saiyad Ali, Somnath Bharadwaj and Biswajit Pandey (IIT, Kharagpur,   India)