  The late-time growth of large scale structures (LSS) is imprinted in the CMBR
anisotropy through the Integrated Sachs Wolfe (ISW) effect. This is perceived
to be a very important observational probe of dark energy. Future observations
of redshifted 21-cm radiation from the cosmological neutral hydrogen (HI)
distribution hold the potential of probing the LSS over a large redshift range.
We have investigated the possibility of detecting the ISW through
cross-correlations between the CMBR anisotropies and redshifted 21-cm
observations. Assuming that the HI traces the dark matter, we find that the
ISW-HI cross-correlation angular power spectrum at an angular multipole l is
proportional to the dark matter power spectrum evaluated at the comoving wave
number l/r, where r is the comoving distance to the redshift from which the HI
signal originated. The amplitude of the cross-correlation signal depends on
parameters related to the HI distribution and the growth of cosmological
perturbations. However the cross-correlation is extremely weak as compared to
the CMBR anisotropies and the predicted HI signal. As a consequence the
cross-correlation signal is smaller than the cosmic variance, and a
statistically significant detection is not very likely.
