  The redshifted 21-cm radiation from the atomic hydrogen (HI) provides an
excellent direct probe to study the evolution of HI in IGM and thus reveal the
nature of the first luminous objects, their evolution and role during Cosmic
Dawn (CD) and Epoch of Reionization (EoR), and formation and evolution of the
structures thereafter. Direct mapping of the HI density during the CD-EoR is
rather difficult with the current and future instruments due to large
foreground and other observational contamination. The first detection of this
redshifted HI signal is planned through statistical estimators. Given the
utmost importance of the detection and analysis of the redshifted 21-cm signal,
physics of CD-EoR is one of the objectives of the upcoming SKA-Low telescope.
This paper summarizes the collective effort of Indian astronomers to understand
the origin of the redshifted 21-cm signal, sources of first ionizing photons,
their propagation through the IGM, various cosmological effects on the expected
21-cm signal, various statistical measures of the signal like power spectrum,
bispectrum, etc. A collective effort on detection of the signal by developing
estimators of the statistical measures with rigorous assessment of their
expected uncertainties, various challenges like that of the large foreground
emission and calibration issues are also discussed. Various versions of the
detection methods discussed here have also been used in practice with the GMRT
with successful assessment of the foreground contamination and upper limits on
the matter density in EoR and post-EoR. The collective efforts compiled here
has been a large part of the global effort to prepare proper observational
technique and analysis procedure for the first light of the CD-EoR through the
SKA-Low.
