The Orsay 21cm Cosmology 2019 workshop aim is to provide a theoretical and observational overview of cosmology at 21 cm. This neutral hydrogen line is the probe of the Epoch of Reionisation (EoR) at high redshift, and to observe the birth of the first stars. It can also be used to observe the universe at lower redshifts, in particular through intensity mapping to constrain cosmological models (dark energy, modified theories of gravity, inflation…).
The scientific perspectives offered by SKA for the study of reionization, as well as the results obtained by pathfinder experiments (LOFAR, MWA, HERA, MeerKat…) in this field will be discussed. Several projects are underway to explore the method of intensity mapping at lower redshift (z ~ 1-3) for cosmology. The scientific results and prospects of these projects, including TIANLAI (China/US/France), CHIME (Canada/US), HIRAX (South Africa) or BINGO (UK/Brazil) will also be presented.