Current status and future of cosmology with 21cm Intensity Mapping
  21cm Intensity Mapping (IM) has been proposed about 15 years ago as a cost
effective method to carry out cosmological surveys and to map the 3D
distribution of matter in the universe, over a large range of post EoR
redshifts, from z=0 to z=6. Since then a number of pathfinder instruments have
been built, such as CHIME or Tianlai. Several other ones will be commissioned
in the next few years (HIRAX, CHORD, BINGO), while even larger arrays, with
several thousand antennae are being considered for the next generation
experiments. We will briefly review the 21cm cosmology of the Epoch of
Reionisation (EoR), and we will then focus on IM for late time cosmology. After
presenting some of the promises of this technique to constrain the cosmological
model, dark energy and inflation, we will review some of the instrumental and
scientific challenges of IM surveys. The second part of the paper presents an
overview of the ongoing and future experiments, as well as recent results by
GBT, CHIME and Tianlai.
