Cosmology with the 21 cm line

Abstract

The 21 cm line corresponds to an excited state of the neutral hydrogen. Therefore observations of that cosmologically redshifted line can unveil how the intergalactic gas became reionized at some redshift higher than 6. The absorption 21 cm line has been recently detected for the first time. However, its profile is highly unexpected as it implies a gas temperature lower than what is expected by all current cosmological models. In this contribution, the possibility that such a discrepancy arises from the fact that the observed 21 cm line was emitted from a typical low-density region is analysed. This has allowed us to substantially reduce the gas temperature discrepancy, which alleviates but not fully solves the problem.