The Cosmic 21-cm Revolution

The redshifted 21-cm signal is set to transform astrophysical cosmology, bringing a historically data-starved field into the era of Big Data. Corresponding to the spin-flip transition of neutral hydrogen, the 21-cm line is sensitive to the temperature and ionization state of the cosmic gas, as well as to cosmological parameters. Crucially, with the development of new interferometers, it will allow us to map out the first billion years of our universe, enabling us to learn about the properties of the unseen first generations of galaxies.