  The cosmic microwave background provides an image of the Universe 0.4 million
years after the big bang, when atomic hydrogen formed out of free electrons and
protons. One of the primary goals of observational cosmology is to obtain
follow-up images of the Universe during the epoch of reionization, hundreds of
millions of years later, when cosmic hydrogen was ionized once again by the UV
photons emitted from the first galaxies. To achieve this goal, new
observatories are being constructed, including low-frequency radio arrays
capable of mapping cosmic hydrogen through its redshifted 21cm emission, as
well as imagers of the first galaxies such as the James Webb Space Telescope
(JWST) and large aperture ground-based telescopes. The construction of these
observatories is being motivated by a rapidly growing body of theoretical work.
Numerical simulations of reionization are starting to achieve the dynamical
range required to resolve galactic sources across the scale of hundreds of
comoving Mpc, larger than the biggest ionized regions.
