The Frontier of Reionization: Theory and Forthcoming Observations

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.