Mapping gas across cosmic time with REX

Imaging with narrow bandpass filters allows us to efficiently capture nearly monochromatic emission from gas in the universe. Using wide-field but narrow-band imaging across 100 square degrees, REX (the Reionization Explorer) will map cosmic dawn and the reionization of intergalactic gas in the first billion years after the Big Bang. This mission, to be proposed as a NASA Small Explorer, will also map gas in the cosmic web around galaxies at cosmic noon. It will additionally show star-forming regions in nearby galaxies including our own. By building its deep images through a series of exposures spread over a two year prime mission, REX will also enable a survey of all things variable - including black holes in active galactic nuclei, tidal disruption events, supernova explosions, novae, and more.