Reionization is the period in the early universe, several hundred million years after the Big Bang, during which ultraviolet radiation from the first stars and galaxies ionized the neutral hydrogen gas that had filled space since recombination, rendering the universe transparent to that radiation once again. It marks the end of the so-called cosmic dark ages and the beginning of the universe as it appears today, filled with luminous structure. Astronomers study reionization through the redshifted 21-centimeter signal of neutral hydrogen and through the light of the most distant known quasars and galaxies, whose spectra show the fading imprint of the intervening neutral gas.
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