Reionization

Reionization was the cosmic epoch when radiation from the first luminous sources ionized most hydrogen between galaxies, ending the universe’s long neutral phase.

Cosmic dawn: The period when the first stars and galaxies formed after the cosmic dark ages. Its first luminous sources supplied the radiation that began reionizing intergalactic hydrogen.
Population III stars: The hypothetical first generation of stars, formed from nearly pristine hydrogen and helium. Their high temperatures could have made them powerful ionizing sources before ordinary galaxies emerged.
Recombination (cosmology): The early-universe epoch when electrons and nuclei combined into neutral atoms as the cosmos cooled. It created the neutral hydrogen that persisted until radiation from later sources began reionizing it.
Epoch of Reionization 21-centimeter line: The redshifted radio signal from neutral hydrogen’s 21-centimeter transition during the early universe. Its spatial fluctuations could map neutral gas directly and constrain when reionization progressed.
Lyman-alpha photon: A photon emitted or absorbed when a hydrogen atom changes between its first and second energy levels. Neutral hydrogen strongly absorbs these photons, making their changing visibility a probe of reionization.
First galaxies: The earliest gravitationally bound systems of stars and gas formed in the universe. Their accumulated stellar populations are leading candidates for driving the later stages of reionization.
Hydrogen atom: The simplest atom, consisting of one proton and one electron. Its ionization state defines what reionization means for the dominant ordinary matter between galaxies.
Thomson optical depth: A measure of the probability that photons scatter from free electrons along a line of sight. Cosmic microwave background measurements constrain the total amount of ionization, but not its detailed history.
H II region: A region of ionized hydrogen produced by energetic radiation, usually from hot stars. Early H II regions expanded around sources and eventually overlapped across the intergalactic medium.
Quasar: An extremely luminous active galactic nucleus powered by matter accreting onto a supermassive black hole. Quasars emit abundant ionizing radiation, though their scarcity may limit their contribution at early times.

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