epoch of reionization
/ EE-pok of ree-EYE-on-ih-ZAY-shun /
Recombination had stitched the universe's electrons and nuclei into neutral atoms, leaving a cold, transparent fog of neutral hydrogen gas. Then the first stars and galaxies switched on and flooded that gas with fierce ultraviolet light — and the light tore the electrons back off the atoms, ionizing the hydrogen all over again. The epoch of reionization is this great turning-on of the cosmos: the period when the first luminous objects re-ionized nearly all the neutral hydrogen filling the universe, ending the dark ages for good.
Reionization unfolded gradually, mostly between about 200 million and one billion years after the Big Bang. It did not happen everywhere at once. Around each early galaxy and quasar, the ultraviolet radiation carved out an expanding bubble of ionized gas — electrons stripped free again. As more sources lit up, these bubbles grew and grew until they overlapped and merged, and the entire intergalactic medium was ionized through. By about a billion years after the Big Bang the job was essentially complete, and the universe has remained ionized between the galaxies ever since. This is, in effect, the universe's second ionization — hence 're'-ionization — undoing the neutralizing that recombination had achieved.
Reionization is one of the last great unmapped chapters in cosmic history, and a major target of current astronomy. It marks the transition from a dark, simple, neutral universe to the lit, structured one of galaxies we know. Pinning down exactly when it happened, and what powered it (the first stars, early galaxies, growing black holes, or some mix), tells us about the births of the first generations of objects. Astronomers probe it through the faint 21-centimeter radio signal of neutral hydrogen, through the absorption that intervening gas imprints on the light of the most distant quasars (the Gunn-Peterson effect), and through deep images from the James Webb Space Telescope. Much remains uncertain, which is exactly why it is such an active frontier.
Picture early galaxies as streetlamps flickering on across a dark fog. Each one clears a glowing bubble of ionized gas around itself; as more lamps light up, the bubbles swell and merge until the whole fog is burned through. By about a billion years after the Big Bang the intergalactic gas was almost entirely ionized — and it has stayed that way ever since.
The first galaxies ionized the universe in growing bubbles that eventually merged.
Reionization was gradual and patchy, not a single instant, and its exact timeline and main power source are still being pinned down. The 're' is because the universe was ionized before recombination, became neutral, and was then ionized again.