Reionization: starlight burns away the cosmic fog
What happened
As more and more of the first stars and galaxies switched on, their ultraviolet light did more than just illuminate the universe, it began tearing electrons back off the neutral hydrogen atoms that had formed at recombination, ionizing the gas that filled all of space. NASA's account of this epoch of reionization describes the ultraviolet light from those young stars breaking down hydrogen atoms into free electrons and protons, spreading in expanding bubbles around each new source of starlight. Over several hundred million years those bubbles grew and merged, and by the time the universe was about a billion years old, essentially all of that primordial hydrogen fog had been cleared.
Why it matters
Reionization is the moment the universe became transparent to light the way it is today, letting starlight and galaxy light travel across billions of light-years without being absorbed by intervening fog. Without it, the universe astronomers observe now, full of clearly visible, sharply resolved galaxies, would look completely different.
How we know
The clearest evidence comes from the light of extremely distant quasars: hydrogen that is still neutral absorbs a very specific wavelength of light passing through it, producing a dark gap in the quasar's spectrum called the Gunn-Peterson trough. Quasars from the universe's first billion years show that trough; quasars from later eras do not, bracketing reionization's completion. The cosmic microwave background itself carries a subtler, independent confirmation, in how reionization's free electrons scatter that ancient light.
Sources
- NASA Science. Overview (Universe) · Reputable sourcescience.nasa.gov · The domain "science.nasa.gov" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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Part of a timelineThe Early Universe7 events · The first billion years, from the fusion furnace of the Big Bang's opening minutes to the collision that reshaped the Milky Way, told through the instruments and satellites that found the evidence.View all →