Science › Cosmic timeline
Reionization: starlight clears the fog
1965· Cosmic event
ProvenTested many times; part of working physics.
From about 150 million to about a billion years, ultraviolet light from stars and galaxies stripped electrons from hydrogen across the universe.
See it on the Science map →How it works
Hydrogen gas had been neutral since 380,000 years. Ultraviolet light from the first stars and galaxies knocked its electrons off again, heating it to about 10,000 degrees. Ionized bubbles grew around each galaxy and merged until almost all gas between galaxies was ionized, as it remains today. Neutral hydrogen absorbs ultraviolet light; once it was gone, that light could travel freely. Space was now too thin for the freed electrons to form a new fog.
Why it matters
It was the last big change in the state of the universe's gas, and its timing records how fast the first galaxies switched on.
How we know
James Gunn and Bruce Peterson predicted in 1965 that neutral hydrogen would absorb light from quasars (bright, black-hole-powered galaxy cores); it was first seen in 2001, in a quasar from about 900 million years. Planck's microwave polarization puts the halfway point near 700 million years.
Still a mystery
Did numerous faint galaxies or growing black holes supply most of the ultraviolet? JWST suggests early galaxies emitted more than expected, perhaps too much to fit the measured timing.
Connections (2)
▸The first stars start Reionization: starlight clears the fogWell supported
The relationship
The first stars were so hot, with surfaces near 100,000 degrees, that most of their light was ultraviolet, energetic enough to strip electrons from hydrogen. Each carved out a bubble of ionized gas thousands of light-years across. These first bubbles were scattered, and many recombined after their star died; lasting, universe-wide reionization needed whole galaxies.
How the link was found
Volker Bromm, Rolf-Peter Kudritzki and Abraham Loeb calculated the first stars' ultraviolet output in 2001. Tom Abel, John Wise and Greg Bryan simulated their ionized bubbles in 2007.
Is it proven?
Inferred from simulations. Microwave-background polarization shows reionization was underway early, but cannot separate the first stars' share from galaxies'.
Open questions and mysteries
How much did first stars contribute compared with later galaxies or black holes? Probably a small share, but nobody knows.
▸The first galaxies (seen by JWST) finish Reionization: starlight clears the fogWell supported
The relationship
Galaxies, packed with hot young stars, produced most of the ultraviolet light that ionized gas between them. Their ionized bubbles grew and merged, like holes in Swiss cheese until little cheese is left. By about a billion years, almost all intergalactic hydrogen was ionized. Faint small galaxies, being so numerous, probably supplied much of the light; growing black holes may have added some.
How the link was found
The galaxy-driven picture was developed in the 1990s and 2000s by Piero Madau, Abraham Loeb, Rennan Barkana and others. Since 2022 JWST has measured how much ionizing light early galaxies produce.
Is it proven?
Quasar spectra show the last neutral patches vanishing at about 1.1 billion years, later than once thought (2022 analyses). JWST counts of early galaxies give enough ultraviolet to do the job.
Open questions and mysteries
What share of ionizing light escapes its home galaxy? The galaxy's own gas absorbs much of it, and at these distances the escaping share cannot be measured directly, because intergalactic hydrogen absorbs it too. JWST's light-budget puzzle hinges on it.