AAS Nova
Glossary · Galaxies & Cosmology
Reionization
The era when the first stars lit up and burned the fog out of the universe.
After the microwave background was released the universe went dark: neutral hydrogen, no stars. Somewhere in the first few hundred million years the first stars and galaxies formed and their ultraviolet light stripped the electrons back off that hydrogen, making the universe transparent again. Exactly when and what did it is one of the questions Webb was built for, and its early galaxies have already turned out brighter and more mature than the models expected.
Where it has come up lately
Related terms
Fast radio burst
A millisecond flash of radio energy from another galaxy, cause still unsettled.
Redshift
The stretching of light from distant objects, and the universe's odometer.
Hubble tension
Two good measurements of the universe's expansion rate that stubbornly disagree.
Dark energy
The name for whatever is making the expansion of the universe speed up.
Questions
What is Reionization?
The era when the first stars lit up and burned the fog out of the universe.
Why does it matter?
Somewhere in the first few hundred million years the first stars and galaxies formed and their ultraviolet light stripped the electrons back off that hydrogen, making the universe transparent again. Exactly when and what did it is one of the questions Webb was built for, and its early galaxies have already turned out brighter and more mature than the models expected.