There were no stars to make light. No familiar swirls of galaxies. Certainly no planets. And the entire universe was shrouded in neutral hydrogen gas.
Then, perhaps million years or so in, everything started to change. Over the next billion-odd years, the universe went from a bland, unimpressive landscape to a rich and dynamic one. This profound shift began when the first stars lit up. As they burned, generating heat and forging new matter, their intense light began tearing apart the hydrogen that pervaded the universe. Everywhere electrons were ripped from these atoms, leaving the bulk of hydrogen—the most abundant element in the universe—in the ionized state it remains in today.
This pivotal period—when all that hydrogen went from one form to another—is known as the epoch of reionization.
It began with our cosmic dawn and ushered in the modern era with all its marvelous textures and features. It serves as the backdrop for when the universe grew up. Everything changed over that billion years or so and nothing much has changed in the billions of years since. While there are models that describe how this great transition might have happened, giant gaps in our picture remain.
When did the first stars form and when did light, escaping their host galaxies, kick off reionization? What kinds of galaxies were most responsible and what was the role of black holes? How did reionization proceed across time and space? And what clues might it hold to other cosmic mysteries, like the nature of dark matter? The James Webb Space Telescope JWST , launched in , is peering at the galaxies that existed only hundreds of millions of years after the Big Bang and is already turning up surprises.
