Cosmic reionization
Ultraviolet light from early stars and galaxies strips electrons from the hydrogen filling space. By about a billion years after the Big Bang, the gas between galaxies is almost fully ionized.
The story
After the cosmic microwave background was released, the universe entered the 'dark ages': neutral hydrogen and helium gas, with no stars to light it. When the first stars and galaxies formed, perhaps 100 to 200 million years after the Big Bang, their ultraviolet light began to knock electrons off the hydrogen atoms around them, which is called ionizing. Bubbles of ionized gas grew around early galaxies, merged, and spread until the gas between galaxies was almost fully ionized, about a billion years after the Big Bang. This phase change is called reionization.
The evidence comes from several observations. Spectra of distant quasars (extremely bright, faraway galactic cores) show that around 12.8 billion years ago, light at the wavelength hydrogen absorbs was being blocked by neutral gas, which is a sign that reionization was just ending. The cosmic microwave background, measured by the Planck satellite, shows that the process was about halfway around 700 million years after the Big Bang. What did most of the work is debated: the likely candidates are massive stars in small galaxies, while some role for black holes is possible. The James Webb Space Telescope is now finding many faint early galaxies, which helps address the question.
Why it mattered
- The intergalactic gas became transparent to ultraviolet light, which let astronomers observe distant galaxies and quasars.
- Understanding the era tells us when and how the first stars and galaxies formed, and what they were like.
- The heating of the gas by ionization affected how later small galaxies could form stars.
Community notes
Loading notes…