The processes that converted the universe from normal hydrogen to ionized hydrogen is called reionization.
The theory has it that, as the number of stars, galaxies, black holes, accretion disks and jets increased in number and size, the ultraviolet, x-ray and gamma ray radiation produced became intense enough to drive the electrons out of their orbits around protons.
Light absorption by hydrogen atoms ceased, and light started to travel across the universe. The key to understanding how we can measure the size and growth rates of these ionized regions is in the spectral analysis of Lyman-alpha photons.
An ultraviolet Lyman-alpha photon created by a quasar in the latter stages of the reionization process would have shifted into the near-infrared by the time it reached us. This is why Webb is needed to fully explore this key cosmological process.
Here’s an image take by Webb in 2023. There are more than 20,000 galaxies in this field. The Hyper-luminous quasar J0100+2802 is at the center. A deep study of this area was conducted by the Emission-line galaxies and Intergalactic Gas in the Epoch of Reionization survey – EIGER for short.
As the light from these galaxies passed through transparent ionized space, the absorption lines disappear. The survey clearly shows that the expected transparent regions do exist around galaxies. The results showed that galaxies near the quasar had fully ionized the gas within a 2 million light-year radius. That’s approximately the same distance as the space between our Milky Way galaxy and Andromeda.
And it showed that the ionization volumes increased over time as the light approached the expected timeframe for the fully ionized and transparent universe we have today.