 In the How Old Is It video book, we covered the early stages of the universe through the dark ages following photon decoupling. It is estimated that the dark ages extended from 370,000 to 150 million years after the Big Bang. During this time, the universe was filled with dark matter, hydrogen, and some helium. With no stars having formed to give off visible light, the universe was literally dark. The caustic process worked the dark matter into filaments, with the baryonic matter, hydrogen and helium, tagging along. Eventually, the dense clouds of hydrogen gas in the filaments started to collapse under their own gravity, becoming hot enough to trigger nuclear fusion reactions between the hydrogen atoms, creating the very first stars. The light from these stars traveled through a sea of free molecular hydrogen, which over time absorbed it. Consequently, during that period, light did not travel far, and the universe remained mostly dark. Today the universe is completely transparent, with light traveling freely over billions of years through a sea of ionized hydrogen that does not absorb light. The process that converted the universe from a normal hydrogen to ionized hydrogen is called reionization. The theory is 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. In this simulation, the dark regions are filled with hydrogen atoms. The light areas have been ionized, and light can travel through it with minimal losses. Over time, the ionized regions around stars cleared entire galaxies. And ionized regions around galaxies cleared entire galaxy clusters. This process ended around 1.1 billion years after the Big Bang, with all of space cleared for light travel. In order to better understand how this reionization actually worked, the James Webb Space Telescope was built with infrared capabilities designed to observe this process that began 13 billion years ago. The key to understanding how we can measure the size and growth rates of these ionized regions is in the spectral analysis of the light we receive. In the How Small Is It? video book, we covered how light is created and absorbed when electrons change energy levels in an atom. For example, when an electron in a hydrogen atom drops from the energy level 2 to the base energy level 1, a photon is emitted with a wavelength small enough to carry away exactly the amount of energy lost by the atom. This is called a Lyman-alpha photon. It's in the ultraviolet range of the electromagnetic spectrum, and in large numbers they create the Lyman-alpha emission line in a star's spectra. In the other direction, such a photon would be absorbed when it encounters a hydrogen atom in its ground state, driving the electron into the higher energy level. In large numbers, they would create a Lyman-alpha absorption line in the light's spectra. The majority of ordinary matter, hydrogen, in the universe, exists in the space between galaxies, called an intergalactic medium, or IGM for short. As light from a distant galaxy travels through a cloud of molecular hydrogen, photons with this wavelength will be absorbed, creating a dip in the light's spectra. As the light continues to move through expanding space, its wavelength stretches with the expansion, so photons that originated with a shorter wavelength in the Lyman-alpha will stretch to become Lyman-alpha photons. Should they flow through neutral hydrogen, they will be absorbed, creating a second Lyman-alpha absorption line. The process will continue creating a forest of absorption lines as the light travels through various normal and ionized hydrogen clouds. 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 reaches us. This is why Webb is needed to fully explore this key cosmological process. Here's an image taken by Webb in 2023. There are more than 20,000 galaxies in this field. The hyperluminous quasar J0100 plus 2802 is at the center. It is one of the most luminous quasars known. Its supermassive black hole is 10 billion times more massive than our sun. His redshift is 6.327. That gives us the distance the light travels to reach us at 12.8 billion light years. A deep study of this area was conducted by the emission line galaxies and intergalactic gas in the Epic of Reionization Survey. Iger for short. The survey used Webb's two near-infrared camera modules, each with a 2.2 by 2.2 arc second field of view. Here's their exposure map around the quasar. The study covered 117 galaxies within 650,000 light years of the quasar. Some are closer to us than the quasar and some are further away. All these galaxies existed near the end of the era of reionization. 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 two million light-year radius. That's approximately the same distance as the space between the Milky Way 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.