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The universe is full of diffuse gas made of Hydrogen and Helium. While we know from observations of the late-time universe that the gas at late times is highly ionized, our theories predict that the gas is neutral shortly after the big bang. Astronomers believe that it is the ultraviolet photons emitted by the first stars and galaxies that ionized the gas in the universe, turning it from cold and neutral to hot and highly ionized. This is the “epoch of reionization“, which is believed to have occurred when the universe was only hundreds of millions of years old. The physics of reionization is basically photon-Hydrogen interaction. Stars emit ultraviolet photons, which get absorbed by neutral Hydrogen and ionize them. Since ionized Hydrogen does not interact with photons anymore, photons can travel further away in an ionized gas. As more stars and galaxies form, more photons are emitted, and more Hydrogen gets ionized, the photons are then able to travel more and more freely into the uni
