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Astronomy

Thermal History of the Intergalactic Medium During the Cosmic Dawn

Quick fact

During the cosmic dawn, the intergalactic medium was first cooled to just a few tens of Kelvin by the expansion of the universe, but then the first stars and black holes heated it to thousands of Kelvin, creating the conditions for the 21-centimeter radio signal that we can detect today.

Why this is interesting

You might think the universe became dark after the Big Bang, but the gas between galaxies tells a more dramatic story of heating and cooling. What made this intergalactic gas switch from a cold, opaque fog to a warm, transparent one?