Astronomy
The Thermal History of the Universe from Recombination to Reionization
Quick fact
The universe became transparent about 380,000 years after the Big Bang, and then stayed dark for over 100 million years before the first stars turned on and reionized it.
Why this is interesting
The universe was once a hot, foggy plasma, but today it's transparent. What made it switch?
Read the full explanation
Understanding The Thermal History of the Universe from Recombination to Reionization
Imagine a dense fog where light can't travel far. That was the early universe, filled with a hot soup of protons, electrons, and photons. As it expanded, it cooled. When it dropped to about 3000 K, protons and electrons could combine to form neutral hydrogen atoms. This event, called recombination, removed free electrons from the scene. Photons, previously scattered constantly, could now stream freely—this is the cosmic microwave background we see today. After recombination, the universe became dark and foggy no more, but it was also cold and filled with neutral hydrogen. This period is called the cosmic dark ages. Then, gravity pulled matter together to form the first stars and galaxies. These first luminous objects emitted ultraviolet light, which stripped electrons from the neutral hydrogen—a process called reionization. This brought the universe back to a hot, ionized state, but this time it was transparent to light.
A deeper explanation
The thermal history is driven by the interplay between cosmic expansion (which cools) and energy injection (which heats). Initially, the universe was a hot plasma where photons and matter were coupled via Thomson scattering. Recombination occurs when the temperature falls below the ionization energy of hydrogen (~13.6 eV, but effectively ~0.3 eV due to the high photon-to-baryon ratio). As electrons and protons capture to form neutral hydrogen, the baryon density drops dramatically, and the mean free path of photons becomes vastly larger than the horizon—this is photon decoupling, which freezes the CMB. Following recombination, the universe enters the dark ages: gravity amplifies tiny density fluctuations, baryons fall into dark matter halos, and eventually the first stars ignite. These stars and early galaxies emit ionizing radiation (Lyα, UV) that reionizes the intergalactic medium. This transition heats the gas and affects subsequent structure formation. The thermal history thus encompasses the recombination epoch, the subsequent cooling, and the reionization epoch, which is the last major phase change of the universe's baryonic matter.