Decoupling (cosmology)

In cosmology, decoupling refers to a period in the development of the universe when different types of particles fall out of thermal equilibrium with each other. This occurs as a result of the expansion of the universe, as their interaction rates decrease (and mean free paths increase) up to this critical point. The two verified instances of decoupling since the Big Bang which are most often discussed are photon decoupling and neutrino decoupling, as these led to the cosmic microwave background and cosmic neutrino background, respectively.

Photon decoupling occurred during the epoch known as the recombination. During this time, electrons combined with protons to form hydrogen atoms, resulting in a sudden drop in free electron density. Decoupling occurred abruptly when the rate of Compton scattering of photons ${\displaystyle \Gamma }$ was approximately equal to the rate of expansion of the universe ${\displaystyle H}$, or alternatively when the mean free path of the photons ${\displaystyle \lambda }$ was approximately equal to the horizon size of the universe ${\displaystyle H^{-1}}$. After this photons were able to stream freely, producing the cosmic microwave background as we know it, and the universe became transparent.

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