The stage early in the history of the Universe when, according to the Big Bang theory, particles of matter ceased to interact with radiation. Decoupling happened at different times, and therefore at different temperatures, for different particles. Neutrinos, for example, decoupled from the background radiation at a temperature of about 1010 K (about 1 s after the Big Bang) while ordinary matter decoupled at a temperature of a few thousand degrees K (after about 300 000 years). After matter and radiation decoupled, the background radiation propagated freely through the expanding Universe.
Subjects: Astronomy and Astrophysics.