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Decoupling (in cosmology) refers to the end of a balanced interaction between a type of particle and one or more other types. An example would be a change from a balance where as many of this type of particle are created as destroyed by the particles that it interacts with to one where more are created than destroyed. Before the decoupling, the type of particle is in thermodynamic equilibrium with the type of particle(s) that emit and absorb it: if for any reason the sum of all of one type gains energy, so does that of the other, and vice versa, i.e., the two groups share their energy. After the decoupling, the type of particle decoupled may be free streaming, i.e., a significant number are not absorbed and the particle's mean free path has lengthened to exceed the horizon size of the universe. Decouplings are theorized to have occurred in the early universe (Big Bang theories), including:
Though decoupling can lead to free streaming, that is not necessarily the case. One alternative is a decoupling event leading to separate equilibria of independent sets of particles. For example, someone might theorize that when WIMPs decoupled from baryonic matter, each still interacted within its own equilibrium, but without affecting the other.