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(some type of particle stops being destroyed as often as it is created)

Decoupling (in cosmology) is the change in some type of particle's relationship with the type of particles that interact with it (i.e., create and destroy it): specifically a change from a previous condition when the particle was being destroyed as often as it is created to a later (lack of) balance when 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 is free streaming, i.e., a significant number are not absorbed. Another way to put this is that 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:

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