In cosmology, decoupling is 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.
Photon decoupling is closely related to recombination, which occurred about 378,000 years after the Big Bang (at a redshift of z = 1100), when the universe was a hot opaque ("foggy") plasma.
Because direct recombinations to the ground state (lowest energy) of hydrogen are very inefficient, these hydrogen atoms generally form with the electrons in a high energy state, and the electrons quickly transition to their low energy state by emitting photons.
They can still be detected today, although they now appear as radio waves, and form the cosmic microwave background ("CMB").
Decoupling occurred abruptly when the rate of Compton scattering of photons
After this photons were able to stream freely, producing the cosmic microwave background as we know it, and the universe became transparent.
, it is possible to show that photon decoupling occurred approximately 380,000 years after the Big Bang, at a redshift of
[3] when the universe was at a temperature around 3000 K. Another example is the neutrino decoupling which occurred within one second of the Big Bang.
An important consequence of neutrino decoupling is that the temperature of this neutrino background is lower than the temperature of the cosmic microwave background, since they were no more heated by the shortly following annihilation of positrons.
By calculating the hypothetical time and temperature of decoupling for non-relativistic WIMPs of a particular mass, it is possible to find their density.
[5] Comparing this to the measured density parameter of cold dark matter today of 0.222
0.0026 [6] it is possible to rule out WIMPs of certain masses as reasonable dark matter candidates.