Prolonged Decoupling

We discuss decay of unstable particles and pair annihilation of stable heavy particles that occur in the cosmic medium, from the point of the fundamental microscopic theory. A fully quantum mechanical treatment shows that the effect of thermal environment on these processes cannot be described in terms of quantities on the mass shell alone, thus requiring an extention of the Boltzmann-like equation. The off-shell effect tends to prolong physical processes that take place subsequent to the decay.