Barbara Ryden , Introduction to Cosmology , Chapter 9 ( The Cosmic Microwave
نویسنده
چکیده
The formula for the entropy density of black-body radiation is given in Lecture Notes 7. The derivation of this formula has been left to the statistical mechanics course that you either have taken or hopefully will take. For our purposes, the important point is that the early universe remains very close to thermal equilibrium, and therefore entropy is conserved. The conservation of entropy applies even during periods when particles, such as electron-positron pairs, are “freezing out” of the thermal equilibrium mix. Since total entropy is conserved, the entropy density falls off as 1/a(t).
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