Thermodynamic Properties of the CMB

The Big Bang model explains the origin of the CMB as the moment of decoupling between light and matter. This event occurred when the Universe was a fraction of its present size and thus much more thermally active. The physics of particle interactions under these hot, dense conditions are described by equilibrium thermodynamics, which predict that frequently interacting photons will exhibit a black body distribution. Thus the Big Bang model simply and elegantly accounts for the CMB's observed emission spectrum.

Alternative cosmologies have been far less successful. Under the Steady State theory all of the Universe's radiation originates at higher frequencies inside of stars and does not exhibit black body behavior. One must introduce ad hoc theoretical assumptions in order to account for the observed properties of the CMB.