Consider the gas of photons of the thermal equilibrium radiation in a cube of volume V at temperature τ. Let the cavity volume increase; the radiation pressure performs work during the expansion and the temperature of the radiation will drop. From the result for the entropy we know that τV1/3 is constant in such an expansion.
(a) Assume that the temperature of the cosmic black-body radiation was decoupled from the temperature of the matter when both were at 3000 K. What was the radius of the universe at that time, compared to now? If the radius has increased linearly with time, at what fraction of the present age of the universe did the decoupling take place?
(b) Show that the work done by the photons during the expansion is

W = (π2/15h3c3)Viτi3(τi – τf)

The subscripts i and f refer to the initial and final states.