Compute the solution of the following problem

Problem- The Universe is full of black-body radiation at a temperature of 2.73 K. Suppose that the radius of the Universe is given Ht where t is the age of the Universe and H is Hubble's constant. Assume that the total internal energy of black-body radiation in the Universe is constant.

Part 1- Give an expression for the internal energy of the radiation and show that the entropy of black-body radiation in the Universe increases in time as T^3/4.

I want help to determine the entropy of black-body radiation.