Dear all, I am using stress-energy tensor to derive equation of state of photon gas (assuming it as a perfect fluid). I completed all the steps except one: average value of [cos(θ)]^2 over unit sphere = 1/3. I have no idea how this is so. (θ is polar angle). I tried integrating over 0<θ<∏ and 0<φ<2∏, and then divided by the surface area of the sphere, but no luck with that either... Please help!