1. The problem statement, all variables and given/known data At what rate would stars have to be producing light (how many photons per second per solar mass start) in order for the energy density of photons in the universe be constant? Assume current values of cosmological parameters. Do it for current time. 2. Relevant equations e = c^2/(8*pi*G) 3. The attempt at a solution So I understand that the photon density decreases linearly as time goes on. I am able to calculate the photon energy density for current time and parameters. But now how do I calculate the amount of light that needs to be produced for that number to be constant throughout all of time.