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Mar9-12, 11:04 AM
PF Gold
P: 184
Just remember that the average energy/photon is proportional to 1/a. I'm guessing that you should assume zero curvature, that the density is always at critical, and that the kinetic energy of the matter can be neglected. You'll need to assume some value for the amount of matter per photon (about 10-35kg/photon).