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I would like to know the ratio of cosmic microwave background radiation to normal radiation in the universe. I am considering cosmic microwave background radiation to include the microwave, and any other radiation that is being emanated from near the "edge of the universe", while normal radiation is radiation emmitted by stars, nebulae, and other sources within the universe (excluding the cosmic microwave background). Since we are in a galaxy, I know that the normal radiation here far exceeds the cosmic microwave background, but I am interested in the "average" value over the universe. For example: total cosmic microwave background radation in the universe / total normal radiation in the universe.

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  • $\begingroup$ Measured in what way? In energy density? $\endgroup$ Jul 4 '14 at 14:44
  • $\begingroup$ Yes, energy density would probably be the best way to measure it. $\endgroup$
    – Jonathan
    Jul 4 '14 at 15:23
  • $\begingroup$ What do you mean by "edge of the universe"? $\endgroup$
    – HDE 226868
    Sep 9 '14 at 1:13
  • $\begingroup$ @HDE 226868 By "edge of the universe", I am referring to the area where it took so long for the light to reach us that the universe was opaque at this location. $\endgroup$
    – Jonathan
    Sep 12 '14 at 17:30
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    $\begingroup$ @Jonathan: The "area" you are describing is called the surface of last scattering. See cosmic microwave background on Wikipedia. $\endgroup$
    – pabouk
    Oct 9 '14 at 14:40
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Photons from the CMB outnumber all other photons by more than 200:1. This is contained in Pela's excellent answer to: Does the number density of photons nγ≈108m−3 refer to CMB photons only?. Pela uses this graph from Hill et al. 2018 to derive this ratio. Note that the CMB forms the largest contribution to the electromagnetic spectrum. enter image description here

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