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? $\endgroup$– adrianmcmenaminJul 4, 2014 at 14:44
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$\begingroup$ Yes, energy density would probably be the best way to measure it. $\endgroup$– JonathanJul 4, 2014 at 15:23
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$\begingroup$ What do you mean by "edge of the universe"? $\endgroup$– HDE 226868 ♦Sep 9, 2014 at 1:13
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$\begingroup$ @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. $\endgroup$– JonathanSep 12, 2014 at 17:30
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2$\begingroup$ @Jonathan: The "area" you are describing is called the surface of last scattering. See cosmic microwave background on Wikipedia. $\endgroup$– pabouk - Ukraine stay strongOct 9, 2014 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.
answered Nov 12, 2020 at 19:36