As I understand it there can be said to be two "kinds" of electromagnetic radiation filling up the universe. First you have the primordial radiation from a time soon after Big Bang. Then you have "newer" radiation from the fact that you start up with very diffuse clouds of mainly hydrogen that falls together and forms planets, stars, black holes etc. basically converting potential energy to heat radiation and in the stars you of course also have fusion producing heat that radiates away.

Can you say something like:

X percent of the energy of the electromagnetic radiation in the universe is primordial and (100 -x) percent of the energy of the electromagnetic radiation in the universe is of newer origin, and if so, what is X?

I mean if you "magically removed" all "primordial radiation" some 13 billion years ago we would still have a lot of radiation in the universe today.

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    $\begingroup$ I answered exactly this question over at physics.SE. The answer is that there are roughly 220 CMB photons for every non-CMB photon, with the largest non-CMB contributions being IR, then radio, then optical (depending a bit on the defined threshold between the bands). $\endgroup$
    – pela
    May 2 '19 at 15:56
  • $\begingroup$ Do we know how much of the microwave radiation that must be "primordial"? I mean if the universe is filled with a very dilute gas that is hit by radiation from stars etc., radiation that is not primordial, that gas should re-emit radiation, some of it in the microwave band. Maybe there is also some other important source of microwave radiation that is non-primordial? $\endgroup$
    – Agerhell
    May 2 '19 at 16:25
  • $\begingroup$ @Agerhell I think you're misunderstanding a crucial aspect of the CMB. It is not just microwave photons, that's just where the wavelength distribution peaks. The CMB is made out of every wavelength of photons in varying numbers. As pela said, for every 1 "non-primordial" photon, there are 220 "primordial" photons (that make up the CMB). Pela's answer already distinguishes between a "primordial" microwave photon and a "non-primordial" microwave photon. $\endgroup$
    – zephyr
    May 2 '19 at 19:16
  • $\begingroup$ Actually it doesn't really. Agerhell is right that other processes emit in the mm region as well, so part of the mm photons we see are not of cosmological origin. There are both individual sources (like supernova remnants), and more diffuse sources (like dust), and if you point your telescope toward one of these sources, they will dominate in the mm. On average, however, the radiation is completely dominated by the CMB (which is why we see such a near-perfect Planck curve described by a single temperature), but because we are inside the Milky Way, we need to correct for foreground noise. $\endgroup$
    – pela
    May 3 '19 at 8:09

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