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From each point in the universe, the light of billions of stars, galaxies, supernovae etc. can be detected. So there seems to be a lot of energy/momentum "in flight".

Is it possible to estimate how much energy this adds up to in total? And with that estimate, what fraction of the universe's total energy is in this form?

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    $\begingroup$ Related question: physics.stackexchange.com/q/453118 $\endgroup$
    – scaphys
    Apr 4 at 19:16
  • $\begingroup$ Regarding the fraction of the photon energy density: You might want to look at Friedmann's equations and the energy density parameters. $\endgroup$
    – scaphys
    Apr 4 at 19:21
  • $\begingroup$ Also related (photons in milky way) physics.stackexchange.com/q/518236/226902 $\endgroup$
    – Quillo
    Apr 4 at 19:40
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    $\begingroup$ In the various estimates given in this thread consider to specify what types of energies are included in the tally of total energy given that the notion of total energy of the universe is usually viewed as ill-defined. $\endgroup$
    – Qmechanic
    Apr 5 at 15:30
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    $\begingroup$ Methinks a difficult question. As I understand, within stars, much of the energy resulting from the fusion reactions is in the form of photons, which take many thousands of years to escape the heavenly body. Do you mean to include this energy? $\endgroup$
    – ttonon
    Apr 7 at 20:32

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See (Fukugita & Peebles 2004), which estimates that $10^{-4.3}$ of the energy density of the universe is in the CMB photons, and $10^{-5.7\pm 0.1}$ in stellar and poststellar radiation.

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