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What is the average density of photon energy (over all wavelengths) in intergalactic space? In other words, if you took a typical cubic meter of intergalactic space, how much electromagnetic radiation energy would be contained in that cubic meter at any given time?

MarkVonTexas
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    Even in interstellar space, most of the photon energy is due to the CMB. There are some relevant numbers here: https://astronomy.stackexchange.com/a/19395/16685 – PM 2Ring May 08 '22 at 04:25
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    2nd result for your 1st paragraph on google... Very in depth info inc. nrg. https://physics.stackexchange.com/questions/233801/how-many-photons-are-there-in-free-space-on-average – bandybabboon May 08 '22 at 06:07
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    https://physics.stackexchange.com/questions/233801/how-many-photons-are-there-in-free-space-on-average https://physics.stackexchange.com/questions/196366/does-the-number-density-of-photons-n-gamma-approx-108-mathrm-m-3-refer – ProfRob May 08 '22 at 07:21
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    We can have fun with this, since lots of photons don't even "fit into" a m^3 if you take their wavelength into account. – Carl Witthoft May 09 '22 at 12:57
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    @CarlWitthoft The fact that photons with λ > 1 m comprise ~0.15% of all photons slightly ruins the fun, though… ;-) – pela May 09 '22 at 14:02
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    Hi Mark, do any of the suggested answers answer your question? I think this discussion on the total energy of photons per volume is what you're after, but you may also find this discussion on the number density of photons interesting. – pela May 10 '22 at 09:51
  • Hi Pela, yes, the suggested answers address my question quite well. Thanks! – MarkVonTexas May 11 '22 at 18:01
  • Great, I'll vote to close this one, then. We aim to have as few duplicates as possible. The phrase in the comment below "Does this answer your question?" is a standard automatic text, not because I didn't hear you :) – pela May 12 '22 at 06:15

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