<p>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?</p>

CC BY-SA 4.0
2022-05-08T04:25:34.363Z
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
CC BY-SA 4.0
2022-05-08T06:07:35.483Z
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
CC BY-SA 4.0
2022-05-08T07:21:26.757Z
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
CC BY-SA 4.0
2022-05-09T12:57:41.193Z
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
CC BY-SA 4.0
2022-05-09T14:02:24.877Z
@CarlWitthoft The fact that photons with λ > 1 m comprise ~0.15% of all photons slightly ruins the fun, though… ;-)
pela
CC BY-SA 4.0
2022-05-10T09:51:23.363Z
Hi Mark, do any of the suggested answers answer your question? I think [this discussion on the total energy of photons per volume](https://astronomy.stackexchange.com/a/19395/5264) is what you're after, but you may also find [this discussion on the number density of photons](https://physics.stackexchange.com/a/196373/70207) interesting.
pela
CC BY-SA 4.0
2022-05-11T18:01:38.540Z
Hi Pela, yes, the suggested answers address my question quite well.  Thanks!
MarkVonTexas
CC BY-SA 4.0
2022-05-12T06:15:22.613Z
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
<radiation><photons><intergalactic-space>
What is the density of photon energy in intergalactic space?
MarkVonTexas
# What is the density of photon energy in intergalactic space?

(Asked by: username_0 on 2022-05-08)

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?

**Tags:** radiation, photons, intergalactic-space

> **Comments:**
> **username_1**: 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  
> **username_2**: 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  
> **username_3**: 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  
> **username_4**: 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.  
> **username_5**: @CarlWitthoft The fact that photons with λ > 1 m comprise ~0.15% of all photons slightly ruins the fun, though… ;-)  
> **username_5**: Hi Mark, do any of the suggested answers answer your question? I think [this discussion on the total energy of photons per volume](https://astronomy.stackexchange.com/a/19395/5264) is what you're after, but you may also find [this discussion on the number density of photons](https://physics.stackexchange.com/a/196373/70207) interesting.  
> **username_0**: Hi Pela, yes, the suggested answers address my question quite well.  Thanks!  
> **username_5**: 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 :)  

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