# Is cosmic background radiation dark-matter and/or dark-energy?

Dumb question alert: Is it possible that the cosmic background radiation might be the source of dark-matter and/or dark-energy? What is the mass of the background radiation in the known universe?

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I think the names 'dark matter' and 'dark energy' should give you a hint. ;) I think a similar question has been asked on here before, if I can find it I'll link to it here. –  Kitchi Feb 20 '13 at 13:37
Just to be sure that Kitchi's point is clear, background radiation is called radiation because it radiates so it's "bright". The adjective "dark" is the opposite of "bright" - it doesn't radiate, it's invisible, it doesn't interact electromagnetically. Dark matter and dark energy are not radiation - neither electromagnetic nor any other - because they have different pressure. Radiation has $p=\rho/3$, dark matter has $p=0$ and dark energy has $p=-\rho$ - clearly three different forms of "matter". –  Luboš Motl Feb 20 '13 at 14:01

No.

Neither dark matter nor dark energy can be seen in the electromagnetic spectrum---that's why it's "dark"---whereas the cosmic background radiation is electromagnetic radiation.

We are able to deduce a number of facts about about dark energy and dark matter from their effects on observable stuff (including the CMB in the case of baryon acoustic resonance), but both are strongly excluded from being "stuff as we know it".

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Does EM radiation in space predict acceleration of the same kind as dark-energy? If so, has this been quantified, and how does it compare to the total dark-energy acceleration? You wave through these steps when you say it's not dark energy because it's observed. Well I agree physicists have no-doubt investigated it as a source of dark-energy, but it's unclear from what you write if it's excluded because of type of effect or by magnitude of effect. –  AlanSE Feb 20 '13 at 14:03
@AlanSE EM radiation has positive pressure the same as normal matter, therefore it contributes to a decelerating universal expansion. That is clearly different from the observed acceleration. To my knowledge the only observation contributing to the belief in dark energy is the observed time dependence of the Hubble (non-)constant which is incompatible with the known and understood categories of stuff. See Lubos' notes in the comments to the question. –  dmckee Feb 20 '13 at 14:12
Ah, indeed, this is what I'm getting from Lubos' comment, and the previous question which covered that in the answer. Here, "positive" pressure indicates a force that tends to a contraction of the universe. Don't photons also contribute a negative pressure due to absorption? The Milky Way and Andromeda pull closer to each other through gravity, but don't they also push a little through exchange of photons? I've read speculation about this exact effect contributing to the universal acceleration. As we generalize to isotropic, is the logic flawed to begin with? –  AlanSE Feb 20 '13 at 14:16
@AlanSE Well, we know enough about photons in GR to have corrected computed the family of light curves associated with MACHO microlensing events in advance. I find it difficult to believe that e could do that and misunderstand the sign of the pressure related to photons. –  dmckee Feb 20 '13 at 14:55
You lost me there. I don't think MACHO is related at all, since it detects the gravitational influence of matter. EM radiation is no mystery, and the only measurement that might be non-trivial is the amount absorbed by a galaxy - reason being that the amount it emits dwarfs the amount it absorbs. Could the emission alone contribute to negative pressure? I don't know (but a good question). No one is getting the sign of the photon pressure wrong, and that statement makes me think I've not communicated. It has both positive gravitational and negative radiative contributions. –  AlanSE Feb 20 '13 at 15:11

I can only elaborate on what's already been said, but no, dark matter and dark energy do not exist as part as the CBR. The CBR we conventionally think of was constructed based on observed EM radiation. Dark matter and energy are only detected currently through their gravitational effects, and won't show up as radiation in the conventional effects, (although I am no expert, I am led to understand that under some models of the theoretical graviton particle, dark matter might radiate gravitons, but don't cite me on that).

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