# Why can't missing mass be photons?

After a star lives and dies, I assume virtually all of it's mass would be photons. If enough stars have already lived and died, couldn't there be enough photon energy out there to account for all the "missing mass" (=dark matter) in the universe?

And if there were enough photons to account for all the missing mass, what would it look like to us?

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Only a tiny fraction of a star's mass ever converts into photons. – David Zaslavsky Nov 29 '12 at 7:28

As a general rule, zero mass particles which travel with the velocity of light are not good for dark matter, because dark matter concentrates around gravitational attractors. It has to be particles with some mass that can be at rest in order to stay around a galactic center from the beginning . In addition they have to be controlled by weak interactions, if they decay, because the dark matter halo is stable for long periods.

Maybe I should add that very cool and slow photons from the beginning of the formation of the observed universe exist and have been detected as Comsmic Microwave Background radiation , very low frequency photons, uniformly distributed in the cosmos.

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Thanks for your answer. If there were a sea of photons, would they not also concentrate around gravitational attractors? Wouldn't there be more photons in the potential energy well of the gravitational attractor than there would be in flat space? Wouldn't this situation be stable? Are particles with mass really the only candidates? – Tom Fangrow Nov 29 '12 at 6:52
Aside from the unstable orbit at $r = 3M$ you can't bind photons in a gravitational well. The reason is simply that they always travel at $c$, while particles with mass can take any speed. That's why planets can have stable orbits round the Sun. There is a single distance from a mass where the velocity of light matches it's orbital velocity, but even this orbit is unstable so you'd never find any significant concentration of photons there. – John Rennie Nov 29 '12 at 7:15
As John says; a particle has to have some mass to stay at an orbit.CMB does not concentrate around galaxies, and that is the coldest photons we have observed, because they still travel at the velocity of light. – anna v Nov 29 '12 at 7:31

There is a simple argument why photons emitted by stars can't be dark matter, and that's because there is about ten times more dark matter than normal matter. If all the stars created at the Big Bang had turned into photons there still wouldn't be enough of them.

You might argue that maybe more normal matter than we think was created during the Big Bang, but the theory of Big Bang Nucleosynthesis places a limit on how much normal matter was created, and this limit is four times smaller than the amount of dark matter. The dark matter has to be something odd.

If you're interested in more info this paper is a good review, though harder going than the answers here!

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 Thanks for this, I'll check it out. – Tom Fangrow Nov 29 '12 at 7:31

Photons don't have mass. So your assumption's incorrect, although I don't know how much of a (say) main sequence star's mass gets converted into photons over its lifespan.

Photon's can't account for, say, dark matter, because dark matter has mass.

Thermal energy (in the vacuum) is comprised of photons, which can spontaneously form particle-antiparticle pairs. Usually these quickly annihilate, so this is also not a good source of mass.

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-1. Photons don't have mass, but they do have energy. It is fully possible that during a star's lifespan, some amount of it's mass is converted into energy (i.e., photons). – Kitchi Nov 29 '12 at 6:32
The assumption was that all a star's mass is converted to energy in the form of photons. Humor me for a moment. You say dark matter has mass. How do you know? Is it because it affects gravity? Do we know for certain that photons do not affect gravity? – Tom Fangrow Nov 29 '12 at 6:44
Photons do affect gravity, but that's because gravity is affected by energy, not mass. – David Zaslavsky Nov 29 '12 at 7:28
Photons do affect and are affected by gravity ( gravitational lensing) they cannot be captured by gravitational wells because of the velocity c. See Johhn's comment in my answer. – anna v Nov 29 '12 at 7:33
@Ryan - yes, in the absence of mass gravity is affected by energy and even unlikely things like pressure. The source of the curvature is the stress-energy tensor (en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor) and this makes no distinction between mass and energy. They are treated as related by the (in)famous $E = mc^2$. – John Rennie Nov 29 '12 at 10:50