# 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?

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

Photons are easily detectable. We can count how many photons are there at any distance of us by just counting the photons reaching us from there. It is impossible that the hidden photons ramble the whole universe but mysteriously avoid us.

-

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!

-
 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.

-
-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
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