# Photons as dark matter

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1. Jul 22, 2015

### Docon

In general photons are not candidate for dark matter.
From the other hand stars constantly change matter into radiations (photons among them). And we can not measure radiation that does not hit us (although we can try to calculate it because radiation from star goes with same intensity in each spherical direction).

So my question is - are these answers below make sense? Is there any better answer why photons are not dark matter?

"Yeah, the only problem there is that we can observe photons. They make up about 0.001% of the energy density of the universe, far too little to explain dark matter."

"Since we know that the universe is filled with a bath of light we can figure out how much energy is in it and account for it in the models of the universe. This is done, and the dark matter that we talk about is extra missing mass on top of that."
(http://curious.astro.cornell.edu/ab...660-could-photons-be-dark-matter-intermediate )

2. Jul 22, 2015

### Staff: Mentor

Photons are not candidates for dark matter because dark matter is non-relativistic--i.e., whatever it is, it's composed of things moving much, much slower than the speed of light. Or, to put it another way, the pressure of dark matter is negligible compared to its energy density. Photons obviously do not satisfy this requirement.

3. Jul 22, 2015

### Chalnoth

Yes, those answers make sense. Another way to state it would be that photons cannot be captured by gravitational potential wells other than black holes (and then only very near to the event horizon). There is a lot of dark matter in galaxy clusters in particular. If that dark matter were made of photons, it couldn't stay within the cluster and would stream off into space.

4. Jul 22, 2015

### Chronos

This goes back to the hot dark matter issue which has already been dismissed as it fails to explain large scale structure formation
.

5. Jul 23, 2015

### newjerseyrunner

Photons have no mass, whatever dark matter is, it contributes gravity, only mass does that.

I assume that if you bind a photon to something, it won't add any extra mass. If you have a perfect fiber optic loop (meaning it never loses any photons) at exactly 1 gram, trapping any number of photons within it will not make the fiber any more massive will it?

6. Jul 23, 2015

### Staff: Mentor

This is not correct. Photons do have energy, and energy contributes to gravity. There are good reasons to rule out photons as candidates for dark matter, but this is not one of them.

You assume wrongly. See below.

Yes, it will. Each photon will add mass $E / c^2$ to the loop, where $E$ is the photon's energy in the loop's rest frame.

7. Jul 23, 2015

### newjerseyrunner

Thank you, very helpful.

8. Jul 23, 2015

### Docon

Thanks.

9. Jul 28, 2015

### ohwilleke

Somewhat restating the other objections, but if photons were dark matter then objects with lots of dark matter in them would be brighter at some wavelength than objections with little dark matter in them. If anything, the reverse holds true. Dwarf galaxies which are very dim have proportionately lots of dark matter, while nearly spherical elliptical galaxies which have proportionately little dark matter are very bright.