# Is Dark Matter light?

1. May 27, 2012

### excession

If this is a stupid question, just tell me.

As I understand it, even forms of pure energy can create a gravitational effect in high concentrations due to E=mc2. Would this have enough of an effect to create some of the anomalies prompting the search for DM, or has this already been taken into account?

Or, as is more likely, what have I misunderstood?

Cheers people

2. May 27, 2012

### Simon Bridge

It has already been taken into account.

Lay-friendly description:
http://curious.astro.cornell.edu/question.php?number=446 [Broken]

Last edited by a moderator: May 6, 2017
3. May 27, 2012

### Dickfore

Dark matter does not participate in electromagnetic interactions, hence it is dark (does not emit any form of radiation that can be absorbed). Photons interact through the electromagnetic interaction, but not ordinary Coulomb interaction because they are not charged. The main http://mightylib.mit.edu/Course%20Materials/22.01/Fall%202001/photons%20part%201.pdf [Broken] through which a flux of photons loses energy are:
• Photoelectric effect
• Compton scattering
• Pair production

Last edited by a moderator: May 6, 2017
4. May 27, 2012

### ImaLooser

Radiation always travels in a "straight" line at the "speed of light." It cannot remain in one place, it disperses if not contained. Dark matter isn't contained and doesn't disperse, so it can't be radiation.

5. May 28, 2012

### excession

Cheers for that, makes a bit more sense now.

Thanks for taking time to reply.

6. May 28, 2012

### twofish-quant

One other thing is that you can model different types of matter by their what's called an "equation of state". Basically it's how much pressure that you get if you squeeze it, and that results in a number called w. For ordinary matter, w=0, which means that the pressure is inversely proportional to the volume. For light, w=1/3, which means that if you squeeze it, then the pressure goes up faster than the inverse of the volume.

One way of thinking about it, is that if you compress gas, it gets hot, when something gets hot, the number of photons it produces goes up very, very quickly.

So you put all of this into your computer program, you figure out the lumpiness factor, and you find out that if dark matter was something like photons, then the universe would be pretty smooth. That rules out dark matter not only being photons, but it also means that it's not a particle like weird neutrinos or anything else "hot." Whatever dark matter is, it's "cold."