# Temperature of Dark Matter

1. Feb 8, 2006

### marcus

http://news.bbc.co.uk/1/hi/sci/tech/4679220.stm

Gerry Gilmore's Cambridge team did a survey of clouds of dark matter, using several telescopes including VLT in Chile. they found no blobs of DM smaller than about 30 million solar masses, or less than 1000 lightyear diameter.

They inferred from this a characteristic TEMPERATURE of dark matter. the particles must be moving on average 9 kilometer per second, so that clouds that are not big enough DISPERSE by random motion.

The clouds that are not big enough don't have enough gravity to hold themselves together, given the particles' speed of random motion.

2. Feb 8, 2006

### SpaceTiger

Staff Emeritus
Just to prevent confusion, these blobs of dark matter are dwarf galaxies -- that is, they contain baryons as well and emit visible light.

3. Feb 8, 2006

### marcus

the baryonic matter is how they see the DM (to recap SpaceTiger previous)
but the DM is overwhelmingly more of the mass in the cloud

IIRC more than an order of magnitude more like two orders
(the dwarf galaxy baryonic fraction is roughly like a percent)

here is the journal article preprint by Gilmore et al
http://arxiv.org/abs/astro-ph/0602186

4. Feb 8, 2006

### SpaceTiger

Staff Emeritus
Yes, but this is the case with pretty much all galaxies. I just didn't want people to think that these objects were completely dark. There have been several claims in the literature of "dark galaxies" (no detectable starlight), which are another thing entirely.

5. Feb 9, 2006

### Chronos

Interstingly enough, a related paper was released today. It relies on velocity distributions in galactic clusters as an indicator. It's a good study, IMO:

Velocity distributions in clusters of galaxies
http://www.arxiv.org/abs/astro-ph/0602197

6. Feb 10, 2006

### Entropy

:grumpy: I really don't like people labeling clouds of gas that don't reflect visible light as dark matter. Dark matter is supposed to be an unknown material that makes up 90% of the universe that we can't see (possibly non-baryonic matter), and this stuff isn't that 90%. This stuff is just regular old dust floating in space that doesn't happen to reflect visible light.

7. Feb 10, 2006

### SpaceTiger

Staff Emeritus
Oh no, I don't think anyone is doing that. Dwarf galaxies do have a lot of dark matter and that is what they're talking about in the article.

8. Feb 11, 2006

### Chronos

I second that notion, ST. Spectral analyses rule out gas clouds.

9. Feb 11, 2006

### Garth

Nevertheless, in the standard $\Lambda$CDM model there is a lot of unseen ordinary baryonic matter, over an OOM greater than that which can be seen.
$\Omega_b$ = 0.04
$\Omega_{visible}$ = 0.003

Garth

10. Feb 11, 2006

### jhe1984

So does dark matter have a temperature (or temperature range)?

11. Feb 12, 2006

### Mk

We kind of beated around the bush so far, according to the BBC article, the particles were 10000 Kelvins.

12. Feb 12, 2006

### jhe1984

Whoa, that's hot!

If this makes up cold, dark matter - how hot is hot dark matter?

And why is dark matter so hot?

Is it because it absorbs e/m radiation but does not emit any - like the ultimate black t-shirt?

13. Feb 12, 2006

### marcus

I am waiting for Spacetiger to take over and clarify these issues. I will tell you very briefly and unauthoritatively what I think.

I think that the word "temperature" applied to DM refers mostly to the VELOCITY of the presumed particles (with a little mathematical finagling like squareroot)
thus, in the scientist discussion, the word is being used in a way that may be unfamiliar.

1. there may be no DM particles, the whole DM effect may be due to a modified gravity law like MOND, but assume for sake of argument that there are particles

2. since they have mass, you can assume a mass and compute their KINETIC ENERGY in the usual way

3. "temperature" is more or less equiv to AVERAGE KINETIC ENERGY of random motion of a bunch of things

4. so if you have an idea of the mass of the presumed DM particle you can translate the "9 kilometer per second" information into "kelvin" information.

5. but it is just another way of talking about the speed of random motion and it doesnt add anything to the discussion to make that translation BECAUSE the presumed DM particles DO NOT INTERACT WITH LIGHT

6. since there is no interaction with EM radiant energy the DM can be any "temperature" you want and it will not absorb or radiate heat, so it doesnt do all the equilibrium heat stuff that we are used to and that we expect to have associated with "temperature"

7. so, like, forget ordinary temperature. that is just how they TALK. what matters is if there are DM particles there may be some bounds on their speed that you can infer from observing the size clouds of DM------since more or less they coincide with their galaxies you can more or less see how big they are and make reasonable assumptions and derive a range of speeds for the particles

Last edited: Feb 12, 2006
14. Feb 12, 2006

### jhe1984

Okay, I think I am following you thus far.

But then that brings up another basic question - even though we could not see or feel it's heat, could we 'run into' dark matter?

I understand scale makes this basically impossible, but I'm assuming in this metaphor that we can 'run into' something like a neutrino, even though in reality it'd go right through us.

Or, put another way, if I had a complete wall of dark matter and I shined a flash light at it, would someone on the other side be able to see that light, in the sense that light'd pass through it - not counting things like gravitational lensing?

The answers might be different for each question, I guess.

15. Feb 13, 2006

### Garth

Hot DM refers to a near relativistic average velocity, one so that the DM particles do not clump at all. Neutrinos would be an example of such, only they do not have enough mass to make up $\Omega_{DM} \approx 0.23$, and DM is required to clump to form large scale and galactic structures, so they are the wrong sort of DM.

If DM were too cold then it would clump too readily at galactic centres producing the 'cuspy' problem, it therefore has to be at the right temperature to make it do what it is meant to, 10,0000K is about right.

In model building this intellectual process is called 'parameter fitting', or on the other hand, until that is, a DM particle is discovered in the laboratory with the required properties, you might call it 'adjusting the epicycles to 'save the appearances''.

Garth

Last edited: Feb 13, 2006
16. Feb 13, 2006

### jhe1984

Ahh, so it's a lot like Goldilocks: not too hot and not too cold.

And without getting into specific proofs - are there at least some folks who are concerned that what we're seeing in observations and considering to be fundamental isn't just a consequence of the ways we are measuring (and have these concerns been discussed and ruled out)?