# SUSY and dark matter

1. May 30, 2012

### varphi42

Hello,

How could SUSY provides a candidate to dark matter since it would only appear only above the TeV scale? The galaxies' environments is at such energies?

Thanks

2. Jul 11, 2012

### varphi42

I think this is an answer to this question : the lightest supersymmetric particle (LSP) could be a massive and stable one with no color and no charge.

3. Jul 11, 2012

### phyzguy

The idea is that the dark matter particles were generated during the big bang when the average energy of the particles making up the universe (i.e. temperature) was larger than the mass of the LSP. As the temperature of the universe dropped below that point, these particles "froze out" and, since they are stable, they have just been carried along ever since, only interacting gravitationally.

4. Jul 15, 2012

### friend

If that's true, and production of dark matter ceased soon after the Big Bang, than the average density of dark matter should be decreasing with the expansion of the universe. And this should be measurable in terms of microlensing. Right?

5. Jul 15, 2012

### phyzguy

Yes, this is absolutely the case. Note that the average density of ordinary matter is also decreasing at the same rate as the universe expands.

Not necessarily. The density of gravitationally bound objects, like stars and galaxies, does not decrease as the universe expands. Microlensing refers to lensing by compact objects like stars and planets, and their density is not changing.

6. Jul 15, 2012

### friend

As I understand it, dark matter does not clump. But I see nothing to prevent it from orbiting around each other in tighter and tighter orbits as it looses energy due to gravitational radiation. And maybe there is a history of this in the microlensing.

Last edited: Jul 15, 2012
7. Jul 15, 2012

Staff Emeritus
Dark matter most certainly does clump. What it does not do is clump on scales small compared to a galaxy. While galactic formation is not entirely understood, it is commonly believed that galaxies form in regions of high dark matter density. In any event, this idea of dark matter "orbiting around each other in tighter and tighter orbits as it looses energy due to gravitational radiation" won't work out - if this were to happen outside the galaxy, it would also happen inside the galaxy, and we'd be seeing DM annihilation radiation from the galactic core.

8. Jul 16, 2012

### friend

I'm trying to consider tests that distinguish whether dark matter is a particle or a gravitational modification. My thoughts are: if it is a particle, then it orbits (by definition), and if it orbits, then it radiates gravitational waves and falls closer to the center of mass. I don't see how that can be avoided for any particle with mass. I think this means that it would eventually gravitate to the center of galaxies. I don't know what that means for its overall distribution.

9. Jul 16, 2012

### phyzguy

You're right that any orbiting particle will lose energy due to gravitational radiation and spiral in. However, the timescale for this to happen is extraordinarily long in the case of dark matter orbiting in galaxies. The timescale for orbital decay is give by:
$$\tau = \frac{5c^5 r^4}{256G^3 m_1 m_2(m_1+m_2)}$$

If you plug in numbers for a DM particle of mass 10 GeV, a galaxy of mass 1E12 Msun, and a radius of 1 kpc, this gives an orbital decay timescale of ~10^83 years, a ridiculously long time. Gravitational radiation is negligible except in the case of very massive bodies in very small orbits.

10. Jul 16, 2012

### friend

Thank you. But we don't know how much of the DM orbits on average in the same general location in the same general direction. Some have suggested that there are even small DM dwarf galaxies, that may be orbitting larger visible galaxies. That would change your calculation by quite a bit, I imagine.

Are there other measurable effects of moving matter. Frame dragging comes to mind. If on the average there is a lot of dark matter all orbitting in generally the same direction, then perhaps frame dragging effects may be discernable in the microlensing, right?

11. Jul 16, 2012

### phyzguy

The things you mention would change the calculation quite a bit, as you said. But no reasonable assumptions will get it anywhere close to the age of the universe, which is only 10^10 years. You would need to change the calculation by 73 orders of magnitude - this isn't going to happen. Frame dragging is a similarly weak effect which is completely negligible except in the close vicinity of very massive objects.

12. Jul 16, 2012

### friend

When we are talking potentially about accumulations about 5 times the mass of a host galaxy, I have to think there may be 73 orders of magnitude from a single DM particle. So by comparison with normal matter, should we expect gravitational radiation and/or frame dragging associated with the mass of an entire galaxy? Or is it moving much to slowly for that?

13. Jul 16, 2012

### phyzguy

Try putting in the numbers. Give me a proposed m1, m2, and r that brings the gravitational radiation lifetime down to 10^10 years.

14. Jul 16, 2012

### friend

If you can give me the units of measure used in the formula you gave, I might be able to work with that.

So the question is how close to the supermassive black hole at the center of a galaxy do you have to be in order to notice frame dragging, etc? And could the frame dragging effect be detected by microlensing?

Last edited: Jul 16, 2012
15. Jul 16, 2012

### phyzguy

Any units will work as long as you use a consistent set. Try using SI units (effectively the same as MKS units), for a start.

16. Jul 17, 2012

### Blackforest

I don't known so much on DM. Does it gravitate? If yes: does it follow the Newtonian law of motion? If no: can we built a link with some off-shell masses?

Concerning the above discussion: do you mean that a Thiring Lense effect could be the origin for the DM?

Another proposition: do you think DM could be a kind of relativistic effect (similar to the muon time of life problem?) and I mean it does not really exist but is a kind of artefact.