1. Dec 13, 2011

### Routaran

http://hitoshi.berkeley.edu/public_html/susy/susy.html

and one of the things it suggested was that "One of the best candidates for Dark Matter is the lightest supersymmetric particle."

So i have a question. (doesnt have anything to do directly with the article itself but...)
Given: We have a lot more dark matter than regular matter.
There's enough gravitation generated by regular matter so that it can collapses into black holes. I realize that dark matter is supposed to be very weakly interacting with normal matter but i haven't heard/read anyone comment on how it interacts with itself. We have enough dark matter so whats preventing it from collapsing into a dark matter black hole for example?

Basically, why is gravity able to lump up regular matter on a small scale (stars & black holes) and dark matter only appears to lump up when viewed spread over large distances like the scales of galaxies or clusters of galaxies?

Last edited: Dec 13, 2011
2. Dec 13, 2011

### Nabeshin

Precisely because Dark Matter does not interact even with itself very strongly (if at all). What allows normal matter to condense into such dense blobs (stars, black holes, planets), is the fact that as it runs into itself, via friction it loses energy as heat. Thus, the 'orbits' if you will, tend to decay until everything has just fallen together. Without this process, things simply do not collapse.

3. Dec 13, 2011

Interesting question and interesting answer. Wouldn't the lack of friction in dark matter prevent it from ever settling into a stable position with baryonic matter? That is to say the dark matter would forever oscillate back and forth past a stationary mass making it near impossible for dark matter to surround a mass, only visit it like a comet visits the sun.

4. Dec 13, 2011

### Nabeshin

Probably, but when you have a whole lot of dark matter, the motion of the individual particles is irrelevant and taken in the bulk it forms a large halo, as we see in our galaxy. The thing to note is that the kinetic energy of the DM particles is too high to be confined to a small radius (Solar system size), but does fall into the much larger potential well formed by the galaxy (which is of course composed mostly of dark matter).

5. Dec 14, 2011

### Chronos

Dark matter is essentially frictionless so it cannot shed velocity via collisions as does baryonic matter. So, as Nabeshin noted, it does not achieve any meaningful core condensity. For that reason it tends to spend more time near the periphery of a gravitating system [e.g., form a halo].

6. Dec 14, 2011

### sandu123

Would anybody please tell me why dark matter has remained so elusive? Whats the actual composition and what are the practical difficulties when dealing with studying it??

7. Dec 14, 2011

### phinds

It is elusive. We don't KNOW the composition. If we did, it would not be elusive.

8. Dec 14, 2011

### phinds

Routaran, you might find it interesting to google "the bullet cluster"

9. Dec 14, 2011

### sandu123

Thanks for replying.I have heard that there are a lot of contenders for dark matter like neutralinos.If there is so much dark matter,then why is it difficult to detect?I also have a doubt regarding whether dark energy aids in accelerating the expansion of universe.Would someone please find some time to clear my doubts?

10. Dec 14, 2011

### phinds

It is difficult to detect because it is elusive. That is, it does not interact with normal matter or with itself in any detectable way other than gravity. The current consensus seems to be that it will turn out to be some form of WIMP.

THe universe is expanding. That expansion is accelerating. Those are facts.

We do not know WHY the universe is accelerating its expansion, so we call the cause of it "dark energy" which is shorthand for "we do not have a clue WHY it is happening". Thus to say that dark energy does not aid in the acceleration is just silly. Dark energy is just the name we give to WHATEVER it is that is causing the acceleration.

11. Dec 14, 2011

### bapowell

The term "dark energy" is more than a stand-in for "whatever" is causing the expansion. For example, there have been investigations into whether the geometry side of the Einstein Equations can be modified to accommodate accelerated expansion on large scales. Higher-order curvature terms and other modifications to the geometry that lead to acceleration would not be considered "dark energy".

As an energy source in Einstein Equations, dark energy is required to satisfy certain properties: it must be homogeneous and satisfy $p < -\rho/3$. Indeed, we don't know what this stuff is, because we have no empirical traction on that question as of yet. But it's not quite correct to say we have no idea; vacuum energy (modulo a pesky 120 orders of magnitude) has the right properties to cause accelerated expansion. And if fundamental scalars do exist in nature (I'm looking at you Higgs) that would open the plausibility door to the presence of rolling scalars in false vacua.

12. Dec 18, 2011

### sandu123

Thank you all for your valuable replies.
Is dark energy same as the cosmological constant Einstein mentioned in his equations? If so,it would turn out that he was right,after all.

13. Dec 18, 2011

### bapowell

In general, dark energy density can vary in time as the universe evolves. The special case of constant dark energy density behaves like the cosmological constant in Einstein's Equations.

Einstein introduced the cosmological constant to render a static universe, not an accelerating one. So, no. He did not anticipate the current accelerated expansion of the universe with this introduction of the CC.

14. Dec 19, 2011

### sandu123

Thanks a lot:)