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I think I saw in a video that if dark matter wasn't repulsive to dark matter, it would have formed dense massive objects or even black holes which we should have detected.

So, could dark matter be repulsive to dark matter? If so, what are the reasons? Could it be like the opposite pole of gravity that attracts ordinary matter and repulses dark matter?

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Wait. Isn't it sufficient that the non-gravitational interaction cross-section is mindbogglingly low? Because in the current model it certainly is gravitationally attractive. – dmckee Apr 30 '11 at 15:59

2 Answers 2

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Lubos Motl's answer is exactly right. Dark matter has "ordinary" gravitational properties: it attracts other matter, and it attracts itself (i.e., each dark matter particle attracts each other one, as you'd expect).

But it's true that dark matter doesn't seem to have collapsed into very dense structures -- that is, things like stars and planets. Dark matter does cluster, collapsing gravitationally into clumps, but those clumps are much larger and more diffuse than the clumps of ordinary matter we're so familiar with. Why not?

The answer seems to be that dark matter has few ways to dissipate energy. Imagine that you have a diffuse cloud of stuff that starts to collapse under its own weight. If there's no way for it to dissipate its energy, it can't form a stable, dense structure. All the particles will fall in towards the center, but then they'll have so much kinetic energy that they'll pop right back out again. In order to collapse to a dense structure, things need the ability to "cool."

Ordinary atomic matter has various ways of dissipating energy and cooling, such as emitting radiation, which allow it to collapse and not rebound. As far as we can tell, dark matter is weakly interacting: it doesn't emit or absorb radiation, and collisions between dark matter particles are rare. Since it's hard for it to cool, it doesn't form these structures.

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Thank you for your clear answer :) So assuming something like our sun is dense enough to pull dark matter towards itself, dark matter would get attracted, but it would pass through the sun because it can't collide with it. What about black holes? Aren't they dense enough to hold dark matter in? And also, if dark matter doesn't radiate as it is falling into a black hole, then large amounts of dark matter would fall into black holes, and that would make the black hole grow too fast, perhaps faster than what we have observed, right? – Aria Apr 30 '11 at 18:41
@Aria that's actually a good point. I've seen some work on the topic of dark matter in stellar cores which stars have swept up in their passage through space. – user346 Apr 30 '11 at 18:48
In the simplest models, dark matter particles are attracted to the Sun and fall into it but aren't trapped. The density of DM particles in the Sun is higher than elsewhere, but not vastly higher. The same goes for the Earth, to a lesser degree. This is relevant to people who do laboratory searches for dark matter particles. There are some DM models in which self-interaction (essentially collisions between DM particles) allow significant numbers of DM particles to get trapped in the core of the Sun. These models should have measurable effects on the Sun's behavior and so can be tested. – Ted Bunn Apr 30 '11 at 19:55
Oh, and for black holes: if anything crosses the horizon, then it doesn't escape. That applies to dark matter as well as ordinary matter. But you still don't expect black holes to swallow up vast amounts of dark matter particles, compared to the amount of ordinary matter. A lot of the stuff that falls into a black hole does it via an accretion disk: the matter swirls around the black hole, losing energy by friction until it falls in. Dark matter particles don't have friction, so this doesn't happen to them. For a dark matter particle to fall in, it has to be aimed correctly to hit the horizon. – Ted Bunn Apr 30 '11 at 19:58
and just to add to @TedBunn's last comment, note that you have to be EXTREMELY accurate to hit the horizon -- the sun's Schwarzschild radius is abut 3 km. The size of that bullseye on a Earth's - orbit sized dartboard is smaller than an atom (earth's orbit is 1,500,000 km). – Jerry Schirmer Sep 10 '14 at 17:09

Dark matter surely has to carry a positive mass, and by the equivalence principle, all positive masses have to exert attractive gravity on other masses.

Also, from the viewpoint of phenomenological cosmology, we obviously want dark matter to attract itself. It has to attract visible matter because this is why dark matter was introduced in the first place: it helps to keep the stars in a galaxy even though they're orbiting more quickly than one would expect from the distribution of visible mass in the galaxy.

For this reason, the force between dark matter and ordinary is surely attractive. The force between dark matter and dark matter has to be attractive, too. In fact, dark matter has played the dominant role in the structure formation - the creation of the initial non-uniformities that ultimately became galaxies, clusters of galaxies, and so on. The dark matter halos are larger than the visible parts of the galaxies: the visible stars arose as "cherries on the pie" near the centers of the dark matter halos.

There's no doubt that the gravitational force between any pair of particle-like entities is attractive. This is linked to the positive mass i.e. positive energy - which is needed for stability of the vacuum (if there existed negative-energy states, the vacuum would decay into them spontaneously which would be catastrophic and is not happening) - and the basic properties of general relativity. In particular, there's a lot of confusion among the laymen whether antimatter has an attractive gravity. Yes, of course, the matter-antimatter and antimatter-antimatter gravitational forces are known to be attractive, too.

The non-gravitational forces between dark matter are almost certainly short-range forces. In particular, dark matter doesn't interact with electromagnetism, the only long-range non-gravitational force (mediated by a massless photon) we know - that's why it's dark (emits no light).

The only repulsive force that arises in similar cosmological discussions is one due to dark energy - or the cosmological constant, to be more specific. Dark energy is something very different than dark matter. This force makes the expansion of the Universe accelerate and it is due to the negative pressure of dark energy which may be argued to cause this "repulsive gravity". However, dark energy is not composed of any particles. It's just a number uniformly attached to every volume of space.

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Exactly. But we still need to know why dark matter doesn't form dense objects itself. It has attractive gravity, and there is no electromagnetism to push it's particles apart. – Aria Apr 30 '11 at 17:59
I like your usage of the term "dark matter was introduced". Not discovered, not invented, but introduced. Very profound! – dotancohen Mar 2 '12 at 8:04

protected by Qmechanic Sep 10 '14 at 16:59

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