Is the behavior of dark matter contradictory to the formation of black holes?

[cealicu_ca]

Hello,

Firstly, i would like to say that i am by no means an educated person (as in taking specialty courses or an university degree in cosmology or physics), at least regarding this subject. From this point of view, what i am about to ask might seem silly or just plain stupid, so feel free to "bash" this "noob" as much as you like .

Nevertheless, i am quite enthusiastic when it comes to the universe, and i try as much as i can to keep me informed on the subject, watching every documentary i can put my hands on and so on an so forth.

Also, as my primary language is not English, please forgive all, if any mistakes i make. I trust there won't be that many as to alter the idea or message i am trying to deliver.

Getting to the point:

Regarding dark matter there seems to be something wrong about it. As far as i know (and this might be the problem - that i don't know or understand enough), dark matter behaves just like ordinary matter when it comes to gravity but does not interact (or at least interacts very hard) with both ordinary matter and itself. The thing is, that considering how much dark matter is supposed to be, and due to the fact that it does not interact with itself or ordinary matter, how come dark matter does not "clump" together to form massive objects that, in turn, would lead to singularities?

Since it does not interact with itself, there would be no force or pressure that would oppose gravity, so the formation of singularities would be pretty favored.

Such large masses would "suck" up, indiscriminately, both dark matter and normal matter. So how come there are still galaxies, and stars, and, well... any kind of matter (for that matter ) in the universe. How come there is no "abundance" of black holes?

Thank you.

 

[stevebd1]

Welcome to PF. You might be interested in the following thread that looks at the notion of dark matter black holes (the discussion starts to focus on DM BHs at about post #7)-

https://www.physicsforums.com/showthread.php?t=278374"

 

[phyzguy]

The simple answer is that because dark matter does not interact (or at least not very strongly), it has no way to shed its orbital energy and orbital angular momentum and fall into a black hole. Think of the solar system - why don't the planets fall into the sun? The answer is because they have orbital angular momentum around the sun, and since they have no way to lose it, they orbit the sun more or less forever. If a dark matter particle approaches a black hole, unless it is aimed directly at the black hole, it just orbits around it and goes back out again. A small amount of the dark matter is aimed directly at a black hole, and it does fall in, but most of it doesn't. There are many simulations confirming this behavior.

 

[cealicu_ca]

The simple answer is that because dark matter does not interact (or at least not very strongly), it has no way to shed its orbital energy and orbital angular momentum and fall into a black hole. Think of the solar system - why don't the planets fall into the sun? The answer is because they have orbital angular momentum around the sun, and since they have no way to lose it, they orbit the sun more or less forever. If a dark matter particle approaches a black hole, unless it is aimed directly at the black hole, it just orbits around it and goes back out again. A small amount of the dark matter is aimed directly at a black hole, and it does fall in, but most of it doesn't. There are many simulations confirming this behavior.

Ok, it seems clear that it would be improbable for dark matter, by itself to form black holes. But it still leaves the problem of dark matter and existing black holes.

The planets don't fall into the sun because they conserved the momentum of the particles or gas and/or rubble (as a system) from the time when they were created - and thus they are in stable orbits around the sun, the gravitational pull of the sun is canceled out by their orbital momentum. They don't just "get" their momentum out of nowhere.

You are mentioning the orbital momentum, that is to say that the particle is already in an orbit in respect to the singularity.

When you calculate the momentum (L = r x p = r x m x v; sorry for the scalar notation) you use the singularity as origin. But the particle does not need to be "aimed" at the singularity to fall into the black hole, it simply has to have it's speed vector (v) on a path that intersects the event horizon (or at least tangent to it).

If we represent the event horizon on a small scale (relative to the size of the particle), in a 3 dimensional way, we could use a plane as a model (in 2 dimensions, a simple line):

...| -> event horizon
...|
O...| X - dark matter particle
...|
...|

Now, the only way such a particle would escape the black hole is to have it's vector parallel in respect to the plane (otherwise it would intersect the event horizon, either in the past - if the vector's direction is intersecting the event horizon and it's way is from the origin (O) towards x, or in the future - if the vector's direction is intersecting the event horizon and it's way is from X towards the origin (O)).

So dark matter near the event horizon would behave in 3 ways:

a) Most of it falls into the black hole (upwards conditions)
b) Some of it is accelerated by the gravitational forces of the black hole and shot out into space.
c) Very few of it orbits it endlessly (if it just happens that it enters a stable orbit).

True, the more we move away from the event horizon, the more of it escapes the black hole (either it's shot of - accelerated by the gravitational forces, or enters a stable orbit). but that still should leave a huge amount of dark matter falling into the black hole. To me it's like the event horizon of a black hole should have something like a halo around it (the more you approach it - the event horizon - the less dark matter there should be). Wow, we might use this to detect the event horizon

 

[Chalnoth]

Well, black holes make up a tiny, tiny fraction of the matter in the universe. Plus they're really tiny, so it's very easy for matter to miss one. They don't "suck" matter in.

 

[phyzguy]

You're right that if the particle's velocity vector intersects the black hole event horizon, then it will be captured. But like Chalnoth said, black holes are quite small. The event horizon of a solar mass black hole is only 3 km in radius. So for a random particle approaching a black hole from infinity, the probability that its orbit intersects the event horizon is quite small, and most particles just make a hyperbolic orbit around the black hole and go back out.