Why doesn't dark matter fall into black holes ?

[Robin]

Why doesn't dark matter fall into black holes ?
Assuming it doesn't of course.
If it did then surely huge amounts would have fallen into black holes.
By now they'd be full of the stuff & more massive than they are.
So I'm assuming for some reason dark matter doesn't fall into black holes.
In which case it doesn't obey the normal laws of gravity like ordinary matter which falls towards centres of mass thus forming clumps that we call planets, stars, black holes etc.
Why is this ?

 

[George Jones]

Dark matter does fall into black holes. As normal matter falls towards a black hole (electromagnetic) frictional forces heat up the matter, which then radiates away energy. This helps normal matter spiral into a black hole. Dark matter does not interact through electromagnetism and doesn't experience these frictional forces. Dark matter does fall into black holes, but, because of the lack of friction, a black hole presents a smaller effective target for dark matter than it does for normal matter.

 

[marcus]

...
In which case it doesn't obey the normal laws of gravity like ordinary matter which falls towards centres of mass thus forming clumps that we call planets, stars, black holes etc.
...

George Jones completely answered your question about DM falling into BH, but the general topic of how DM gathers and coagulates is fascinating. According to computer simulations, DM was able to play an important role in structure formation in the early universe

Google "smoot TED" for an 18 minute video talk about this, with good slides and animations.

It is indeed harder for DM (than for ordinary matter) to gather and fall into higher density regions and stay there, bound.

It has to be able to blow off the extra energy, otherwise it will fall into the region of concentration and then just sail right on thru and out again.

Nevertheless DM is able to collect, and the models show it actually being the BASIS of structure formation in the early U! Because there was more of it. According to the model, it was the first to form the wispy cobweb armature of crisscross strands which then permitted ordinary matter to collect. Where DM strands crossed, clusters of galaxies formed. DM curdling provided the seeds of structure. At least that is the story George Smoot tells, and illustrates with computer sims.

So how does DM manage to collapse into higherdensity cobwebby strands? How does some of the infalling stuff manage to blow off excess kinetic energy so that it can stick around?

By gravitational interaction within the DM cloud which randomly drains energy from some of the DM and pumps it into other DM. The loser DM can stay and become bound in the growing structure, the gainer DM sails out of there with an extra kick.

Expansion will eventually cool or drain kinetic even from that (initially hyperactive) portion. So eventually all the DM settles down and participates in structure formation. Expansion is the overall main cooling engine.

That's just an impressionistic take on Nobelist George Smoot's talk at the TED event. Not an expert myself on early U structure formation, all I can do is say that the puzzles are intriguing and the ideas being tried out are exciting. I't a beautiful subject.
I'd encourage anyone to check it out and get acquainted with it.

 

[Robin]

Dark matter does fall into black holes. As normal matter falls towards a black hole (electromagnetic) frictional forces heat up the matter, which then radiates away energy. This helps normal matter spiral into a black hole. Dark matter does not interact through electromagnetism and doesn't experience these frictional forces. Dark matter does fall into black holes, but, because of the lack of friction, a black hole presents a smaller effective target for dark matter than it does for normal matter.

Some of the spiralling in due to gen relativity effects I believe.
I guess dark matter will be subject to this effect.
Still it's interesting that when people talk of a black hole being 4 million solar masses, or however much, as calculated by observing the movement of stars at the centre of our galaxy. Does anyone know how much of the mass is from stars & dust that's been gobbled up & how much of the mass is dark matter ?
I mean I've never heard any reference whatsoever on documentaries (often dumbed down of course) to dark matter falling into black holes !
If DM doesn't interact with normal matter then I imagine it can fall in one side of a black hole & practically come out the opposite side as there's no friction as it passes through.

 

[Robin]

By gravitational interaction within the DM cloud which randomly drains energy from some of the DM and pumps it into other DM. The loser DM can stay and become bound in the growing structure, the gainer DM sails out of there with an extra kick.

That's a fascinating phenomenon. Rather like evaporation. I'll Google "smoot TED". DM is fascinating as it's so different to normal matter. Makes one realize how much a part EM forces play in the way ordinary matter behaves in space. Thanks for your reply.
One other question. Why doesn't DM fall along the filaments to wind up being all located at the beads where the filaments meet ?