We believe that Dark Matter (DM) interacts with ordinary matter gravitationally, as evidenced by galactic rotation curves among other things. But if DM affects matter then shouldn’t the reverse be true, particularly with reference to the super-massive black holes at galactic cores? Shouldn’t we be able to detect the consumption of dark matter by black holes? If a black hole were spewing forth energy without a matter accretion disk, would that not be actual evidence of DM?
Blackholes don’t emit energetic jets of energy (leave aside Hawking radiation for right now). It’s the interaction of matter through electromagnetic forces in the accretion disk that leads to the heating and emission of light. If you had a concentration of dark matter it would seem to me, given it only seems to interact gravitationally, the only evidence would be an expansion in the event horizon at a rate great than that expected by just consuming matter.
So the dark matter would not form an accretion disk?
- Maybe but I’m trying to think of an efficient mechanism for DM to transfer angular momentum with other particles so it can flatten out into a disk
- Even if it does, the fact that there is a DM disk doesn’t lead to an emission of energy. In a regular one yes, gas particle collide, get energized through EM interaction etc etc etc., but DM only interacts gravitationally - it can’t emit light.
I’m thinking that the DM would have a very hard time actually falling in to the black hole, since it can’t shed gravitational potential energy by interacting with other particles (except very weakly through gravity). Most particles would just orbit the black hole for a very long time.
So the action between matter and dark matter is not reflexive? That is, is it the case that DM gravitationally affects ordinary matter but the reverse is not true?
The energetic jets don’t arise from gravitational interaction, they arise from electromagnetic interactions that also allows for energy to escape from the particles. DM doesn’t interact outside of gravity. So sure rub a M particle against a DM particle and you’ll get a gravitation wave but of such a mind numbingly tiny amount you would never ever notice it.
No, gravity works between matter and dark matter the same way it works between matter and matter, but it doesn’t appear to interact through the other forces, and it is mostly interactions through those forces (or friction if you will) that makes the matter in the accretion disk heat up, emit energy and spiral into the black hole.
If a dark matter particle happens to stumble upon a black hole, it’ll be eaten, just like anything else. But black holes are very small targets, and so it’s very difficult for things to just stumble upon them, absent friction with other matter.
This is one way that physics in space works differently than our common experience. In most “human” situations, all systems leak energy indiscriminately to friction as heat, so, for example, thrown balls always stop eventually. The energy that was in the ball is all still there, but distributed between so many other particles that we’ve lost track of it.
In most situations in space, systems don’t leak energy. If a particle is spiraling into a black hole, the gravitational potential energy it is losing in doing so must be transferred to some other particle(s). Ejecting stuff from the vicinity of the black hole is an intrinsic part of the process of other stuff falling in, because the ejected stuff carries away the energy the falling in stuff loses.
For non-dark matter, the interactions which transfer energy from the in-falling stuff to the ejected stuff are electromagnetic (which includes “bumping into each other”, viscosity, friction and all that). For dark matter there are no electromagnetic interactions, so there is no way for it to shed energy to fall into the black hole (except gravitational waves, which is a super slow way to shed energy).
OK, so the dark matter doesn’t interact electromagnetically, but doesn’t it tug on the normal matter, which does shed energy that way? If a dark matter particle interacts gravitationally with a normal matter particle, and the normal matter particle can then radiate away some energy gained by the interaction, doesn’t that act like a ratchet that gradually reduces the energy of the system? After all, the dark matter can’t gain energy by absorbing a photon or through collision.
I suppose a dark matter particle could shed some angular momentum that way - but gravity is much much weaker than electromagnetism. An ordinary cloud of matter will shed angular momentum via electromagnetism much more effectively than dark matter can transfer angular momentum to ordinary matter via gravity.
Consider that it takes an entire planet to hold a paper clip down and only a cheap fridge magnet to pick it up.
This is technically incorrect (the worst kind of incorrect). There’s no problem carrying energy into a black hole. Point a particle directly at a black hole and it will keep accelerating (turning gravitational potential energy into kinetic energy) all the way in, carrying that (now kinetic) energy right into the hole. No need to carry any energy away at all.
What needs to be transferred away is angular momentum. If the particle is moving sideways relative to the hole, and it’s interacting only with the hole, there’s no way for that sideways motion to go away (since gravity only pulls straight towards the hole), and the particle will stay in orbit around the black hole. The only way for it to fall in is to bang into something, slowing its sideways motion.
Thanks. I was trying to come up with a clear way of saying that - but now you’ve done it, so I don’t have to…
How Black Holes Gobble Up Dark Matter
A very interesting article and worth the read.
This makes sense if you think about it. The Super Massive Black Hole in the center of the giant elliptical galaxy messier 87 is estimated to have a mass equivalent to six and a half BILLION solar masses! Really? It swallowed six and half billion stars to get that way? There couldn’t be that much regular matter within its gravitational influence to begin with.
Evidently there could be. The sun is a pretty small reference star, though. There are stars that are hundreds of solar masses all on their own.
But the article also says:
Definitely.
Some of the things it mentions should be eminently testable: for example for the runaway black hole scenario, look for massive but dim galaxies
Those would be incredibly difficult to find, though.