Dark matter around black holes.

[halfeye]

https://science.slashdot.org/story/23/03/28/0036248/black-holes-may-be-swallowing-invisible-matter-that-slows-the-movement-of-stars

Does this make a difference to how we envision the universe?

[Anymage]

From a skim of the paper, it might allow us to better model orbital mechanics of sufficiently massive objects. Although given that we haven't seen the result in our solar system the result is likely insignificant on the scale of a planet in a solar system. And of course the paper is still early and speculative so there's a good chance that it doesn't stand up to more rigorous testing. (Which is no shade against the paper or the researchers. Throwing out interesting ideas and then being open to harsh scrutiny is how science works.)

It doesn't posit any new characteristics of dark matter so doesn't change our thoughts on that in the slightest.

[Mastikator]

If true then dark matter is capable of friction. Which AFAIK is new information about dark matter. Most models of what dark matter might be proposes that it doesn't do friction since it doesn't seem to clump up into stars and planets like atoms do.

[gbaji]

If true then dark matter is capable of friction. Which AFAIK is new information about dark matter. Most models of what dark matter might be proposes that it doesn't do friction since it doesn't seem to clump up into stars and planets like atoms do.

I think you may be misunderstanding their use of the word "friction". They aren't talking about what we normally think of as friction (two objects physicaly interacting, which we model using EM interactions), but "gravitational friction", which is already exactly how we infer the existence of dark matter in the first place.

The key takeway from this isn't some new property of dark matter, but a possiblity that large amounts of dark matter "clump" around black holes, inferred by the same gravitational effects that we use to detect dark matter in general (ie: We see an effect that can only be explained by a gravitational force the source of which we can't see, we say "must be dark matter").

And honestly, it's not that far out of the predicted behavior of dark matter anyway. Assuming we're even remotely correct with our models, dark matter is "stuff" that interacts gravitationally (has mass, or something similar to mass), but not via EM. So the usual means we detect matter (shiine some kind of light at it), doesn't show up. And other methods (see what other matter collides with it), also doesn't work. The real trick and question about dark matter isn't that it collects around other gravity wells (because "gravity sucks" right?), but why every scrap of it isn't already embedded within every single existing gravitational source in the universe already. If that were the case, we'd just think that "matter" has higher gravitational effect then it does based on what we detect, and we'd never notice it.

It's the fact that dark matter does appear to actually be "spread out" across the universe, and only seems to clump around gravity wells "somewhat, but not completely like we might expect" is what is really the mystery here. It does appear as though dark matter itself has some sort of repulsion force that only actually affects other dark matter. How strong that force is? Still up to discovery. But yeah, detecting it and maybe even measuring it around black holes can help us get some measurements on the stuff.

[Chronos]

Nah, you wouldn't expect dark matter to clump up around other masses. It might fall down towards a black hole, but unless it hit the hole dead-on (which is extremely unlikely; black holes are a very small target), it'd just swoop past and fly back up out of the gravity well again. Stuff only accumulates in gravity wells if there's something to slow it down so it can't get back out. Dynamical friction can do that to a degree, if the neighborhood of some object is especially lumpy and cluttered. So that's where you'll see dark matter accumulating, around objects that already have a cluttered neighborhood.

[gbaji]

Nah, you wouldn't expect dark matter to clump up around other masses. It might fall down towards a black hole, but unless it hit the hole dead-on (which is extremely unlikely; black holes are a very small target), it'd just swoop past and fly back up out of the gravity well again. Stuff only accumulates in gravity wells if there's something to slow it down so it can't get back out. Dynamical friction can do that to a degree, if the neighborhood of some object is especially lumpy and cluttered. So that's where you'll see dark matter accumulating, around objects that already have a cluttered neighborhood.

I suppose that depends on how we model the initial momentum of dark matter, how discrete it is (large objects moving around, or a gazillion teeny tiny individual ones), and to what degree dark matter interacts with other dark matter. One would expect it would start out in the same "cloud" form as the rest of matter, gaining spin via gravity initially, and as "normal matter" coalesced over time into discrete gravity wells, one would expect the same gravitational effects to cause dark matter to gather around the same locations as well. We should expect to see clouds of dark matter orbiting gravity wells, but with different densities most likely in direct proportion to the size of said well.

We should also expect a relatively uniform distribution of dark matter. Otherwise we'd see much more obvious and varied gravitational effects resulting from it. Such a uniform distribution can cause the effects we've observed, since as we move farther away from the "center" of a gravity well, the density of the "real" objects orbiting it decreases (take the total mass orbiting at a given distance from a gravity well relative to its obit's "size" and it'll always be smaller. Space gets more empty the "farther out" you go from any "center point"). So in "really large" systems (like galaxies), the effect of the gravitational force at the center of that system is too small to account for the orbital velocity of distant star systems orbiting it. But, if we fill in all that "empty" space with dark matter, the velocities work.

That model somewhat requires relatively "sedate" movement by dark matter itself. Well, or I suppose super active, but so uniform and in all directions at once, that it makes no difference in terms of how it affects other objects gravitationally. But I have problems with the super active concept, simply because we should expect to see an entirely uniform spread, but if that was true, then objects would be as likely to be pulled "away" from the center of a galazy as towards it. Which suggests that there must be some sort of higher density of dark matter near the center and less as we get farther out. But for that to work, we must also postulate some reason why dark matter hasn't formed into larger bodies itself (or make other adjustments to account for that if it is the case, but I also find that problematic).

One of the easier ways to model this is to assume that DM also has a very strong negative/repulsive force on other DM. This would cause it to form densities relative to the gravitational force pulling it to the center, but only somewhat. Basically, if we assume DM repels DM and is just evenly distributed in a cloud through the entire universe, then the only thing that will pull them closer together is gravity from other objects. Resulting in said "cloud" being more dense near a gravity well, and maybe only really noticably dense near a "really strong gravity well" (like a black hole). But it'll still always be more dense "downward" towards a center of gravity than "outward". And also fill in all the spaces within an orbital path, which can cause the exact rotational effects we've seen in large systems (like galaxies), while not being significant enough to be measurable in smaller ones (like solar systems).

I tend to like this model of DM. It also has the interesting feature that it's entirely possible that solar system sized effects are actually measurable, but they're just accounted for already in our gravity calculations. We assume gravity works the way it does, and with the effects it has because of how we see objects move on an interplanetary scale. So whatever effect the DM "filling in the spaces" in our own solar system is there, is already baked into our orbital math. That is the "normal" scale upon which we litereally created our measurements. So of course we can't see it there.

We can even get really weird with this and postulate that DM is actually what defines "space" itself (as in "what is distance in the first place" space). So when space curves, it's really just a density shift in DM. Which then takes us into other strange space/time correlations, concepts about what expansion really is, etc, and goes well beyond my physics understanding and well into great ideas for science fiction stuff.