Black holes may be swallowing invisible matter that slows the movement of stars

So, now "dark matter" does interact with regular matter via "friction"? Up to this point, the story was always that dark matter does not interact with regular matter except via gravity.

Friction is basically an electrostatic effect, so that would seem to imply that dark matter has some electrical properties. So, why does it not interact with photons?
 
Dynamical friction is a purely gravitational effect. This explanation is at the end of part 1 in the article.

"If a star is moving inside a collisionless dark matter background, the star would exert a gravitational force to pull the dark matter particles toward it. Then a concentration of the dark matter particles would locate behind the star and exert a collective gravitational force on the star. This collective gravitational force would slow down the star, and the resulting effect is called dynamical friction. The idea of dynamical friction was proposed by Chandrasekhar more than 70 yr ago (Chandrasekhar 1943)."


Indirect Evidence for Dark Matter Density Spikes around Stellar-mass Black Holes - IOPscience
 
Bill,
That still doesn't make sense to me. Why isn't the dark matter in the vicinity of the normal matter also orbiting the black hole at about the same velocity of the normal matter? That explanation makes it sound like the dark matter would just be sitting there with the regular matter trying to pass through it, except that the dark matter is getting dragged along by gravitational attraction to the regular matter.

What keeps the dark matter that is not moving fast enough to be in orbit from simply falling directly into the black hole, so it isn't left there to drag on anything? Does dark matter have a self-repulsive effect like gas pressure? That again sounds like electrostatic repulsion, much like the regular friction processes.

It seems that theorists just dream things up to make their theories "work" without looking at the other implications of the same ideas that aren't so convenient to those theories. "Dark matter does whatever we want it to do, but nothing that we don't want it to do," seems to be the working definition. I guess the temptation is just too great for some to resist, when there is no known material with defined properties that can be called "dark matter".
 
The DM is orbiting the BH. It never loses energy through friction so it does not fall to the event horizon. It is dramatically denser the closer you get to a certain radius, deemed "the spike".
The stars are also orbiting but not in exact circular orbits and not nrcessarily in the same plane of rotation of the DM.
Thus the stars are always moving relative to the DM.
 
Bill,

That really sounds like a fairy tale. The "dark matter" has "gravitational friction" with regular matter, yet it doesn[t get dragged into the same orbit as the regular matter and doesn't fall into the black hole - for13.7 billion years??? Sorry, not logical as proposed.

And, even the article's title is "Black holes may be swallowing invisible matter . . ." They just seem to be postulating that the dark matter doesn't behave like regular matter in a gravitational field near a black hole, even though it is attracted to regular matter in the same way that regular matter is attracted to regular matter by gravity.

I would like to see a consistent set of assumptions across all of these proposed theories about dark matter. What I seem to be seeing is really a dispute among different groups with different concepts about how dark matter behaves. I am left to wonder if what is really going on is that there are several different obserrvations that don't match established Relativity Theory that may or may not be due to the same thing, but the only currently acceptable "thing" to blame those unexplained obervations on is the unexplained "dark matter". Maybe they will eventually need to introduce yet another tuning variable when they can't make dark matter logically simultaneously do everything they need it to do to match all observations.
 
Well, that is sorta kinda how it worked. In a nicely behaved keplarian galaxy, stars somewhat closer toward the center, would move faster and those farther out would move slower, like our planets, and you predict their motion, using Kepler’s laws. But, they didn’t. They moved more like the galaxy was something like ten times as massive than the stuff they could measure. So, it was theorized that there was a whole bunch of mass they couldn’t see.
You can’t see it, touch it, taste it, or put a voltmeter on it. And it’s not neutrinos, they know a little bit about them, and they ain’t it.
So, they can’t observe anything except it’s gravitational effect.
Now they discover some new property. In the early days of the Solar System, bodies moved by their orbital motion, but, the system was pretty thick with dust and gas, enough to have an effect on the motions of these bodies. The dust particles don’t orbit in the same fashion as larger bodies, they were a bit slower, and they also created a drag on the bodies. It seems that dark matter does something similar where it’s apparently really thick.
So, like every new concept, the current model of how the universe works will be adjusted to accommodate the new observations. That’s how science works and how our knowledge improves. No doubt we will discover more amazing things about dark matter and dark energy in the years to come.
Or, we’ll discover they are really something else we never knew about.
That’s what keeps science interesting.
 
Pogo,

Is there observational data on planetary nebula that shows that the dust is slower than orbital velocity? Or, is that just something gleened from a computer modeling project.

And, if a measured observation, is that related to infalling action by the dust and or a (very large nunber of) multi-body gravitational interactions? Or, is it electostatic in its dyamamic - i.e., actual friction?

Bill,

Sorry you were offended by my choice of words. I used "fairy tale" as a metaphor for something that is made-up as a story that sounds plausibe - but only if you believe in the characters' actual existence and the fanciful powers attributed to them. Same connotation as "counting the number of fairies dancing on the head of a pin". I did raise some questions that are scientific in nature. No matter my choice of words, those questions still stand on their own logic.
 
Looking back, we had a discussion on the behavior of dark matter about a year ago. See https://forums.space.com/threads/does-dark-matter-flow-into-black-holes-and-if-not-why-not.57527/ .

Billslugg and "Ethan" of "Ask Ethan" seemed to believe then that dark matter is not drawn into black holes. But there were others who think that it is, but with some sorts of limits, trying to understand why there was no runaway effect of too much dark matter being taken into the black holes by this time in the universe's theoretical existence.

Now, we are seeing theoretical troubles explaining how black holes became so massive so early in the universe's existence (Webb Telescope data on distant galaxies).

I am willing to consider all sorts of possibilities for dark matter materials and behaviors, including that there could be more than one substance currently being lumped into our "dark matter" concept, so there may be different sets of behaviors for different forms of dark matter.

But, what I want to see in these papers and discussions is some better acknowldgements of the assumptions that go into these analyses, and some evaluation about how those assumptions are consistent or inconsistent with other assumptions about the same material(s). In particular, for the subject of this article, how is dark matter that is not moving fast enough to be in orbit around the black hole still avoiding being drawn into the black hole? Why is it not already moving at the velocity of the regular matter in its vicinity if it tends to build-up around the regular matter that passes through it, and already something like a billion suns worth of matter has already passed through it - wouldn't that have already set the dark matter into the same motions as the regular matter? If not, why not?

This paper infers a peak in the radial distribution of dark matter, basically to match the apparent slowing of the 2 stars as they spiral into the black hole. I wonder if the authors corrected for frame dragging effects and time dilation effects of the observations. I wonder how uncertain the observations are, numerically. I wonder how consistent their theorized solution is with the theories about how black hole masses are or are not affected by the amount of dark matter that is or is not drawn into them over the existence of the universe to-date.
 
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Dynamical friction is a purely gravitational effect. This explanation is at the end of part 1 in the article.

"If a star is moving inside a collisionless dark matter background, the star would exert a gravitational force to pull the dark matter particles toward it. Then a concentration of the dark matter particles would locate behind the star and exert a collective gravitational force on the star. This collective gravitational force would slow down the star, and the resulting effect is called dynamical friction. The idea of dynamical friction was proposed by Chandrasekhar more than 70 yr ago (Chandrasekhar 1943)."


Indirect Evidence for Dark Matter Density Spikes around Stellar-mass Black Holes - IOPscience
There is no point in arguing the math with a guy dead decades ago. What should be pointed out the observations are pretty limited in time, few years at most, the black hole and the stars are very far away and the events occurred a long long long time ago . This is basically a guess using the current PC dark matter claim. Nobody has detected dark matter or energy for that matter on earth.
 
Pogo,

Is there observational data on planetary nebula that shows that the dust is slower than orbital velocity? Or, is that just something gleened from a computer modeling project.

And, if a measured observation, is that related to infalling action by the dust and or a (very large nunber of) multi-body gravitational interactions? Or, is it electostatic in its dyamamic - i.e., actual friction?
The orbital velocity of a body orbiting another is a function of the Standard gravitation paramater which is given as (G(m1+m2))^sqrt2. Mercury will orbit faster than a dust mote in the same orbit, but not near Mercury.

In the early Solar system, gas atoms and molecules are accelerated by the barrage of photons from the Sun and they spiral outward. Dust which is more massive is subject to drag by the orbiting gas much like the ISS slowly spirals in toward Earth due to the thin atmosphere even out there. Mercury, being very large in comparison is not subject to gas drag. But, any dust there may swirl a bit behind Mercury, if there's enough. In the early system there was a lot more gas and dust around to mess around with.

If the Dark Matter is subject to drag in a similar manner as dust, it may swirl behind the observed stars. That seems to be what these observers are leaning toward. This phenomenon is really far away, so, it's not like they're seeing it up close, so they may be just conjecturing what little they can observe. Then they publish a paper with their findings and others not involved look at this to see if it fits what they already know. Sometimes it fits, sometimes it doesn't and someone suggests another process that could also fit. Then sometimes no one else has a contrasting hypothesis, yet it could fit the model, and it could become part of the model theory.
That's how we gain knowledge.

Observe, question, develop a hypothesis, test against the knowns, adjust as needed, throw it out, or adjust it again until the sum total makes sense with known observations and data.
 
Pogo, Regaarding the dust having a lower orbital velocity because each particle has less mass than a planet: I agree that would be true if there was very little dust. But, in a swirling sea of dust, the dust also attracts the other dust particles. So, it is realy a multi-body problem, where the dust probably acts like a massive body, too.

Concerning the "graviational drag" from dark matter on stars, it would seem like there must be a large amount of dark matter mass to produce a measurable effect. Since dark matter is assumed to gravitationally attract its own kind and is thought to not be able to radiate away energy, it seems counter-intuitive to think of it like regular dust in orbit in a planetry nebula disk. Why wouldn't regular matter spiraling into the black hole through the dark matter have spun-up the dark matter over the last several billon year?

With "dark matter" that is supposedly not affected by friction, may not be able to radiate away energy (or maybe it can), and doesn't fall into black holes (or maybe it does), there seems to be a lot of free parameter exploitation in these differing models to make things "fit" while not addressing the other issues raised by the same assumptions, much less addressing conflicting assumptions of other models.

Hopefully, somebody is trying to reconcile all of these papers to actually learn something about dark matter that everybody eventually can agree on.
 
I don’t think we know much of anything about this dark matter stuff to say much about it. If it’s gravity is similar to matter, would not the sun and other bodies have some dark matter in them? And would black holes be comprised of a bunch of both types.
They don’t know much and I know even a lot less. It just seems to be a bunch o’ stuff we can’t see.

Wikipedia calls it hypothetical, which means we don’t know it actually exists, it’s just inferred by the motion of matter.
Matter doesn’t behave as expected, but it does behave as if we add six times as much matter as we can detect, therefore we’ll add the balance and call it dark matter.

I’m sure scientists read one another’s papers and someone will either confirm what they say, or provide another explanation for their observations.
 
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DM is inferred by the bending of light from background galaxies. There is not enough visible matter, by a very large factor, to account for the bending. Something is bending it, we just don't know what.

Average density of DM is 7e-30 kg/m^3.
The Solar System out to the Earth's orbit has 1.4e34 m^3 of volume. In that volume there might be 100 tons of dark matter.
 
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Averages don't tell us much in that case. Using the same logic, the average density of regular matter would give about 20 tons of regular matter inside the Earth's radius, including the Sun. But, we can see that regular matter is not uniformly distributed.

The distribution of dark matter is not close to uniform. And, hopefully, that non-uniformity will tell us something about what dark matter is (or isn't).

There seems to be a lot of conflicting assumptions about what dark matter can and cannot do.