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

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By Robert Lea
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Scientists watched as two stars slowed in their orbits around black holes, concluding it was the result of "drag" generated by dark matter.

An illustration of a supermassive black hole ringed with a fiery orange accretion disk ending in a thick ring of black dust
An illustration of a supermassive black hole ringed with a fiery orange accretion disk ending in a thick ring of black dust (Image credit: NASA/JPL-Caltech)

For the first time, scientists may have discovered indirect evidence that large amounts of invisible dark matter surround black holes. The discovery, if confirmed, could represent a major breakthrough in dark matter research.

Dark matter makes up around 85% of all matter in the universe, but it is almost completely invisible to astronomers. This is because, unlike the matter that comprises stars, planets and everything else around us, dark matter doesn't interact with light and can't be seen.

Fortunately, dark matter does interact gravitationally, enabling researchers to infer the presence of dark matter by looking at its gravitational effects on ordinary matter "proxies." In the new research, a team of scientists from The Education University of Hong Kong (EdUHK) used stars orbiting black holes in binary systems as these proxies.

Related: What's the biggest black hole in the universe?

The team watched as the orbits of two stars decayed, or slightly slowed, by about 1 millisecond per year while moving around their companion black holes, designated A0620–00 and XTE J1118+480. The team concluded that the slow-down was the result of dark matter surrounding the black holes which generated significant friction and a drag on the stars as they whipped around their high-mass partners.

Using computer simulations of the black hole systems, the team applied a widely held model in cosmology called the dark matter dynamical friction model, which predicts a specific loss of momentum on objects interacting gravitationally with dark matter. The simulations revealed that the observed rates of orbital decay matched the predictions of the friction model. The observed rate of orbital decay is around 50 times greater than the theoretical estimation of about 0.02 milliseconds of orbital decay per year for binary systems lacking dark matter. 

"This is the first-ever study to apply the 'dynamical friction model' in an effort to validate and prove the existence of dark matter surrounding black holes," Chan Man Ho, the team leader and an associate professor in the Department of Science and Environmental Studies at EdUHK, said in a statement. 

The team's results, published Jan. 30 in The Astrophysical Journal Letters, help to confirm a long-held theory in cosmology that black holes can swallow dark matter that comes close enough to them. This results in the dark matter being redistributed around the black holes, creating a "density spike" in their immediate vicinity that can subtly influence the orbit of surrounding objects. 

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Chan explained that previous attempts to study dark matter around black holes have relied on the emission of high-energy light in the form of gamma rays, or ripples in space known as gravitational waves. These emissions result from the collision and resulting merger of black holes — a rare event in the universe that can leave astronomers waiting a long time for sufficient data. 

This research gives scientists a new way to study dark matter distributed around black holes that may help them to be more proactive in their search. The EdUHK team intends to hunt for similar black hole binary systems to study in the future. 

"The study provides an important new direction for future dark matter research," Chan said. "In the Milky Way galaxy alone, there are at least 18 binary systems akin to our research subjects, which can provide rich information to help unravel the mystery of dark matter."

This story originally appeared on Livescience.

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Robert Lea
Robert Lea
Senior Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.

15 Comments Comment from the forums
  • Unclear Engineer
    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?
    Reply
  • billslugg
    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
    Reply
  • Unclear Engineer
    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".
    Reply
  • billslugg
    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.
    Reply
  • Unclear Engineer
    Bill,

    That really sounds like a fairy tale. The "dark matter" has "gravitational friction" with regular matter, yet it doesn



    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.
    Reply
  • Pogo
    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.
    Reply
  • billslugg
    Unclear Engineer said:
    Bill,
    That really sounds like a fairy tale.

    That's where you lost me. If you can't have a discussion without making disparaging comments, then I'm outta here. See ya.
    Reply
  • Unclear Engineer
    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.
    Reply
  • Unclear Engineer
    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.
    Reply
  • mbee1
    billslugg said:
    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.
    Reply