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I Dark matter and black hole interaction

  1. Jun 7, 2017 #1
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  3. Jun 7, 2017 #2

    phinds

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    Dark matter contributes to BH formation exactly as does regular matter BUT dark matter passes right through an accretion disk so is less LIKELY to go into a black hole. That is, unless a dark matter particle is either traveling fairly slowly so as to be attracted by gravity before it passes by, or is headed straight for a BH, it won't fall in.
     
  4. Jun 7, 2017 #3

    Janus

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    I'm not quite sure what your question is. The article you linked to contains the suggestion that Black holes left over from the formation of the universe are what what account for what we now call dark matter, it doesn't deal with any interaction between black holes and dark matter.

    If Black holes and dark matter are different in composition, then black holes would ingest any dark matter that crossed their event horizon. The difference between regular matter and dark matter in this respect is due to the fact that normal matter interacts with itself electromagnetically and DM doesn't. This interaction of normal matter causes it radiate away energy as it gets near the black hole, which slows it down and increases the likelihood of it falling into the event horizon . DM, on the other hand, has no such means to radiate away energy, so unless the initial trajectory takes it across the event horizon, it will not be injested by the black hole. In a way, Black holes makes a "larger target" for normal matter than it does for dark matter.
     
  5. Jun 7, 2017 #4
    OK... so my understanding is the DM at the event horizon is not being 'sucked' into the BH but instead is either oozing on by? The article about BH and DM interaction was the only relevant one I found, I am sure there are other articles on the subject but didn't have time to look for them.

    Going back to my original question if gas and stellar materials are being swallowed by the BH, why wouldn't DM be continually fueling the BH? What equivalent energy is released from this DM?

    Accretion onto a black hole is the most efficient process for emitting energy from matter in the Universe, releasing up to 40% of the rest mass energy of the material falling in.
     
  6. Jun 7, 2017 #5

    Janus

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    No. Anything that crosses the event horizon falls into the black hole. It is just that normal matter, due to its interaction with other matter near the black hole is more likely to cross the event horizon
    Th emitted energy is the energy lost that I was talking about above, caused by interaction between matter falling into the black hole. Since DM does not have this mechanism for emitting/losing energy, no equivalent energy will be emitted by the consumption of the Black hole by dark matter. Such consumption will have the effect of increasing the mass of the black hole, which in turn would have an effect on the energy emission by normal matter falling into the black hole.
     
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  7. Jun 7, 2017 #6

    Drakkith

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    The answer is directly linked to orbits. Take the ISS for example. Why doesn't the ISS fall to Earth? The easy answer is of course that it is in orbit. It is moving fast enough that the path it is falling in (yes, it is literally falling. It is constantly in free fall) does not intersect with the Earth's surface. Instead, if we take a rough approximation, the path makes a ellipse and is called a closed orbit. Since the path of objects in closed orbits does not intersect the larger body, the object does not "fall in". We'll talk more about that in a moment.

    But what about other objects that aren't in closed orbits? What about a random space rock that comes speeding in from beyond the solar system? Obviously its path must intersect with the Earth's surface for it to impact. But if it doesn't, then it's likely going fast enough that its path forms a hyperbola (or a parabola, but that's not a realistic orbit), not an ellipse. This is known as an open orbit. Few people other than pedantic astronomers would call this an orbit in regular conversation, though it is still technically an orbit.

    So here we have two classes of orbits, open and closed. But what does this have to do with black holes and dark matter? Well, if we look at a black hole with an appreciable amount of dust, gas, and other matter surrounding it, most of this matter is in a closed orbit about the black hole and is located well away from the event horizon. But we just talked about a closed orbit above and came to the conclusion that objects in closed orbits don't fall in, right?

    The truth is that our conclusion above ignores a very important feature and isn't entirely correct. If it were, then the ISS, satellites, and other objects in orbit wouldn't have to make occasional burns to correct their orbits. The reason they do is that space isn't a perfect vacuum and there is an appreciable amount of gas and dust around large objects like planets and stars. Objects in orbit run into this gas and dust and, due to air friction, gradually lose energy and fall into a lower orbit. Without making orbital corrections, their paths would eventually decay so much that they intersect with the Earth's surface and the objects fall to Earth.

    For black holes with accretion disks, the gas and dust particles are all in slightly varying orbits and constantly bang up against each other, heating up and losing energy as EM radiation. Since it is losing energy, some portion of this gas and dust ends up spiraling further and further down towards the event horizon until it eventually crosses it. Over time, black holes can grow to huge sizes through this mechanism.

    However, Dark Matter doesn't interact through electromagnetism. Instead of colliding with itself or other matter, it passes right through. This means that it cannot lose energy like regular matter can. So dark matter cannot form accretion disks and spiral down into the event horizon. It has a hard time even getting into a closed orbit around the black hole. The vast majority of it simply passes by the black hole on an open orbit and continues on its way. It's only if its path happens to intersect the event horizon that dark matter gets sucked into the black hole. And since black holes are extremely compact, only a few dozen kilometers in diameter, it is extremely unlikely that significant amounts of dark matter get pulled in. You'd have a better chance of landing a hole-in-one during a hurricane while blindfolded.
     
  8. Jun 7, 2017 #7

    russ_watters

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    If dark matter doesn't radiate energy, does it keep that feature once inside a black hole and if so does that mean such a black hole can't produce Hawking radiation and evaporate?
     
  9. Jun 7, 2017 #8

    phinds

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    If I understand Hawking Radiation correctly, it does not originate inside the EH and so it doesn't matter what's in the BH, just how strong the tidal force is at the EH.
     
  10. Jun 9, 2017 #9

    stefan r

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    I thought dark matter could effect the black hole's spin. The angular momentum then effects normal matter and can be radiated.
     
  11. Jun 9, 2017 #10
    Let's use something more familiar as an example: an asteroid and a star. If you have an asteroid moving in the direction of a star, what will happen? Sometimes sometimes it'll get sucked into the star, but the most likely case is that it'll make a near miss of the star and get shot away. Depending on it's original trajectory, it may be thrown out of orbit, or it may be captured. Once captured, it's not going to fall into the star, it has too much momentum to carry it past the star each time. In order for it to fall into the star, it has to slow down.

    A gas atom is the same way, if it misses the star, it'll be slingshotted around and has to slow down before it can get sucked in. With gas, thes is largely due to friction with other gas that's also trapped in orbit. That's the accretion disk.

    Evidence from the Bullet Cluster shows us that dark matter doesn't experience friction, so it can't really slow down, so the most likely case is that it'll miss the black hole and be slingshotted away. Dark matter is certainly attracted to the black hole, but now without any way to gently fall into it, it will most likely miss it. Let's remember that the EH of most black holes are tiny compared to stars.
     
  12. Jun 9, 2017 #11

    Janus

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    Yes, but that is a indirect effect and does not necessarily lead to an increase of radiation from the black hole. The in-falling DM can just as easily decrease the black hole's spin, decreasing its angular momentum. The point is, that while the DM can have effects on the nature of the Black hole, which in turn effects how the Black Hole interacts with normal matter and how that matter radiates, there is no direct radiation of energy by the in fall of the DM.
     
  13. Jun 9, 2017 #12

    phinds

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    Unless I'm mistaken, this is slightly misleading because it implies that there IS direct radiation of energy by the in-fall of regular matter. My understanding is that neither DM nor regular matter causes any immediate radiation on in-fall but rather they contribute to the mass of the BH which in the long run contributes to the total amount Hawking Radiation.
     
  14. Jun 9, 2017 #13

    Janus

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    During the in-fall, regular matter interacts with other matter falling in prior to crossing the event horizon to radiate away energy. Thus the process of feeding regular matter into a black hole can produce energy. Dark matter being fed into a black hole does not have this mechanism of energy release, so its only effect will be that caused by the increase of the mass of the black hole.
     
  15. Jun 9, 2017 #14

    phinds

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    Ah. I thought you were saying that it radiates at the EH. I understand about the accretion disk as I explained in an earlier post.
     
  16. Jun 9, 2017 #15

    stefan r

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    My understanding was that you can extract energy from a rotating black hole. That energy is taken from the black hole's angular momentum. Penrose process. The wikipedia article says a particle goes in and a particle comes back out with energy added.

    I expect a black hole accretion disk will radiate energy same as any accretion disk. But is this jet from an accretion disk?
    250px-Messier_87_Hubble_WikiSky.jpg
    messier 87
     
  17. Jun 10, 2017 #16

    phinds

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    Into the accretion disk and back out, NOT into the BH itself and back out
     
  18. Jun 10, 2017 #17

    Janus

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    With such a process the particle passes into the ergosphere (an oblate spheroid region extending outside of the event horizon). Inside of this region, matter is required to travel in the same direction as the black hole rotates. Such matter can rob the black hole of angular momentum and leave the ergosphere with more KE than it entered with. Now if it is your purpose to "use" this energy, then you would have to extract that KE from the exiting matter. I see no reason why dark matter can't rob a black hole of angular momentum through the same process, However, since DM doesn't interact other than through gravitation, there would be no easy way to usefully extract the KE it gained. The Black hole would lose energy without us being able to "harvest" it.
     
  19. Jun 11, 2017 #18
    What if dark matter is the product of black holes? That is, another form of matter that exists in our universe but is undetectable by the normal forms of observation.
     
  20. Jun 11, 2017 #19

    Drakkith

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    If you can find a peer-reviewed source that discusses this, then we can talk about it. Otherwise we cannot, as per PF rules.
     
  21. Jun 12, 2017 #20

    stefan r

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    It is in the original post:
    "The Astrophysical Journal Letters" is peer reviewed.

    When people say 80% dark matter and 20% observable matter they have rounded. So if future astronomers discover that dark matter is only 76% of milky way's mass we cannot claim the figure was wrong.

    My impression was that black holes are almost certainly some of the dark matter. Black holes without accretion disks are a subset of MACHOs (massive compact halo objects). Free comets and rogue planets are also MACHOs. Some papers on MACHOs say they do not have enough mass to explain all of dark matter. For example:

    That is fairly strong evidence for believing that part of dark matter is MACHOs.

    For the original post question:
    Black holes do merge. LIGO detected several.
     
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