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Dark matter in black holes

  1. Jul 26, 2013 #1
    We know that DM interacts gravitationally with ordinary baryonic matter, so we should assume that any DM particles close enough to a black hole will also fall into it, shouldn't we?

    If so we should assume that black holes must have some DM inside them, perhaps impossible to estimate in which ratio?
    Many calculations have been done regarding the properties of black holes related to their mass, size surface area etc. Does the "mass" considered in all those calculation include potential contribution by DM in the black hole (it simply doesn't matter whether the mass comes from baryonic matter or from DM)? Or if it does, making all the calculations based on baryonic matter only might not be possibly missing something?
     
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  3. Jul 26, 2013 #2

    mfb

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    Sure.

    Once matter is in the black hole, the type of matter does not matter any more.

    Dark matter does not clump on small scales, so the capture process is less effective compared to ordinary matter.

    The mass of black holes is mainly a measurement result, and those measurements don't care about the origin of the mass.
     
  4. Jul 26, 2013 #3
    Thanks. For the famous statement that "85% of the matter in the universe is supposed to be DM and 15% OBM" (what's the standard abbreviation for ordinary baryonic matter? I'l use use OBM for this post), was it assumed that some DM may (or must?) also be present inside black holes?
    If so how could they possibly make a guess as to what might be the ratio of DM / OBM inside black holes?
    Or perhaps for simplicity it was assumed that DM is black holes can be neglected for the purpose of the statement? (the total estimated mass in black holes is considered to come from OBM so black holes are entirely taken as contributing to the 15% OBM).

    And now that we are on this, is there any estimation of, within the 15% of OBM, which percentage is in black holes and which as normal matter outside black holes?
     
  5. Jul 26, 2013 #4
    What do you mean "it does not clump on small scales"? that it interacts gravitationally with ordinary baryonic matter but not so much with itself?
     
  6. Jul 26, 2013 #5

    mfb

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    I think black holes count as ordinary matter, independent of the type of matter which formed it.

    That is not a meaningful quantity.

    Sure: The cosmic energy inventory
    The contribution from black holes is about 0.00007 (as part of the 0.04 of ordinary matter).

    It interacts normally via gravity, but gravity is not sufficient to get things like stars, planets and other dense objects - you need some way to cool the material, which does not exist for dark matter. Regular matter can radiate, and cool down, to allow further compression.
     
  7. Jul 26, 2013 #6
    Thanks again. Sorry I'm afraid I didn't understand this bit. Stars don't look very cold and yet they are held together by gravity, and it is frequently said that if they do not collapse gravitationally is because the energy radiated from the fusion keeps them from collapsing under their own gravity?
     
  8. Jul 26, 2013 #7

    mfb

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    Well, stars start with big clouds of gas.
    If those clouds collapse gravitationally, the particles attract each other so they gain kinetic energy. They interact with each other, and you get a hot, sparse gas. Without any cooling mechanism, this can be an equilibrium, and no star forms.
    On the other hand, if you have electric charges in your medium, the hot gas will radiate (thermal radiation), lose energy, and continue to reduce its size. The energy reduces, but the temperature increases as the compression is faster than the radiative losses.
    In that way, a star is "cooler" than a gas cloud: if you put the gas cloud in some gigantic container and compress it, it would probably get hotter than a star (if we consider the phase before fusion starts).

    In stars, fusion releases new energy, and the compression process stops as soon as the fusion power matches the radiated power.

    Dark matter cannot reach a thermal equilibrium within the same timescale, and it cannot radiate away energy. In addition, it cannot slow down incoming new particles - they just fly through anything (apart from black holes).
     
  9. Jul 26, 2013 #8

    Chronos

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    The dark matter content of black holes is considered negligible. Since it is effectively collisionless, it has no way to shed kinetic energy. So, instead of inspiralling in like an accretion disk, it simply passes right through the entire mess and out the other side. Only the tiny amount that is on a direct collision course would be captured.
     
  10. Jul 26, 2013 #9
    That's interesting thanks. Not sure what do you mean by "effectivelly collitionless" though, if it interacts gravitationally. Intuitivelly I would say that "collitionallity" is related to gravity and friction. DM may not interact electromagnetically, weak or strong force -wise, but if it interacts gravitationally it should be subject to inertia and friction, doesn't it? Would you mind elaborating a bit more for this ignorant soul? Thanks!
     
  11. Jul 27, 2013 #10

    Drakkith

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    Friction is a result of the electromagnetic interaction between particles. Since dark matter does not interact via this force, it passes straight through everything. It should have inertia however.
     
  12. Jul 29, 2013 #11
    Hello.

    so...you think that if dark matter falls into the black hole, it suddenly becomes normal matter? :)
     
  13. Jul 29, 2013 #12

    mfb

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    As counted for the energy content, yes.

    Possible dark matter annihilation or decays would do the same.
     
  14. Mar 17, 2015 #13
    Can't a dark-matter cloud "radiate" fast particles gravitationally while cooler ones remain in the cloud? The reverse of gravitational capture. Thus a cloud that is mostly or entirely dark matter could collapse into a low-mass BH without being impeded by radiation.
     
  15. Mar 17, 2015 #14

    mfb

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    On the scale of a black hole and its environment, that process is not effective enough (no electromagnetic interaction to exchange energy between particles).

    This thread is from 2013, by the way.
     
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