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Dark Matter in Black Holes

  1. Feb 16, 2016 #1
    Since we think that 85% of the matter in the universe is Dark Matter, does it follow that around 85% of the mass of a typical black hole should be of dark matter origin?

    If not, why not?

    And if so, black holes are defined by only 3 parameters, mass, electric charge and spin. As far as we can tell the dark matter component of a black hole should only contribute to its mass, not to its electrical charge nor spin, right?

    If 85% of the mass of a black hole has no net influence in its electrical charge and spin, does this have any significance in the total observed properties of the black hole? I'm thinking that it might not have any influence on the electrical charge but yes on the spin of the black hole?

    TX
     
  2. jcsd
  3. Feb 16, 2016 #2

    mfb

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    No. Why should it?

    Matter can easily clump (e.g. forming stars), and fall into black holes as result. Dark matter cannot, as dark matter cannot cool via the emission of electromagnetic radiation. Dark matter falls into black holes only if it directly hits the black hole, which is rare as black holes are tiny. Yes, it does contribute to the overall black hole mass, but the contribution is tiny.
     
  4. Feb 16, 2016 #3

    Janus

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    In addition to what mfb said. The dark matter of a galaxy is spread out a lot more thinly than visible matter. There may be more total mass in the DM halo of the Milky way, but this Halo extends out to a volume that is much much larger than the visible disk of the galaxy. In the disk itself, where the vast majority of stars, planets etc are, the concentration of visible matter is many many times that of DM. For example, the total DM you would expect to find in the sphere with a radius equal to that of the Solar system adds up to being about the same mass as a small to medium asteroid. In other words, a tiny, tiny fraction of the mass of the Solar system. So if we take a typical star system with a star massive enough to go supernova and leave behind a stellar black hole, and taking into account that the star itself has the vast majority of the mass of total system mass and that the DM in the system is a tiny fraction of that total mass, you would expect a pretty insignificant amount of the mass for the final black hole to have come from DM.
     
  5. Feb 24, 2016 #4
    I'm a little late but I have a related question. I'm wondering if any estimates are made as to the composition of a black hole. During the evolution of a black hole from a stellar system there is an increasing amount of energy as opposed to mass concentration due to the binding energy of atoms created by fusion over billions of years. Due to the equivalence of mass and energy General relativity takes both into account, but what about when determining the properties of black holes? are there any or could there be any differences in how we perceive a black hole due to the relative concentration of mass vs. energy?
     
  6. Feb 24, 2016 #5
    According to currently accepted theories, no.
     
  7. Feb 24, 2016 #6
    So when we speak of the mass needed to create a black hole, as for example with a supernova, the energy is completely irrelevant? In other words, you are not changing the black hole by adding energy?
     
  8. Feb 24, 2016 #7
    You yourself said earlier: "...the equivalence of mass and energy General relativity takes both into account". I'm confused about what you do or don't know.
     
  9. Feb 24, 2016 #8
    The Schwarzshild radius is calculated using mass not the stress-energy tensor of the Einstein equation which would potentially give a more exact solution. So I am assuming that the Schwarzshild solution must give an approximate result. From what I can tell calculations of the mass needed to create a black hole use only mass not energy. No I don't know a lot about black hole formation, but I don't see a lot written about the fundamentals either.
     
  10. Feb 24, 2016 #9
    Mass and energy is the same thing. In GR, it's T00 element of stress-energy tensor.
     
  11. Feb 24, 2016 #10
    OK, I think I get it. As fusion is creating more binding energy the mass of the star is not decreasing as it would be if mass and energy were not the same. Thanks.
     
  12. Feb 24, 2016 #11
    Fusion releases energy. Binding energy of any bound system is negative, and during fusion it is going *down* - it becomes more negative.
     
  13. Feb 24, 2016 #12

    mfb

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    Much more than 99% of the energy of a star is in the mass of its nuclei (this mass depends on nuclear binding energies, of course). There is a small positive contribution from electrons, a small positive contribution from heat, and a very small negative contribution from gravitational binding energy.
     
  14. Feb 24, 2016 #13
    Then can we assume that a black hole is also 99% particles such as neutrons and protons? or do we just call this an unknown?
     
  15. Feb 24, 2016 #14

    mfb

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    As seen from the outside, it (probably) does not matter what lead to the formation of a black hole. Inside: In general relativity it does not matter, what happens if we add quantum mechanics is unclear. It is unlikely that the original contribution would be relevant, however.
    For all practical purposes, it does not matter what lead to the formation of a black hole.
     
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