Looking at Dark Matter and Energy

New research has been published using supermassive black holes to probe the nature of Dark Matter. Dark Matter is the stuff used to explain observations that stars in the outer bits of the discs of galaxies (and in clusters in the halos) corotate with the inner bit, rather than spinning more slowly due to the extra velocity required to keep up. Our best model of dark matter is that it is cold – ie low energy – particles called WIMPs – weakly interacting massive particles – that interact with other matter through gravity, but not very much in any other way.

Black holes are sources of intense gravitational fields. Bodies compressed to such high densities that their surface fields have escape velocities beyond the speed of light. As black holes accrete mass through gravitational attraction, it stands to reason that they absorb dark matter as well as ordinary matter. This is being used as a way of differentiating between different dark matter scenarios – how much dark matter can a black hole absorb? The answer is anything up to seven solar masses per cubic light year. If there’s more than this in the area of a black hole, it has a runaway growth spurt that affects how the entire galaxy is shaped. As we don’t see this, there is an upper limit in the density of dark matter that can gather in the middle of a galaxy.

Meanwhile, on the subject of Dark Energy, there is a five page website on the Smithsonian with a bit of an explanation of what it is and how it is also being characterised.

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2 responses to “Looking at Dark Matter and Energy

  1. You said: “The answer is anything up to seven solar masses per cubic light year. If there’s more than this in the area of a black hole, it has a runaway growth spurt that affects how the entire galaxy is shaped. As we don’t see this, there is an upper limit in the density of dark matter that can gather in the middle of a galaxy.”

    Can you point me to any papers or articles discussing this specific concept and the reasoning behind the limit?

    • Certainly. The paper calculating the critical density is available here. You can download the full thing in pdf or other formats from the right hand corner. The title is:
      An upper limit to the central density of dark matter haloes from consistency with the presence of massive central black holes
      X. Hernandez, William H. Lee
      and it has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.

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