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Dark Matter

  1. Aug 30, 2006 #1
    I don't know if this belongs in Astronomy or Cosmology, mods, move it if you see fit.

    As far as I'm aware the idea of Dark Matter was introduced because of the rotation of galaxies. Closer to a galaxy's centre matter should move faster because of the stronger gravitational pull, and further away from the centre matter should move more slowly. However, matter at the edge of a galaxy appears to be moving at the same speed as matter near the centre of a galaxy. This is only explainable if we have dark matter - the further away from a galaxy's centre, the more mass is needed to create a stronger gravitational pull and so the further away from a galaxy's centre the more dark matter there should be to balance out the gravitational pull and keep everything moving at the same speed.

    However in 2003 WMAP showed that small density fluctuations in the CMBR were too slight to create galaxies and large-scale structure in the Universe, and could be explained by having clumps of dark matter which photons (the CMBR) didn't interact with. Visible "light" matter then was attracted to these dark matter cores, causing the fluctuations in the CMBR, and then later evolved into galaxies. This would imply that dark matter should be at the centre of galaxies, which contradicts the findings shown in spiral arms.

    A theory is that quasars are (or were; they are billions of light years away so the light left them billions of years ago and we are seeing them as they were back then) young galaxies exploding into existence. Quasars give out huge amounts of energy and radiation in two jets from the top and bottom - my question is, when galaxies first formed, did they contain a lot of central dark matter? Then when they became active, did the quasar jets effectively clear the dark matter from the centre of the galaxy and the surroundings, leaving the central part of the galaxy virtually devoid of dark matter and the outer regions full of it, meaning that the WMAP findings and the spiral arm rotation curve can get on in perfect harmony?

    Bear in mind that I am nearly 16 years old and have had no real education in this; my knowledge comes entirely from books, magazines and the internet, so if there is a flaw in my physics I'd be more than happy to have this pointed out to me.
  2. jcsd
  3. Aug 30, 2006 #2


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    I don't think the jets could have cleared the Dark Matter out of the center, as these jets shoot straight out from the poles of the galactic core, whereas most of the dark matter (and every other kind of matter) would be along the plane of rotation (sort-of the galactic equator).
  4. Aug 30, 2006 #3
    Then why is there more dark matter at the edges of galaxies than the centres? Can someone explain that without having to say something cleared it from the centre?
  5. Aug 30, 2006 #4


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    Dark matter is weakly interacting with ordinary matter (like the stuff in quasar jets), so it can't have been blown out of the center of the galaxy. Also, the dark matter is not thought to be distributed in a disk, but rather an approximately spherical halo.
  6. Aug 30, 2006 #5


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    The density of dark matter is higher toward the center than at the edges. There is a discrepancy between how much dark matter is predicted to be in the center and how much is observed, but this is most likely a problem with the predictions (numerical simulations).

    More info here:

    Last edited: Aug 30, 2006
  7. Aug 30, 2006 #6
    How is that possible?
    I have drawn a diagram explaining why we need more dark matter further from a galaxy's centre:
  8. Aug 30, 2006 #7


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    The speed of a star in its orbit is determined by the mass enclosed by its orbit (a sphere with the same radius as the orbit), as well as the radius of the orbit. Stars at larger distances from the center do require more enclosed mass to orbit at the same speed. However, this doesn't mean that the density has to increase with radius because the size of the enclosing sphere increases with radius. If, for example, the density of dark matter were uniform, the mass enclosed by the orbit would increase as the radius cubed (i.e. the volume of the sphere).
  9. Aug 30, 2006 #8
    Ahh, I get it. Everything makes sense now.
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