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I Is the influence of dark matter symmetric?

  1. Apr 27, 2016 #1
    I am more familiar with quantum physics than cosmology so it occurred to me that I hadn't heard anyone talk about the question of symmetry with respect to dark matter. If its influence is restricted to the disc of a galaxy, but it is particle-like in structure than symmetry is violated isn't it? In other words, it exhibits gravitational attraction but not radially like ordinary matter.
  2. jcsd
  3. Apr 27, 2016 #2


    Staff: Mentor

    Its density is larger in the disc of a galaxy (at least, in some cases--in others I believe the dark matter "halo" around a galaxy is much more spherical than the galaxy itself is), but that doesn't make the gravity of any particular piece of dark matter asymmetrical, any more than the fact that the ordinary matter of a galaxy lies in a disc makes the gravity of ordinary matter asymmetrical.
  4. Apr 27, 2016 #3
    So apparently the disc-like structure of a typical galaxy is not explained by a property of dark matter and there is no reason, theoretically speaking, for the predominant presence of dark matter in one particular area as opposed to another any more than there is for ordinary matter. If the asymmetry of the galaxy, when it is not spherically shaped that is, is due to something other than dark matter can it be explained by something else?
  5. Apr 27, 2016 #4


    Staff: Mentor

    Yes, rotation. Galaxies that are flattened are flattened because they are rotating. There are also galaxies that are much more spherical, because they are not rotating, or are doing so very slowly, comparatively speaking.
  6. Apr 27, 2016 #5


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    Gold Member

    Not true at all. What you are missing is the ramifications of the fact that dark matter does not interact with anything except gravitationally. What this means is that a given particle of dark matter will travel from the halo in through the center of the galaxy, drawn there by gravity, but since it doesn't interact with anything, it just keeps going, slowing down slowly until it gets to the other side of the halo. Its speed is greatest at the center of the galaxy and lowest at the outer edges of the halo. That means that it spends far more time in the halo than in towards the center. Take it that all the particles do that and you have a distribution that has higher density in the halo than in the center.

    Normal matter, of course, would not act that way because even with the vast distances between object involved, it is at least somewhat likely to smack into something during the trips back and forth so you would not get (and in fact do not get) a normal matter halo around galaxies in the same way that you do for dark matter.
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