Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Dark matter interacts with visible matter via gravity

  1. Dec 20, 2016 #1

    Sanborn Chase

    User Avatar
    Gold Member

    Thanks to all of you for your kind attention to my questions. May I ask another?
    If what we're calling dark matter interacts with visible matter via gravity, wouldn't it be concentrated along these arms if density waves are a feature of gravitational force?
  2. jcsd
  3. Dec 20, 2016 #2


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Hi there

    Dark matter/energy and it's interactions are outside my field
    I will let others see if they can answer that for your :smile:
  4. Dec 20, 2016 #3


    User Avatar

    Staff: Mentor

    I don't think rex is saying the stars or the arms don't exist. Just that they are formed from density waves. https://en.wikipedia.org/wiki/Density_wave_theory

    No, because instead of staying in the spiral arms, it will pass right through them and continue on unimpeded until gravity slows it down and stops it in the dark matter "halo" that surrounds the galaxy. It will then fall back through the galaxy in another pass. This is because most dark matter is falling into the galaxy from this halo, whereas most of the normal matter is rotating in a disk shape (the latter is due to normal matter also interacting via the EM force).
  5. Dec 21, 2016 #4


    User Avatar
    Science Advisor
    Gold Member

    Since dark matter does not [or, at best, very feebly] interact via the EM force, its distribution is unaffected by friction and collisions with other things that causes ordinary matter to organize itself in the taxonomy of galaxies. DM appears content to coexist as an amorphous blog surrounding most galaxies.
  6. Dec 22, 2016 #5

    Ken G

    User Avatar
    Gold Member

    Put differently, since dark matter does not contract into a disk in the first place, there's no density enhancement there to make spiral density waves in.
  7. Dec 22, 2016 #6
    I think you are thinking about galaxies and dark matter backwards. Common mistake, I used to too. You envision dark matter being attracted to the galaxies and clumping there right? Wrong. Because there is so much more dark matter than ordinary matter, it's the dark matter that formed clumps and the galaxies bob around inside of them.

    This is something bobbing around in water on the international space station. This analogy helps visualize scale, the galaxies are the small bubbles and the dark matter is the dominate blob.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted