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Why dark matter distribution is so different in galaxies?

  1. Jul 17, 2015 #1
    I learned that dark matter distrubition is homogeneius and isotrophic in cosmic scales.

    I searched some galaxies dark matter distrubition.And I am actually suprised.
    Why cause every galaxy has a different ratio of dark matter baryonic matter distrubition.
    In cosmic scales baryonic matter/dark matter=1/5 but in galaxies its varies.
    Whats the reason for that ? Why every galaxy didnt contein five times bigger dark matter.

    My second question is we know that from planck results baryonic matter density is 0.0456 and dark matter density 0.26.
    It means we have a galaxy and the mass of the galaxy is will be 6m.Cause m from baryonic mass 5m from dark matter=6m.

    I know that both questions are relavent each other.

    Thank you
  2. jcsd
  3. Jul 17, 2015 #2

    Vanadium 50

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    Why would you expect the dark matter distribution to be the same? That's not the case for luminous matter.
  4. Jul 17, 2015 #3
    I want to give an example.A galaxy baryonic matter /dark matter=1/3.B galaxy baryonic matter /dark matter=1/5. its different galaxy to galaxy.
    Why their ratio is not same.I dont know why.

    Your answer is why not.But Is there any other explanation ?
    Dark matter Homogenenity is not what I mean.I guess I used wrong word to describe myself

    I wanted to mean Why ratio is different every galaxy
  5. Jul 17, 2015 #4
    If you understand my question right,I expected to be same.Thats more make sense and reasonable for the other one
    Last edited: Jul 17, 2015
  6. Jul 17, 2015 #5
    Any ideas ?
  7. Jul 17, 2015 #6

    Ken G

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    I wonder if the main cause is just randomness, or if there is a systematic trend. A random aspect might be, you have an excess gravity from a contracting dark matter region, and it pulls in baryonic matter. But the details of how the baryonic matter gets pulled in could have details that lead to random variations. I don't know exactly what would matter, but gravitational instability could be a messy process with many things that could vary. Or, we might have systematic trends, like a larger dark matter mass might produce a gravity that pulls in an unusually large amount of baryonic matter, such that the ratio of baryonic matter to dark matter increases for galaxies with large dark-matter mass. I don't know if there are correlations like that, or if it's all just random, but those are the kinds of things that would come to mind to explain it. But you do have a reasonable question there-- since gravity affects all the "cold" matter similarly, we might expect the baryonic matter to do more or less exactly the same things that the dark matter does, so we need to explain why that does not happen.
  8. Jul 18, 2015 #7
    I know that we can assume earth is in the center of universe.Every observer in the universe will gonna think like that but know lets suppose we are in the center of universe.

    Is there any source that.I can find the galaxy position for us(assuming we are in the center of universe).And the dark matter/baryonic matter ratio.

    Its a mystery now I guess.
  9. Jul 18, 2015 #8
  10. Jul 18, 2015 #9


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    We are the center of the OBSERVABLE universe, not of "the universe"
    Last edited: Jul 18, 2015
  11. Jul 18, 2015 #10
    When I say universe I mean observable universe.I know the difference between them.Thanks for recall
  12. Jul 18, 2015 #11


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    Some galaxies are known to be relatively devoid of dark matter, and they look different than DM rich galaxies. We already know the assumption of homogeneity does not apply to things as small as a galaxy, or even galactic clusters - so I agree with Vanadium's comment.
  13. Jul 18, 2015 #12

    Ken G

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    I actually agree that there is a question here that needs answering. In the early universe, we expect some ratio between dark matter and baryonic matter that is extremely uniform, just like the CMB is. I don't know of any theory what would expect anything different there. So then we have gravitational instabilities, which create density variations in both dark matter and baryonic matter. But if both are "cold", it's hard to see why their ratio could be so different. Certainly the baryonic matter can condense faster, by virtue of its ability to emit light. But that comes late in the process, after the gravitational wells have already been established, and presumably, so has the ratio of dark matter to baryonic matter in the general vicinity. So what allows the dark matter and baryonic matter to part company enough to show such variations? Is it just that the baryonic matter flows into different dark matter potential wells, like how water might find different lakes after a flash flood? That would mean that the baryonic matter needs to have considerable mobility relative to the dark matter in which it was originally embedded.
  14. Jul 18, 2015 #13
    In theory Cold Dark Matter Made by WIMP.So they would be interect weakly.And they will move more slowly due to baryonic matter and that will cause this ratio differences.
    This is the answer I guess.

    But this situation is only true for cold dark matter I guess.Hot dark matter never interects with baryonic matter.

    And Is Hot dark matter can make extra mass ?

    Cause maybe baryonic matter/mixed dark matter ratio is same for all galaxies maybe.

    Hot dark matter moves very fast so maybe their energy makes mass effect.
  15. Jul 19, 2015 #14

    Ken G

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    Current models don't give a significant role to hot dark matter. How firmly that has been established I could not say, and that it is hard to establish is perhaps why we've seen a Nobel prize for dark energy but not dark matter.
  16. Jul 19, 2015 #15
    I searched and it seems cold dark matter is also does not explain everything.

    In my opinion baryonic matter/mixed dark matter ratio can be same for all galaxies.Why ? Cause maybe dark matter moves slowly due to baryonic matter (Which I will call later BM).But the hot dark matter moves faster due to BM(Cause its made of neutrinos and they are high velocity particles).So mixed of them maybe makes the BM/MDM(Mixed Dark Matter) ratio same for all galaxies.

    Dark matter is a real mystery I guess.I dont know your degree in physics.Is there any person which I can ask these question If you dont know the answer ?
  17. Jul 19, 2015 #16


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    The modern consensus is dark matter cannot be 'hot' [relatavistically speaking] due to the large scale structure of the universe. It is less clear how 'warm' it may be without disrupting large scale structure. This is 'hotly' debated in the cosmology community and is about as much as we know about it.
  18. Jul 19, 2015 #17
  19. Jul 19, 2015 #18


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    Hot and warm is the term to distinguish particles with highly relativistic velocities from those with lesser velocities.
    Last edited: Jul 19, 2015
  20. Jul 19, 2015 #19
    I am really confused.Is there any good source that I can understand the subject better.

    The answer of my question seems like given.But I am not sure that I understand the answer.

    I understand cold matter and why the ratio is different from galaxy to galaxy.But I didnt understand the other types of dark matter

    I made some reaserch and mixed dark matter is good approximation of observation


    Or this article says MDM is not good I guess


    And Is this a good source to understand DM ? Which I asked above


    My english is middle level.So excuse me please.
  21. Jul 19, 2015 #20


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  22. Jul 19, 2015 #21
    If I imagine a galaxy as a small object bobbing around a cloud of dark matter, with gravity pulling it all together, it would collide with nearby clouds and galaxies may or may not and pull random amounts of dark matter and matter with it depending on how they collided? Wouldn't the stirring of the early universe create randomly sized clumps of dark matter and matter in random ratios like shaking a lava lamp, even if the initial distribution was nearly evenly distributed? It wouldn't really matter how hot dark matter is, only that it behave differently than matter in collisions, which we know it does.
  23. Jul 20, 2015 #22
    Thank you
  24. Jul 20, 2015 #23
    I was actually asking everyone else, it was just a thought I had that seemed to make sense.
  25. Jul 20, 2015 #24
    Maybe someone replies.I dont know
  26. Jul 20, 2015 #25
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