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Properties of Dark Matter

  1. Mar 21, 2014 #1
    Let me start off by stating that I have no formal education in Astrophysics, or any other education beyond high school, so if my question is stupid, just say so!

    Dark Energy, from my understanding Dark Energy is used to explain the expansion of the universe, because when we look at distant galaxies they appear to be moving away from us and this is deduced by measuring the red shift of light from distant galaxies, the Doppler Shift.

    Dark Matter is an invisible matter that wraps all galaxies and it's properties are not understood at all, other than that it exerts gravitational pull within galaxies. Dark Matter is invisible because we look through it in the milky way when observing distant galaxies and we look through it in distant galaxies (because they are also wrapped in it).

    Now if Dark Matter's properties are not understood, how can we be sure that it doesn't affect the red shift of light from distant galaxies?

    I hope its not a stupid question, and if anyone can give me a link to more on Dark Matter I'd appreciate it..

    Thanks
     
  2. jcsd
  3. Mar 21, 2014 #2
    First of all, Dark Energy is an energy that makes the expansion of the Universe accelerate. Second, we do know some properties of dark matter. We just don't know what it is, exactly. We know it creates a gravitational field, that it doesn't interact with electromagnetic radiation and that it doesn't interact with baryonic matter

    I don't think Dark Matter can affect the redshift. The redshift is affected by gravity significantly only when the body itself is emitting and it is dense enough.
     
  4. Mar 21, 2014 #3
    I read a paper on arXiv that what Dark Energy does is described in GR, but Einstein didn't believe it (steady-state universe was still in vogue), so he created the cosmological constant.
     
  5. Mar 21, 2014 #4

    bapowell

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    Einstein believed that the universe was static -- so he added the CC to counteract the contraction that a matter-dominated closed universe would undergo. Once the CC comes to dominate the energy budget, you get accelerated expansion. The CC is a special case of dark energy, not distinct from it.
     
  6. Mar 22, 2014 #5

    Chronos

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    Actually, Einstein did not 'add' the CC to GR, it naturally appears in the equations. He merely ignored it initially because it appeared irrelevant. Dark matter is a horse of a different color. The only relationship between DE and DM is the word 'dark', which basically means we have no idea what the heck it is. It would be an amazing coincidence if it turned out the two are actually related.
     
    Last edited: Mar 22, 2014
  7. Mar 22, 2014 #6
    The topic suddenly changed from DM to DE. What Chronos said is something I've been thinking about for quite some time. I wonder if there is some sort of connection between these two unknown mysteries. Maybe DM is negative DE but concentrated in one place or something. No one can possibly know. At least not at the moment.

    -cb
     
  8. Mar 22, 2014 #7

    phinds

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    Actually, based on their characteristics it seems pretty clear that we CAN say they have nothing to do with each other. Any way, why should they? THEY don't care that humans had the stupidity to call them by names that allow them to get easily mixed up by people who aren't paying attention.

    It would likely have saved hundreds of thousands of keystrokes here on The Physics Forum if "dark matter" had been more appropriately called "Zwicky matter" and "dark energy" had been called "vacuum energy", then people wouldn't keep wanting to conflate the two.
     
    Last edited: Mar 22, 2014
  9. Mar 22, 2014 #8
    Yes, that's true. Those names are awful. However, the names you suggested as a replacement of the actual names are not the best, in my opinon. I didn't even know what "Zwicky" was (now I know it is the name of a scientist) and the name doesn't sound good. Vacuum energy wouldn't be a good name either because it implies that we know what it is, when in reality its nature is not completely understood(however, it is still better than DE). I'd say that simply Missing Matter would be a much better name. Regarding DE, I don't know for sure what to call it. Missing energy would sound weird.

    cb
     
  10. Mar 22, 2014 #9

    phinds

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    Yeah, I agree that those suggestions (which I did not originate ... they come from this forum) are not the best but they are WAY better than "dark ..."
     
  11. Mar 22, 2014 #10
    How do we know for sure that it doesn't interact with light or baryonic matter? How do we know it's not just another form of baryonic matter?
     
  12. Mar 22, 2014 #11

    phinds

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    It does not emit or reflect light, which by definition means it doesn't "interact" with light.

    If it were baryonic matter it would clump. It doesn't. Google "bullet cluster"
     
  13. Mar 23, 2014 #12
    Would it be accurate to say that if dark matter did emit or reflect light even weakly then since it composes 85% of matter we should see a lot of this light?

    Would it also be accurate to say that if dark matter absorbed light then we would still expect it to emit black body radiation that we could observe?
     
  14. Mar 23, 2014 #13

    Drakkith

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    Yes, that's correct, Paisiello. If dark matter interacted via the EM force then we would expect to see it emit and absorb light.
     
  15. Mar 23, 2014 #14
    OK but the key seems to me that even if it reflected or emitted light very weakly such that it was difficult to detect at a distance then it should still be observable because there is so much of the stuff.
     
  16. Mar 23, 2014 #15

    phinds

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    Which is one of the reasons that we believe it DOESN'T reflect or emit light, so what's your point?
     
  17. Mar 23, 2014 #16

    Drakkith

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    It is observable. Through gravity.
     
  18. Mar 23, 2014 #17
    My point is to just to clarify why we believe dark matter doesn't react with light.

    For example, we currently believe that a small portion (5% ?) of dark matter is actually comprised of baryonic matter that we just can't see: MACHOS. Now we can't observe MACHOS directly but that doesnt mean that they don't interact with light.

    So how do we know for sure that non-baryonic matter doesn't interact with light? Suppose that 100% of dark matter were MACHOS. Would there be enough of it that it should then be observable by light?
     
  19. Mar 23, 2014 #18
    Yes, of course, but indirectly.
     
  20. Mar 23, 2014 #19

    Drakkith

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    Because we can't see it, nor can we observe any mysterious extinction of light that would point to it being absorbed by some sort of matter.

    That is incorrect. We can, and do, observe many types of MACHO's. Candidate objects such as brown dwarfs, red dwarfs, white dwarfs, and neutron stars emit light and are directly observable. Others, such as black holes, can have accretion disks which are directly detectable. To my understanding, searches for MACHO's show that they cannot make up more than a very, very small percentage of dark matter.

    We can't suppose that 100% of dark matter is in the form of MACHO's because experiments have ruled that possibility out.
     
  21. Mar 23, 2014 #20
    And there is also some MACHOS that we can't see. Yet we dont claim that these do not interact with light. So there must be another reason that we make this claim about dark matter.



    Are you saying we can see all MACHOS directly even the ones that make up a small part of dark matter?



    I was speaking hypothetically to better understand the reasoning behind the chaim that dark matter doesn't interact with light.
     
    Last edited: Mar 23, 2014
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