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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
  22. Mar 23, 2014 #21

    Drakkith

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    Yes, the reason is that MACHO's cannot make up more than a small percentage of dark matter, which leaves practically all of it as an unknown material that does't appear to emit or absorb EM radiation, at least not in amounts significant enough to detect at this time.

    I'm saying that MACHO's, by their very definition of being normal bayonic matter, are capable of being seen if the conditions are right. We may have to build bigger telescopes, or get closer to them, but they are capable of being directly observed in principle. The only ones that don't quite obey this rule would be black holes.

    And we are attempting to tell you why.

    Your original question was:

    The answer is that we know how normal baryonic matter works and the only situations that could give rise to normal baryonic matter making up a significant portion of dark matter have been shown not to exist. Note that we aren't just dealing with direct observations here. Any theory would have to fit with the Big Bang Theory's model of the universe. This means that we must have some way of showing where this matter came from within the BBT model itself. See below.

    Per wiki: http://en.wikipedia.org/wiki/Massive_compact_halo_object#Theoretical_considerations

    Theoretical work simultaneously also showed that ancient MACHOs are not likely to account for the large amounts of dark matter now thought to be present in the universe.[6] The Big Bang as it is currently understood could not have produced enough baryons and still be consistent with the observed elemental abundances,[7] including the abundance of deuterium.[8] Furthermore, separate observations of baryon acoustic oscillations, both in the cosmic microwave background and large-scale structure of galaxies, set limits on the ratio of baryons to the total amount of matter. These observations show that a large fraction of non-baryonic matter is necessary regardless of the presence or absence of MACHOs
     
  23. Mar 23, 2014 #22
    What then excludes dark matter from also being seen like some of the MACHOS "if the conditions are right"?
     
    Last edited: Mar 23, 2014
  24. Mar 23, 2014 #23

    Drakkith

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    Well, not only can we not see it, but the way dark matter seems to interact makes us believe that it doesn't interact with the EM force. For example, most of the dark matter in our galaxy seems to be on the outside in a large, spherical shell. This makes perfect sense if dark matter doesn't interact through the EM force, since infalling dark matter (matter that falls out of the shell and into the galaxy) doesn't seem to be able to lose this energy and clump together. If it could, then we wouldn't have a halo of dark matter, as it would have clumped together long ago just like normal matter did. Instead, it simply passes through the galaxy without interacting with anything and goes right out the other side where it gradually slows down under gravity, spending most of its time in the halo.

    This also explains situations like the Bullet Cluster. Two galaxies collided with each other and the visible, baryonic matter was drastically slowed down through collisions, heating much of it up, triggering star formations, and generally making a big mess. However, the center of mass of this cluster is way off from where the visible matter is. Most of the mass of the cluster is located away from the visible matter, which is exactly what we'd expect if dark matter doesn't interact with anything except through gravitation. It didn't slow down and lose energy when the galaxies collided. Instead, it oscillates around under gravity.
     
  25. Mar 23, 2014 #24
    Thanks for the explanation. Of course now I want to know even more.

    How does the EMF cause matter in general to clump?
     
  26. Mar 23, 2014 #25

    phinds

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    Basically, it's like this: when tiny bits hit each other because electronic repulsion does not allow them to pass through each other (which is why we can't walk through walls) and there is some loss of momentum to heat energy. Lots of collisions makes for slow hot bits of matter, which is then going slow enough to be clumped together by gravity so the bits get bigger and the process goes on and on and on and that's how planets and stars are formed. If the clumps get big enough they becomes stars and if smaller they become planets and if smaller still they become what amounts to being asteroids and comets but some of those are captured and become moons, and smaller still they just stay as dust.

    DM doesn't ever get to the first stage because the tiniest bits just pass right through each other so no clumping.
     
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