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I What if dark matter is unrelated to WIMPs?

  1. Oct 3, 2016 #1
    Is it conceivable that dark matter only interacts gravitationally? Is SUSY losing her charm? I truly want to know what the experts think. I'm not qualified to have an informed opinion about this subject.
     
  2. jcsd
  3. Oct 3, 2016 #2

    Orodruin

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    If it only interacts gravitationally it would be very difficult to find a theory for how it is created in the right amount in the early Universe. This does not mean that it needs to interact weakly or even with a strength that is of the same order as the weak interactions. There are many models of dark matter where its creation is not attributed to thermal freeze-out.

    To some extent yes in the sense that the more simple and "natural" SUSY models have been ruled out. However, there still remain large parts of parameter space that is still fine so it is certainly possible that there is SUSY. It may just be that it is not so wonderfully elegant as may have been portrayed by SUSY proponents before the LHC turned on.
     
  4. Oct 3, 2016 #3
    Cosmological models or particle physics models?
     
  5. Oct 3, 2016 #4

    mfb

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    For the big bang, both are relevant.

    Dark matter that only interacts gravitationally is the worst case for our attempts to find it on Earth.
     
  6. Oct 3, 2016 #5
    I apologyze in advance for what I am about to ask. You have already tried to explain it to me before, but I seem to be unable to get it:

    - What is the point of using the standard model of particle physics in cosmology? The cosmos is intrinsically curved in space-time, the Poincare group is not a symmetry of the universe. How can it be a good description or even relevant of whatever happened in the early universe?

    - How did Hawking managed to combine general relativity and particle physics to predict that a black hole emits radiation?
     
  7. Oct 3, 2016 #6

    mfb

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    We don't know what happened at the very early phase of the universe, but at the energy scale where our theories work spacetime was extremely flat. QFT in flat spacetime works well there. You don't take the gravity of Earth into account to model LHC collisions for the same reason.
    It is not a full combination, as he did not quantize gravity. QFT on curved spacetime is tricky, but some simple results (like the emission of radiation) can be done there as well.
     
  8. Oct 3, 2016 #7
    Is there any consistent formulation of QFT in a curved space-time? (I'm not thinking about quantum gravity). How dou you describe a fermion field in a curved space-time? Can it be done?
     
  9. Oct 3, 2016 #8

    mfb

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    You can write it down. The question is can you also calculate it.

    But it does not matter for the early universe, as I said (repeatedly). So where is the point?
     
  10. Oct 3, 2016 #9
    Can you? Loosely speaking, in the standard model of particle physics, you start with two irreducible representations of the Poincare group. Then you realize that if the two internal global symmetries of these two representations are forced to be local, you end up with a Lagrangian which looks beautiful and, when second quantization is applied, describes particles that interact and have acquired mass.

    How do you do that with a set of generalized coordinates that change from one 4-dimensional point to the next? Poincare's symmetry group is essential in QFT. What do you do if you don't have that symmetry group to start with?
     
  11. Oct 3, 2016 #10

    Orodruin

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  12. Oct 3, 2016 #11
    Thank you very much for the tip. I just have one question: Am I talking nonsense? Is this a dead-end path?
     
  13. Oct 3, 2016 #12

    fresh_42

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    That's a question about personality. To those being on the hunt for quick and short answers? Probably.
    However, those who gets inflamed by the idea and want to find out, what's behind it, it might as well be the start of a career in physics.
     
  14. Oct 3, 2016 #13
    OK, thank you very much. You've given me a honest answer that I really appreciate.
     
  15. Oct 23, 2016 #14
    Is there any hint, apart from its possible cosmological origin, that dark matter can't be normal matter?
     
  16. Oct 23, 2016 #15

    ChrisVer

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    if it was "normal" matter we would be able to see it.
     
  17. Oct 23, 2016 #16

    mfb

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    It does not interact with the electromagnetic interaction.
    Its distribution is different from the distribution of normal matter.
    It influences the CMB in different ways, and we can see the difference.
    We know the amount of regular matter.
     
  18. Oct 23, 2016 #17
    If it is cold and dense matter like the matter of objects in the Kuiper Belt how would we be able to see it?
    What kind of electromagnetic interaction could we expect from far distant cold objects, which are also far away from sources of electromagnetic energy?
    What kind of influence on the cosmic microwave background can we expect from such objects?
    We know the amount of regular matter...This is related to the question of the possible cosmological origin of such kind of objects.
     
  19. Oct 23, 2016 #18

    ChrisVer

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  20. Oct 23, 2016 #19

    ChrisVer

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    also when we say that they don't interact electromagnetically it doesn't mean that they have to somehow radiate something away- but they are more or maybe less transparent to the light that goes through them.... When light for example pass through a cloud of gas, its spectrum will be altered (eg some frequencies of the light are going to be absorbed)
     
  21. Oct 23, 2016 #20
    Yes, but this is exactly what I mean, according to the current theory, enough "Halo Objects" to explain galaxy dynamics cant exist.
    But it seems that there is no direct proof, that they do not exist.
     
  22. Oct 23, 2016 #21

    mfb

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    There are not enough heavy elements around to form so many solid objects. And gas is interacting with radiation in a detectable way.

    What is a "direct" proof, apart from going there and looking around with a flashlight?
     
  23. Oct 23, 2016 #22

    Chronos

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    Actually, CMB and other data offer overwhelming evidence that the mass 'missing' in the universe cannot consist of objects too faint to be seen with telescopes [i.e., asteroids, planets, dim/failed stars, black holes, etc., aka MACHOS]. For a discussion that does not require an advanced degree in mathematical physics to understand how and why dark matter was tried and convicted for crimes against physics and human intuition, see https://ned.ipac.caltech.edu/level5/March10/Garrett/Garrett3.html
     
  24. Oct 23, 2016 #23
    The text in the link says:
    ....
    B. Cosmological Evidence

    BBN is a period from a few seconds to a few minutes after the Big Bang...
    ........................
    The conclusion is that based on the Big Bang theory, the mass 'missing' in the universe cannot consist of objects too faint to be seen with telescope.
    But the question is: Are there any observations, which directly proof the existence of dark matter.
    The observation could be the presence of a gravitational lense in a region penetrated by intense radiation. If the cause of the gravitational lense is real matter, it should become visible in such a region.
     
  25. Oct 23, 2016 #24

    Chronos

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    The Bullet cluster and similar examples are peculiar in one outstanding respect: gravitational lensing shows the center of mass of colliding galaxies is not located where the apparent center of visible mass in the clusters is located according to x-ray observations. This is generally considered smoking gun evidence that dark matter sailed through unhindered by the collision between galaxies, whereas gas associated with these galaxies was left behind where the collision occurred like derelict cars from a titanic traffic crash. That's about as close to direct evidence of dark matter offered by nature as it gets.
     
  26. Oct 28, 2016 #25
    From extended clouds of larger cold objects, which are only weakly coupled via gravitation with their own galaxy , such a behaviour "sailing through without noticeable collision interaction" also could be expected. The fact that remaining gas can be observed at the collision point, rather indicates that real matter has been involved in the collisions.
    Aren't there any more obvious hints for the existence of something which even dominates the gravitational behaviour of galaxies, but never has been detected within our near environment?
    May be some smoking gun like evidence can be derived as follows:
    After a very long time, the trajectories of all objects in a glaxy must reach some kind of an equilibrium with mass concentrated to attractors. This kind of equilibrium should be accessible via simulations and then be comparable to observations. The simulations can start with equally distributed clouds of real matter objects of random mass (sparsely distributed in the cloud) or with an equally distributed cloud of dark matter.
     
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