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I Is there a consensus about the nature of dark matter?

  1. Apr 5, 2017 #1
    https://www.sciencedaily.com/releases/2016/12/161219085020.htm
    http://www.skyandtelescope.com/astronomy-news/no-dark-matter-from-lux-experiment/

    I believe that WIMP theory has been the leading contender for dark matter in the past, but the null results from LUX, PandaX, Fermi and LHC would all seem to disfavor that model as far as I can tell. Is there a leading mathematical model for "dark matter" today and is it still WIMP theory?

    What happens if LUX-LZ returns null results like it's predecessor? Is there a clear way to falsify WIMP theory, and/or the whole concept of exotic types of "dark matter"?
     
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  3. Apr 5, 2017 #2

    Orodruin

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    The short answer is "no". The WIMP models have been largely favoured, but there is no shortage of other models.
     
  4. Jun 3, 2017 #3
    With reference to other models, I presume two of these are some sort of neutrino or the hypothesized axions. These are both very light particles, so can they really account for the missing dark matter?
     
  5. Jun 3, 2017 #4

    Orodruin

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    The key here is "some sort of". The standard model neutrinos cannot be dark matter. However, there are models where sterile (or "right-handed") neutrinos in the keV mass range are dark matter candidates.

    Axions are not thermally produced in the early Universe like many other dark matter candidates (which would make them hot dark matter, which is ruled out).

    This is not a problem if you have enough of them. What is more of a problem for very light dark matter candidates is that they cannot be thermally produced as they would then be hot dark matter.
     
  6. Jun 3, 2017 #5
    Is the ruling out of axions agreed by all theorists? I thought this was still an active area of research.
     
  7. Jun 3, 2017 #6

    Orodruin

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    Axions are not ruled out.
     
  8. Jun 5, 2017 #7
    Can you clarify the issue of axions. In #4 you said 'Axions are not thermally produced in the early Universe like many other dark matter candidates (which would make them hot dark matter, which is ruled out).' I asked in #5 'Is the ruling out of axions agreed by all theorists?' However you now says it doesn't. I thought that the predominant theory for dark matter was that dark matter was cold so if axions can only be hot dark matter doesn't that in effect exclude them as a major contributor to dark matter?
     
  9. Jun 5, 2017 #8

    Orodruin

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    You are misreading #4. I wrote that axions are not produced thermally and that any particle that light which is thermally produced would be hot dark matter. Axions have a different production mechanism which is non-thermal and therefore can still be cold.
     
  10. Jun 6, 2017 #9
    For all we know, they could all prove to be wrong. Until we can fully test the matter which may be soon
     
  11. Jun 12, 2017 #10
    I have a partial understanding of astrophysics, to put the best face on it. There's a question related to this discussion that I would appreciate some help with:
    1. Is dark matter a consequence of the expanding universe theory?
    2. Is the expanding universe theory a consequence of an assumption that cosmic red shift is a consequence of relative velocity?
    3. If cosmic red shift were simply a consequence of distance, would that simplify or complicate our cosmology?
    4. How do we know the true meaning of cosmic red shift?
     
  12. Jun 12, 2017 #11

    mfb

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    "Expanding universe theory" is a bit like "existence of Sun theory".
    No.
    No. It is not a relative velocity in the way special relativity has it, for example. It is an increase in space in between.
    It is completely incompatible with observations. It was briefly discussed in 1930, but more precise measurements showed it was wrong.
    "True meaning" is something for philosophy.
     
  13. Jun 12, 2017 #12
    Technically dark matter theory is unrelated to the belief that photon redshift is caused by the metric expansion of space, although "dark energy" theory might be afffected. Even if Hubble's "tired light" solution turns out to be the correct explanation for photon redshift/distance, it would still have little or no bearing on the dark matter debate. You'll find some pro and con arguments related to dark matter in this thread:

    https://www.physicsforums.com/threads/questions-regarding-dark-matter-dynamics.911560/
     
  14. Jun 13, 2017 #13
    The reference to "tired light" at least got me into the right reading list on this problem, though I can't agree that the effects of the sun, which are easy to observe, are on the same plane of reality as the theory of an expanding universe, which has received serious discussion within my own lifetime. The reason I asked the question the way I did was that I was considering the existence of dark matter as a possible mechanism for the cosmic red shift; this would be a silly question if dark matter were, in fact, a theoretical extension of the expanding universe theory. My insistence on trying to draw the line between theory and fact relates to my having watched generally-accepted theories come and go in my lifetime, so please accept it as an attempt at humility. I'm certainly not in a position to develop my ideas to an acceptable level of precision, given the theoretical complexity of Tolman's test, etc.
     
  15. Jun 13, 2017 #14
    Which one?

    Expansion of the Universe is as observable as "effects of the Sun". There is no place to agree or disagree, it's just a plain fact.
     
  16. Jun 13, 2017 #15

    Drakkith

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    I'd argue that the degree of certainty about the effect of the Sun are a bit higher than that of the expansion of space.

    The problem is that dark matter has the opposite effect on the universe. It opposes expansion and redshift. If the amount of dark matter and/or regular matter in the universe was much higher, it would reverse the current expansion and we would be observing a blueshift instead of a redshift.
     
  17. Jun 13, 2017 #16

    mfb

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    Gravitational potentials - clumps of dark matter - have a subtle but measurable effect on the redshift: they change the otherwise uniform redshift a bit. This is called Sachs-Wolfe effect.

    To think outside the box, you first have to know where the box is.
     
  18. Jun 14, 2017 #17
    Without getting further into the bushes about what's factual, I'm still looking for an answer to my original question: If we don't know much about dark matter, how do we know its influence on light over a megaparsec or more?
     
  19. Jun 14, 2017 #18

    Drakkith

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    The answer to that is a bit complicated and gets into many different areas of physics, astronomy, and cosmology. I don't have the knowledge necessary to go into detail, but I will try to provide a simple explanation.

    For starters, when we talk about what effect dark matter has on light or anything else we are talking about dark matter as currently modeled by one or more possible theories. One model may make slightly (or hugely) different predictions than another model. Sometimes the predictions are the same at very large scales, but differ drastically at small scales (WIMPs vs MACHOs for example).

    Of course, all dark matter is modeled as interacting gravitationally using General Relativity. But it's possible, though viewed as unlikely, that dark matter could turn out to be explained using a different theory of gravity altogether, which could potentially have enormous consequences for cosmology. In these theories there is no missing matter (dark or not) and all of the observed effects are the result of our incomplete knowledge of gravity. Modified Newtonian Dynamics (MOND) and Tensor–vector–scalar gravity (TeVeS) are two candidate theories of this type.

    Regardless of what ultimately turns out to be correct, all current predictions are the result of combining known physics and hypothetical physics to build a specific model and then seeing what pops out. So any and all answers to your question on what effect dark matter has on light over cosmological distances has to be answered within the framework of one or more specific models. The first class of models, in which dark matter is an undiscovered type of matter, is the current prevailing "go-to" class and most of the models within the class make similar predictions about what happens to light over cosmological distances as far as I know.
     
  20. Jun 14, 2017 #19

    mfb

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    We know it does not interact via the electromagnetic interaction - otherwise we would see it. That leaves the gravitational interaction as only option to interact with light. To determine this, we just need to know the distribution of dark matter - and that is well-studied based on its gravitational effect.

    We don't know the particles that make up dark matter, but that doesn't matter for its influence on light.
     
  21. Jun 15, 2017 #20
    "We know it does not interact via the electromagnetic interaction - otherwise we would see it."

    "So any and all answers to your question on what effect dark matter has on light over cosmological distances has to be answered within the framework of one or more specific models."

    So maybe I'm thinking outside the box... but since we don't know much about dark matter, outside the box is where the solutions may lie. How would we rule out the possibility that cosmic red shift is partly a product of subtle interactions with dark matter? A lot of careers could go up in smoke and a lot of new careers could be born. Plus, to quote my favorite poet,
    "The sky will be much friendlier then than now...."
     
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