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B What's the deal with dark matter

  1. Nov 9, 2017 #1
    Can someone explain why we are certain there is dark matter in our universe?

    I understood it like this. At some Point in our recent history we figured out, that according to our math galaxies (or sth like that) wouldnt have developed like they did or wouldnt even stay in 1 Piece since they do not have enough mass. So we came up with dark matter as a source of mass to Keep it all in place. Now since dark matter does not react with anything other than its gravitational pull u cannot testify its existence.

    If that is the case "dark matter" seems like a pretty far fetched concept to me which maybe is validating bad math or sth just to make it work.

    Im sorry for my bad spelling english isnt my first language and im not very educated. I hope someone maybe takes the time to help me with this even a good link (other than Wikipedia) would help me alot.
     
    Last edited: Nov 9, 2017
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  3. Nov 9, 2017 #2

    russ_watters

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    Your understanding of how we got here is pretty good, but you underestimate how difficult it would be for "bad math" to be the cause. GR has worked exquisitely well for pretty much every situation it has been thrown at except this, so one would have to be able to correct the "bad math" in such a way as to have no effect on all those other calculations.
     
  4. Nov 9, 2017 #3
    So there is no experiment at this Moment which can proof dark matter and its just an assumption based on GR?
     
    Last edited: Nov 9, 2017
  5. Nov 9, 2017 #4

    russ_watters

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    No experiment separate from GR, right.
     
  6. Nov 9, 2017 #5
    If the math error occurred in universal sum calculations and correct answers were obtained in isolation how would we know that we had an error in our universal sum calculations?
     
  7. Nov 10, 2017 #6

    Janus

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    It's a bit more complex than that. It isn't just that there should be more mass, but that mass would have to be distributed around the galaxy in a different shape than the visible matter we see. And you just can't say that the dark matter just happens to collect that way to produce the desired result. Instead you determine what properties dark matter would have to have in order to not be visible. That determines how it would tend to form around galaxies, and then you see whether that prediction matches how the dark would have to form around the galaxy to provide the correct gravity. If the two match, you have good evidence for dark matter to be the reason.
    It's not as people haven't tried to explain things by assuming that our understanding of gravity needs to be modified, its just that all such attempts to come up with a modified theory have come up short in explaining what we observe and the dark matter hypothesis is a much better fit.
    And "dark" matter is not that far fetched. We already know of one type of particle that matches that description. Neutrinos don't interact with light or electromagnetically at all. There are reasons why the neutrinos that we know of aren't what dark matter we are looking for is made off, but it is evidence that something else that shares some of its properties could exist.
     
  8. Nov 10, 2017 #7

    russ_watters

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    I don't understand. Are you saying as opposed to an error in the theory a literal typo repeated thousands of times, identically? That's basically impossible.
     
  9. Nov 11, 2017 #8
    No, I'm saying that the error may be that the universal sum of the individual parts is incorrect even though the calculations for each individual part may well be correct.
     
  10. Nov 11, 2017 #9

    russ_watters

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    That is, in essence, what the problem is. I don't see a good reason to consider that a "math error" instead of "we're missing some parts".
     
  11. Nov 12, 2017 #10

    OCR

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    That almost sounds like the definition of emergence... ?
     
  12. Nov 12, 2017 #11
    A model of the universe that puts the boundary at our observation limit (visible universe has an observational data limit) and excludes everything that could physically exist beyond that boundary may very well have sum problems as you can never actually know if your boundary is correct or not.
     
  13. Nov 12, 2017 #12
    Very true but why could this not apply to dark matter itself?
     
  14. Nov 12, 2017 #13
    Reading between the lines, would "math error" be compatible with MOND and of course "we're missing some parts" be compatible with dark matter? So you appear to be agreeing with one side of the argument, that the only way to solve this problem is with dark matter.
     
  15. Nov 14, 2017 #14
    In my opinion, what we need right now is patience. We do not have the answers we want. The few verified answers we have from experiments contradicts our speculations. We are befuddled as what to do next. How shall we invent, design, develop, trouble-shoot the next generation of experiments? Can the sources of funding the technology be convinced to invest in an uncertain outcome?
     
  16. Nov 14, 2017 #15
    I agree with your post and of course there are ongoing experiments to try to directly detect dark matter, some of which will only be completed in a few years from now. However, I note theorists are moving away from dark matter candidates that can be directly detected in any way and that raises serious philosophical questions. On a technical matter, I find it difficult to see how some of the putative candidates, for example neutrinos or axions, can be responsible for dark matter because of their negligible mass. To my thinking there needs to be a quite astonishing number density of these particles to be responsible for 85% of the universe's mass.
     
  17. Nov 14, 2017 #16
    In the second half or the 19th century electromagnetism was thought to be perfect. It correctly described the results of numerous experiments, much as GR today describes correctly the results of numerous experiments. There were a few issues which were a bit perplexing and so it was necessary to hypothesize a medium through which electromagnetic waves could travel. Bit of a problem detecting the luminiferous aether though, eh? Of course, it is really, really difficult to observe something which was eventually found to not exist! In spite of difficulties with moving forward with gravitation theory, there is still the question as to whether we understand gravity itself as well as we think we do; I occasionally hear reputable cosmologists (which I am most certainly not) acknowledge this possibility. Just sayin'!
     
  18. Nov 14, 2017 #17

    stefan r

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    Not likely. More and better telescopes make pretty pictures. The pictures have independent value even if they do not generate new physics . Like any natural phenomenon the rotation of the galaxy should be explained long before we try changing it. It is hard to argue there is any urgency there. People who are professionally teaching as well as many amateurs attack this question without sending anyone a bill.
     
  19. Nov 14, 2017 #18

    George Jones

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  20. Nov 14, 2017 #19

    Drakkith

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    Scientists are well aware of this question and are doing their best to answer it. It hasn't been forgotten.

    There are many possible ways other than telescopes to search for dark matter. I believe we've set up various particle detectors in different areas of the world to see if we can find dark matter or other new types of particles. The Large Underground Xenon experiment was/is one of them.

    I would think a math error would be incompatible with everything since it's just an error. The "we're missing something" would seem to be compatible with all theories, since we almost certainly have to be missing some pieces if we are observing phenomena that aren't predicted or explained by current theory.
     
  21. Nov 14, 2017 #20
    We observe that the visible matter of galaxies is moving in ways that cannot be attributed to only the gravitational forces associated with this visible matter and conclude that there must be additional gravitational forces caused by matter we can't see. As I understand it, gravitational forces are associated with warping in space-time, according to GR. A question arises, is there some way to cause space-time warping - and hence, forces that attract matter - other than by the presence of massive bodies?
     
  22. Nov 14, 2017 #21
    Nothing that we know of, that is why it is called 'dark'.
     
  23. Nov 15, 2017 #22
    I think the short answer is no, within current accepted theory. The most concise representation of General Relativity is,
    Gμν = - κ Tμν
    where Tμν is the energy-momentum tensor and by implication since mass is energy, it accounts for mass as well. Gμν is the Einstein curvature term and κ is Einstein's constant of gravity which is directly related to Newton's constant G. So curvature of space is directly linked the amount of mass and energy in a locality. Because of the relation E = mc^2, mass is by far the largest contributor to the curvature of space.

    However, the point you make at the start of your post, 'We observe that the visible matter of galaxies is moving in ways that cannot be attributed to only the gravitational forces associated with this visible matter', is not backed up by all observational data!
     
  24. Nov 15, 2017 #23

    stefan r

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    Are you sure? Light going into a black hole increases the gravity of the hole and therefore "space time warping" [link is not peer reviewed, arXiv.org]. Photons of light do not have mass.

    This is not useful information for dark matter. Dark matter is clearly not made of photons.
     
  25. Nov 15, 2017 #24
    Everyone is going to laugh about my opinion in that if we could calculate the frame drag of the galaxy using earths frame drag as a representation and apply that figure with regards to the amount of visible matter contained within our galaxy would we get a figure that is close to what we have calculated for the amount of dark matter needed? Maybe I’m in left field on this or not I don’t know.
     
  26. Nov 15, 2017 #25

    Janus

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    Frame dragging is just too tiny an effect compared to what would be needed.
     
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