Would general relativity be falsified if dark matter is not found?

Discussion in 'Physics & Math' started by pluto2, Jul 23, 2016.

  1. pluto2 Registered Senior Member

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  3. Daecon Kiwi fruit Valued Senior Member

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    Well then we'd need to come up with a different explanation for the observations made about the shape and structure of galaxies.
     
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  5. origin Trump is the best argument against a democracy. Valued Senior Member

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    What do you mean dark matter was predicted by GR?
     
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  7. PhysBang Valued Senior Member

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    If there is absolutely no dark matter, then GR is wildly wrong about the structure of the universe and about the way that galaxies move. Currently, the evidence from the structure of the universe makes it very unlikely that GR is wildly wrong and that there is no dark matter (in the form of a particle).
     
  8. Ultron Registered Senior Member

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    Falsify is probably not the ideal word for it, but it would certainly show that GR is incomplete. This is also what I believe what is true.
     
  9. The God Valued Senior Member

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    No....where is the link ?
     
  10. Confused2 Registered Senior Member

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    Is Dark Matter a prediction of General Relativity?
     
  11. PhysBang Valued Senior Member

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    At this point, one has to say a qualified "yes". Just like at before GR, one had to say that some hidden distribution of matter in the solar system was a prediction of Newtonian mechanics, since something had to explain the precession of Mercury.

    Today, using GR at its best predicts that there must be dark matter surrounding many galaxies and galaxy clusters. It also predicts dark matter at the cosmological level (in several different types of observations).
     
  12. DaveC426913 Valued Senior Member

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    Hang on a sec. GR does not predict DM.

    GR simply models what spacetime does in the presence of mass. The fact that we observe galaxies doing weird stuff does not fall in the lap of GR.
    (Granted, an error in GR might be one way to resolve it.)
     
  13. The God Valued Senior Member

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    It is bad to say that dark matter is the prediction of GR. Neither the theory nor Einstein had any hint about it. In fact Galaxies except MW were not known when GR was proposed. Standard text is available about the need for DM hypothesis.

    PS: BH not considered as DM here.
     
  14. PhysBang Valued Senior Member

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    Yeah, the galaxies doing weird stuff does fall in the lap of GR, just like Mercury doing weird stuff fell in the lap of Newtonian Universal Gravity. GR says that the way that galaxies, galaxy clusters, and the universe at the cosmological level behave point to dark matter, a measurable amount of dark matter. So if there is no dark matter, then the problem is GR.
     
  15. DaveC426913 Valued Senior Member

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    No not directly.
    Yes, the percession of Mercury was eventually traced back to a problem with Newton. Easy to pretend in retrospect that it was open-and-shut.

    And true, as I pointed out, galactic rotation could be the result of GR. That is one of many possible causes.

    But it is not a direct link, (any more than a son robbing a bank automatically falls in the lap of his parental upbringing).
     
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  16. rpenner Fully Wired Staff Member

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    What has been found is gravitational effects as if clumpy matter was there that could not be seen. For example when galaxies collide, something that isn't gas or stars or that collides well still provides gravity for lensing of distant light.

    https://arxiv.org/abs/astro-ph/0312273
    https://arxiv.org/abs/astro-ph/0309303
    https://arxiv.org/abs/astro-ph/0608407

    Likewise the structure of cosmic background radiation and other evidence suggest it has been around a very long time, influencing how the cosmos developed over billions of years.

    https://arxiv.org/abs/1502.01589

    As evidenced by all the above.

    What the OP is about is particle physicists trying to observe dark matter directly for which the astrophysical evidence says that is a very hard thing to do.
     
    Last edited: Jul 23, 2016
  17. paddoboy Valued Senior Member

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    Of course other than the observational evidence of DM when galaxies collide/merge, we also have evidence of DM by the effects it produces and that we see as gravitational lensing, of objects further afield from clumps of DM.


    http://arxiv.org/abs/1001.1739

    The dark matter of gravitational lensing

    We review progress in understanding dark matter by astrophysics, and particularly via the effect of gravitational lensing. Evidence from many different directions now all imply that five sixths of the material content of the universe is in this mysterious form, separate from and beyond the ordinary "baryonic" particles in the standard model of particle physics. Dark matter appears not to interact via the electromagnetic force, and therefore neither emits nor reflects light. However, it definitely does interact via gravity, and has played the most important role in shaping the Universe on large scales. The most successful technique with which to investigate it has so far been the effect of gravitational lensing. The curvature of space-time near any gravitating mass (including dark matter) deflects passing rays of light - observably shifting, distorting and magnifying the images of background galaxies. Measurements of such effects currently provide constraints on the mean density of dark matter, and its density relative to baryonic matter; the size and mass of individual dark matter particles; and its cross section under various fundamental forces.
     
  18. Ultron Registered Senior Member

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    I think Mercury precession is good example. Before GR it was explained by a "dark planet" and many were searching for the planet, but without any luck. Then came GR and showed that Newton theory was incomplete. I think now we have similar situation. There is long and fruitless search for dark matter, but then will come some new theory, which will show that GR is incomplete and explain that there is no dark matter at all. I hope it will be my theory

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  19. The God Valued Senior Member

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    Can you please explain how GR predicts existence of Dark Matter ? Who predicted it ? And which part of GR's original maths hints or implies Dark Matter ?

    PS : Oort work and Vera Rubin work was mostly keeping in view Newtonian Mechanics. Our observations of Galaxy Speed Distribution curves was not as per Keplerian calculations and hence DM was conceptualise, do you call it as prediction of GR and allow the misteaching ? In stricter sense, by DM hypothesis observational anomalies in galaxy Speed Distribution Curves were settled, if someone calls it the prediction of GR, then thats misteaching.
     
  20. paddoboy Valued Senior Member

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    GR is overwhelmingly supported by evidence and having passed all tests thrown its way. DM does not contradict or invalidate GR, it works within GR, just as Einstein's CC, also works within to explain accelerated expansion.
    So no, I fail to see any "misteaching" but with all due respect, I would keep your "misteaching" remark in mind next time you feel like claiming or making some unsupported assumption re mainstream cosmology.
     
  21. The God Valued Senior Member

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    I did not say that failure of DM detection contradicts GR as some are saying. I just said strictly speaking there is no direct link. I also dispute that DM is the prediction of GR, as Rpenner is trying to support. Thus I said it is misteaching on the part of physbang.
     
  22. PhysBang Valued Senior Member

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    I'm not pretending anything. Right now, if there is no dark matter, then there is either an extremely weird set of coincidences going on or there is a problem with GR.
     
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  23. DaveC426913 Valued Senior Member

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    Agreed. That's two options. GR isn't really in question until we eliminate the other options (cuz we're fairly sure about GR).
     
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