Dark matter or variable gravity?

  1. Can someone explain why 'dark matter' is the dominate explanation for the 'excess' gravity seen around galaxies?

    It seems to me that we have made an assumption that mass causes gravity. Isn't it possible that gravity arises from some as yet not understood mechanism?

    I may be way off base but it seems to me that if gravity is space-time density, then it would 'seem' that when the Universe was all energy (no mass formed yet) there would have been some variability in the energy and density of space-time itself.

    In the more dense areas, energy would condense into mass. So maybe higher density space-time gives rise to formation of mass.

    Thoughts?

    Shannon
     
  2. jcsd
  3. This may be a redundant question.
     
  4. DaveC426913

    DaveC426913 16,081
    Gold Member

    The point of dark matter is that, whatever it is made of be it particles or energy, it affects gravity as if it were massive.
     
  5. Chalnoth

    Chalnoth 5,141
    Science Advisor

    Well, the dynamics of dark matter, mainly the way that structures in the early universe formed, demonstrates that dark matter doesn't experience any significant pressure. This is the sort of behavior we associate with free particles that have no electric charge, such as neutrinos (neutrinos just have too little mass, so the dark matter needs to be some sort of neutrino-like particle with more mass).

    And by the way, there is no such thing as pure energy. When you get down to it, energy is a property of matter, full stop. Energy cannot exist on its own separate from matter any more than there can be "pure velocity". If there is energy, then that energy is contained in some field or other.
     
  6. Chronos

    Chronos 9,872
    Science Advisor
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    Variable gravity is inconsistent with general relativity, not to mention observational evidence.
     
  7. There are alternatives to dark matter, where the observed phenomena is not associated to some new kind of matter but to a modification of the gravitational laws. Maybe the more known is MOND (MOdified Newtonian Dynamics) which is well-tested and has been used to make predictions at galactic scale.
     
  8. Chalnoth

    Chalnoth 5,141
    Science Advisor

    MOND is dead and buried. It simply doesn't explain the variety of galactic rotation curves we see, let alone cluster dynamics, the bullet cluster, or the CMB.

    There are, of course, always a few people at the fringes of any science that like to work with zombie ideas. But this is one that really deserves to go the way of the luminiferous aether.
     
  9. PAllen

    PAllen 5,396
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    MOND also doesn't explain gravitational lensing by dark matter, that has been observed.
     
  10. MOND is non-relativistic. Gravitational lensing is a relativistic effect, which is explained by relativistic MOND theories.
     
  11. PAllen

    PAllen 5,396
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    Gold Member

    Correct me if I'm wrong (preferably with an arxiv reference), but the versions of relativistic MOND I've seen incorporate lensing by ordinary matter, but in no way account for lensing by dark matter, as the latter doesn't exist for them, and can't be arbitrarily distributed. My point is that clumped distributions of dark matter have been directly observed by weak gravitational lensing.
     
  12. Those distributions have been inferred from GR-based gravitational lensing plus ordinary matter distribution. That is not a direct observation.
     
  13. Chalnoth

    Chalnoth 5,141
    Science Advisor

    It doesn't really matter. The MOND model doesn't explain the data. Ergo it's wrong.
     
  14. Chalnoth

    Chalnoth 5,141
    Science Advisor

    If you're talking about TeVeS, well, first of all that isn't MOND. But in order for TeVeS to explain the cluster data, they have had to invoke a much greater neutrino density than is expected in the standard model. In other words, they need dark matter anyway, and once you have to have dark matter in the model, what is the point of proposing TeVeS in the first place?

    I'm also not convinced that you can use the exact same parameters in TeVeS to explain dwarf galaxies, large galaxies, and galaxy clusters, let alone CMB anisotropies.
     
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