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Originally Posted by snarkophilus
There's a question for you: what predictions do current MOND hypotheses make that match a wide range of observations?
I don't see how MOND can be reconciled with the fact that galaxies seem to have different amounts of dark matter (or variance from predicted gravity, if you prefer not to speak of DM) in varying distributions. If you could show me that, it'd move me a bit away from the dark (matter) side.
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I don't think MOND is the answer, nor do I believe that it has any wider applicability beyond galactic rotation curves. Its principal value has been in opening the discussion regarding gravitation and quantifying the failure of the standard model to predict the behavior of galactic masses. The mainstream assumes that Newton got gravity right, and that Einstein refined that, so that our understanding of gravity is perfect and unassailable. Given this level of faith, they had to find some way to explain the broad rift between expectation and observation, and they invented DM to solve the problem. If DM is distributed "just so" it can fix the galactic rotation curve problem, if it is distributed "just so" it can explain the excess gravitational binding of clusters, and if it is distributed "just so", it can account for the apparent walls and filaments of large-scale structure. To borrow Nereid's expression, that's a big steaming heap of invisible pink fairies.
How do we get out of this mess? First of all, we go back to what we know. The Standard Model (prior to the addition of the DM epicycle) is not predictive on galactic scales or larger. Gravity in these domains does not follow the simple rules that work so well for simple systems like our solar system. Instead of inventing DM for which there is NO evidence at this time, we must find out why Newtonian gravitation and GR fail on large scales. I believe the key to the problem is that gravitation is dynamic, and the gravitational field is polarized in domains populated by lots of matter, resulting in fields of greater intensity within very massive domains. Gravitation is not a simple kinematic exercise definable by the masses and separations of the matter involved - there is also the matter of field strength.
Until the end of his life, Einstein was looking for a dynamical ether to fix the shortcomings of GR. This ether would not be a fixed thing, but would vary in in its qualities in accordance with the masses embedded in it. He insisted that gravitational attraction, inertia, and centrifugal effects all arise from matter's interaction with the ether. Einstein also insisted that the ether was the transmissive medium by which EM traverses "empty" space. In the mid-1960s Sakharov hypothesised that gravitation and inertia arise from matter's interaction with the quantum vacuum field. This is not a crazy idea, because if quantum theory is correct (and it has a pretty good track record) the vacuum contains the bulk of the mass/energy in the Universe.
I don't want to hijack the MOND discussion with my views on gravitation, but needed to point out that we don't understand gravitation sufficiently well to justify the invention of DM. MOND has quantified the failure of the standard model's gravitation and has made accurate predictions. This tells us that the failure of the mainstream view of gravity is consistent and perhaps understandable, with a bit of work. In this sense, MOND has been one of the most useful exercises in decades. It is never going to explain cluster binding or large-scale structure, but it is pointing us to a more general formulation of gravity that will be able to do so.