LCDM versus MOND

There’s been a lively discussion going on at Tritonstation about recent observations of the cosmic background radiation that were correctly predicted by the MOND (modified Newtonian dynamics) model of cosmology, and incorrectly predicted by the LCDM (Lambda cold dark matter) model. Tritonstation, run by Stacy McGaugh, is a treasure-trove of information about everything cosmological. The discussion has perhaps moved a bit off the original topic, and I am at least partly to blame for that. But arguments between LCDM and MOND do have a tendency to get heated, and sometimes philosophical.

The essential question is whether you should invent things called dark matter and dark energy to make the classical theories of gravity give answers that agree with observation, or whether you should modify the equations of the theory. If you don’t have a great deal of experimental evidence, then there is no good way to decide between these alternatives. But there is a huge amount of experimental evidence, which different people evaluate in different ways. My amateur evaluation of the evidence is that it comes down fairly clearly on the side of MOND.

But MOND is a mathematical theory, obtained by modifying the equations, without an underlying physical theory to underpin it. LCDM, on the other hand, is a physical theory, but without any direct experimental evidence to back up the physical assumptions of the existence of dark matter and dark energy. So neither model can be the final answer to the difficult questions under consideration.

The essential observations make it absolutely clear that something funny happens to gravity when the strength of the gravitational field falls below a certain critical value. Usually this field strength is defined in terms of the acceleration that the field imparts to a test body. It seems to me as an educated amateur that this critical field strength is a tell-tale sign that below this value quantisation of the gravitational field takes over. Above this value, gravity can be adequately described by the classical continuous field theories of Newtonian gravity and/or general relativity, but below this value it cannot.

I am certainly not the first person to make this observation, but I am puzzled as to why it is not part of the mainstream thinking on gravity. It is such a clear “smoking gun” that you would have thought everyone would be onto it.

One commenter on Tritonstation uses the metaphor of a vehicle engine, and symbolises the false assumptions of a theory with a potato in the exhaust pipe. Rather than adding more and more fixes to the engine itself, the suggestion is to remove the potato. I’d like to modify this metaphor slightly, and identify them instead with spanners left in the works. These are the tools (mostly mathematical) that were used to build the engine in the first place. I am not a mechanic, so I don’t understand in detail how the engine works, but I am a tool-maker, and I understand how spanners should be used, and how they should not be used. So when I see a spanner left in the engine where it shouldn’t be, I have no difficulty in recognising it. When the mechanics then tell me it isn’t a spanner, it’s a vital part of the engine, I can see a problem that needs to be dealt with.

14 Responses to “LCDM versus MOND”

  1. Robert A. Wilson Says:

    I should add that I am a specialist toolmaker, specialising in spanners and screwdrivers (yes, you understand, of course, that these are the tools that exploit a “mixing” between the rotation and translation symmetry groups), and have no expertise in sledge-hammers, bolt-cutters, arc-welders or angle-grinders. But I do find it odd when sledge-hammers, bolt-cutters or angle-grinders are used to take things apart, and arc-welders to put them back together again, when spanners and screwdrivers can do both jobs much more simply. And I absolutely cringe when I see spanners being welded into the engine.

  2. Robert A. Wilson Says:

    In case the metaphor is too obscure, I point out that the rotation group U(1) is the gauge group of electromagnetism, and relates to the fundamental unit of electric charge, namely the charge on the electron. Charge is quantised, because you can count how many times your have turned the spanner or the screwdriver. The translation group corresponds to mass, which is not quantised, because you can measure a translation, but you cannot count it. Now one of the fundamental problems of physics is to understand the pitches of the various types of bolts and screws, that is, how the mass relates to the charge. The correct use of spanners and screwdrivers can help in this endeavour. Sledgehammers and bolt-cutters will get you nowhere.

  3. Marni Dee Sheppeard Says:

    Rob, astrophysicists have known for a long time that MOND is empirically correct, for the fit to galactic rotation curves. But other physicists have been loathe to touch something that must alter the classical intuition for gravity. Now, the theorists who know something about quantum inertia, know that MOND can be derived from fundamental principles in particle physics. But again, most are too scared to admit it. So MOND has already won the argument, not least because of the additional discrepancy of the Hubble parameter. LCDM remains as an empirical tool, to be replaced by the correct quantum cosmology as soon as the details are worked out.

    • Ronald M Smith Says:

      Marni:
      Well, you may be right, but that is not my experience. Recently I had an email discussion with the primary gravity researcher in the physics department at a major university that is near where I live. Their Physics department is considered to be one of the top in the USA.
      None the less, he was surprisingly adamant that Dark Matter is a real thing. To quote: “DM is made up of some combination of black holes and WIMPs” … and .. “MoND is nonsense”. Maybe his view is an aberration, I don’t know, but I do think that MoND has a way to go yet to gain the acceptance that it deserves.I agree that, if viewed objectively, MoND has pretty much won (though a physical theory is still needed). I’m just saying that the needed objectivity may not be as common as we would like, yet.

      • Marni Dee Sheppeard Says:

        Ronald, many theorists firmly believe this, because of the details of the power spectrum (CMB) etc. Their logic tells them that DM simply must be there. The correct answer is that the DOF of DM are there, in the empirical LCDM, but they will never manifest themselves locally as particles in the usual sense. The fact that he is from a so called leading institution does not mean anything.

    • Robert A. Wilson Says:

      Marni, can you point me to a good reference where I can find more details about your statement that “MOND can be derived from fundamental principles in particle physics”? My own belief is that it must come somehow out of the properties of neutrinos, and especially neutrino oscillations, but I don’t know enough about this subject to figure out how it works.

    • Robert A. Wilson Says:

      Wow. That’s quite something.

      • Marni Dee Sheppeard Says:

        Yes, I’m sure it can all be put together with your SL(4) quite nicely. Recall that SMCs in dimension 4 are not defined without braiding axioms. One needs to account for the complexity in all the low dimensions, starting with a (3,3) picture for dyonic ribbons.

  4. Ronald M Smith Says:

    Rob, When I was referring to someones ideas as involving adding more and more unverified assumptions just to get their model to work I was referring to the fellow that was going on at some length about Planck lengths and SMBHs. I then tried to point out that the additions of Dark Matter, Dark Energy, Lamda, Inflation, and on and on, to GR are just as much the same sort of ad-hoc carelessness. None of that was aimed at you, and I hope you did not think that.
    Meanwhile I agree that a lot of spanners have been left in the engine, but if we can just find that #%&@ potato then we can get rid of most of the extraneous sections, spanners and all.

    • Robert A. Wilson Says:

      Yes, I do realise that, and I think your comments are spot on. Meanwhile, I found the potato 18 months ago, after years of searching – but can I get anyone to look at it? No way. No-one believes that potatoes have anything to do with the problem.

      • Ronald Smith Says:

        Do you mean neutrino oscillations? I don’t know much about that subject either, but I would be curious to know more about why you think it is relevant. If you already have a post here explaining that, could you point me to it? Thanks

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