Some of dgruss23's posts, in the No Dark Matter thread, provide good material on MOND (MOdified Newtonian Dynamics), a shorthand for a set of ideas, hypotheses, and nascent theories that have arisen from a conjecture by Milgrom, in 1983 {link/ref to be added}.

In particular, #10, , #17, #20, #25, and #28 (and the various links therein).

There is also a good (2002) review paper, by Sanders and McGaugh.

As a discussion of MOND is (largely) OT for the thread in which dgruss23 posted, I have started a new thread, to enable a more general discussion.

Here are some questions I have about MOND, per some of the papers in the links provided by dgruss23. Can any BAUT member reading this point to where answers may be found?

1) In Scarpa (2003), an H₀ of 50 is assumed - how do the various plots and conclusions of this paper change if 72 is used instead?

2) in the Sanders and McGaugh 2002 review paper, discussion of MOND compatibility with lensing observational data is limited to strong lensing (and the paper's claim is, crudely, that if there is strong lensing, then the acceleration regime must be non-MONDian). How does MOND handle weak lensing?

3) ditto, for weak lensing signals in general (e.g. the integrated galaxy weak lensing signal found by SDSS)?

4) What is the current status of the 'no-go' challenges to the various classes of super-MOND theories, which incorporate (modified) GR, wrt gravitational lensing? (this is covered in section 5.4 of the review paper, but it seems things were very tentative in 2002).

5) Has anyone tried fitting the rotation curves of the Milky Way, M31, LMC, SMC, IC5152, and M81 using MOND? (none of these galaxies appears in any of the McGaugh lists).

6) How does MOND fare when it is used to model galaxy collisions? (such as those done by the Toomre brothers, modelling the Antennae)

7) How well does MOND produce tidal tails and streams? (such as those of globular clusters and dwarf satellite galaxies being stripped, by the MW)

First, let me say thanks for breaking this off. MOND is one of the most seriously studied ATM ideas and a thread devoted to it is worth having.

Second, I'm afraid the discussion we were having on the other thread got blown way beyond my intent. Correct me if I'm wrong, but I believe our disagreement boils down to this: You seem to hold the view that MOND is largely irrelevant - that MOND might be a handy formulation for fitting rotation curves but beyond that is of little value - since it is not a cosmological theory on its own.

My view of MOND is that it has already empirically established a fundamental fact about gravitational dynamics - that being the recognition that when applying Newtonian dynamics mass discrepancies appear in the low acceleration regime. I think our disagreement is more about this - I see that empirical reality as very important to the larger DM issue whereas you seem to be implying it is not important. This is a much more fundamental disagreement I have with your comments than the quibbling over MOND right/wrong and in what regimes.

Now let me also say that I am still examining the MOND literature and have been for quite a while. So while I find MOND interesting and will discuss my understanding of it, I do not wish to be seen as a "MONDian". My "defense" of MOND in so much as I may do so is more in regards to clearing up misunderstandings about it and in regards to my belief that there is some importance to what MOND researchers have found that will impact revisions to our models. In other words, I feel the literature has shown that "right" or "wrong", MOND is telling us something about the nature of gravity that theory must be able to account for.

I don't have time right now to examine all your questions in appropriate detail, but I'll provide a few additional links.

MOND fits can be sensitive to the distance scale used. I'll have to look into this. That is something you might do - if you really want to know.

With all these, I'll have to look into it - again, you could too. It should be relevant to your concerns given your position in your DM thread.

You would probably need to ask McGaugh why those have not been modeled with MOND. In the case of the MW, ther may be too much uncertainty in the surface brightness profile to perform a MOND fit (because we are inside the galaxy. A similar problem may arise with M-31,LMC, and SMC because of their large angular dimensions. AS for IC 5152 - is there something so special about the galaxy that we should have expected MOND researchers to have tackled it? Perhaps with M-81 its interaction with M-82 and NGC 3077 has created a problem for modeling it? Again, McGaugh could no doubt tell you.


I don't think it has been.


Here is an example and here is another.

Finally a few unresolved questions I have which I would like answers to.

Could you explain what you meant by this:

If mainstream researchers use Newtonian dynamics, why do you think it is a waste to work on a modification of that dynamic?

and I'd also like an answer to this question:

And I have also linked to papers that show MOND to be consistent with the dynamics of globular clusters, elliptical galaxies, and the research shows MOND reduces the mass discrepancy in galaxy clusters from ~ 10 to only ~2-3. So are you still taking the position that this is all MOND does?:

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"The scientist who asks the right question reconnoiters a new patch of the unknown, and may, with luck, bring it within the constricted but expanding boundaries of the known."

~Timothy Ferris (The Red Limit) 1982

Too much proper motion in the milky way, likewise the rotations of very close galaxies is difficult to model because of proper motions. A candidate for MOND studies must be viewed at an angle that allows both surface brightness measurements and Doppler velocities within the galactic plane. These criteria should not normally introduce a selection bias.

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jwj

If you always believe what you already know, you can't learn anything - Liz

MOND is an ad-hoc correction of Newtonian dynamics that correctly predicted the behavior of low surface brightness galaxies at least a decade in advance of their observation. MOND may not have global applicability, but may instead be a marker of just how and to what extent Newtonian dynamics falls apart on galactic scales. This begs for epistemology. At what point could such an error have crept in? Are our observations in error? Are our assumptions about gravitational attraction in error? Could the fabric of space-time be polarized on large scales such that gravitational forces are not exactly the same in every domain? When you put all this on the table with the fact that clusters are far more gravitationally-bound than we expect given the calculated masses, it is hard to ignore that elephant in the room -- gravity on large scales does not act as we expect from Newton and GR. Since gravity is the dominant force on these scales, it seems rather cavalier to say that Newton got it right, and Einstein refined that, and then press on. Neither of them managed to get beyond a mathematical model of the effects of gravitation, though Einstein struggled for the rest of his life to lay out a mechanical model explaining how gravitation and inertia arise from matter's interaction with space-time. MOND has quantified the error of the Standard Model on galactic scales. If it serves no other purpose, it has served us well, indeed.

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The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924

I am not a cosmologist, but I think it is fair to say that MOND will not go away until dark matter particles are detected. It should be interesting to see how MOND adherents react at the time, if such a time comes, DM particles are discovered. There has been some recent chatter in the cosmology/particle physics community which predicts a DM detection within the next five to ten years, as dark matter direct detection experiments are on the verge of being able to probe a substantial portion of the "dark matter parameter space", and/or the LHC may produce exotic heretofore unseen particles.

If DM is found it will be as much of a fundamental step forward in our understanding of the matter as the discovery of new chemical elements of the periodic table. Some may argue that finding DM will be even more of a breakthrough since it makes up a greater percentage of all matter than do the elements. On the other hand, perhaps particle dark matter will not show up and we will be forced to revise our ideas about gravity. Either way excites me-- since either a definitive detection of particle dark matter or a refutation of particle dark matter will constitute major progress in terms of understanding the Universe.

Turbo-1, a very insightful analysis

[Neither of them managed to get beyond a mathematical model of the effects of gravitation,]

While Newton was certainly about effects, Einstein went beyond 'just effects'.

Although the warping of space/time is an effect, and explains gravity dealing with Baryonic Mass, in a more thorough and useful way, the ultimate warping of space/time; Black Holes and Singularities is cause and effect!


[though Einstein struggled for the rest of his life to lay out a mechanical model explaining how gravitation and inertia arise from matter's interaction with space-time.]

And I now have no doubt what-so-ever, after reading a String/"M" theorists paper (which I am not going to share at this time!), that Einstein, had he known of Massive Black Holes (he definitely would have figured out that they are different) and String/"M" Theory, would have easily figured out the 'rest of the gravity' story!

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

The strongest argument in favor of dark matter is the large scale structure of the universe. Without dark matter, it is difficult to explain galactic clustering. It is also difficult to explain the dark matter based numerical simulations that so elegantly predict the large scale filamentary distribution of galaxies in the universe:
http://en.wikipedia.org/wiki/Dark_ma...ture_formation

This is not an argument for dark matter. It is an argument for stronger-than-expected gravitational attraction in some domains, and the reason for this is unknown, at present. Unfortunately, the mainstream was unwilling or unable to question our assumptions regarding gravitation, and resorted instead to the invention of yet another entity to explain the gap between our expectations and reality. That so many people have bought into the Dark Matter idea instead of questioning our gravitational model is a sad commentary on the lack of epistemology in cosmology today. Despite its inability to interact with baryonic matter, this wonderful entity somehow obediently arranges itself around galaxies, in clusters, and on supercluster scales in just the right ways to make the standard cosmology conform to observations in each instance. How convenient. Let's back up to the one thing that we know - gravity does not behave as we expected on large scales - and start from there, asking why that is.

Einstein was looking for an "ether" to fix the shortcomings of GR - a dynamical entity that defines the structure of space-time and is conditioned by the matter embedded in it. He was on the path that the mainstream should be following now. We already know that the vacuum is not empty, but is a teeming sea of virtual particle pairs popping in and out of existence. Is that enough of a hint?

It is not enough to think of gravitation kinematically, with bodies interacting against a passive space-time field. Gravitation is dynamic. The field that confers gravitational attraction on matter is necessarily polarized and densified by that interaction, which means gravitational attraction varies with field strength, not just with mass and physical separation. Dark matter is just a place-holder - a bookkeeping trick to keep things working until we finally figure this out. We should never make the mistake of thinking that DM is real. That way lies decades of stagnation.

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The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924

Well, no... they say it does interact with baryonic matter. Just not through photons, but through gravity instead.

I'm undecided, and awaiting the next installment in Nereid's thread in the Q&A section. I'm not convinced for or against at the moment, but it appears that there is some very strong observational evidence for it. I'm the sort of person who is predisposed to a MOND-like solution to the problems that dark matter tries to solve, but there's a lot of evidence for dark matter. If I can be shown that DM theory allows us to make predictions, I'll be close to sold on it.

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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"It's turtles all the way down."

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.

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The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924

Okay, that's what I was asking for... what predictions? I've heard LSB observations mentioned somewhere before -- how do these match to MOND? Are they uniformly supportive of it, or are there contradictory results as well?

Besides, MOND isn't really necessary to quantify the difference between the SM and observation. One can just look at where things are and where they ought to be.

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"It's turtles all the way down."

Yes, MOND predicted the rotation curves of LSB galaxies a decade before they were measured. I don't know if these results have been uniformly accepted.

You're right, of course, but the fact that MOND was proposed is a watershed. It shows that at least some people are resistant to tacking epicycle after epicycle onto an already bloated model. Also, the fact that MOND can apply some simple rules and undo the failure of the SM to predict the rotation curves of galaxies is encouraging, because it suggests that correcting our gravitational model is not going to be too daunting.

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The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924

Turbo-1, whether you call it DM + DE, aether, quintesence, quantum vacuum field, or mass/energy field, or stress-energy tensor, as Celestial Mechanic did to try to explain how it works, and how it 'anchors' (they are anchored, but this does not explain it) galaxies to the space they are in, it is all the same thing...space/spacetime, and it is made of something physical, it is not a pure vacuum.

Since it is impossible to escape the necessity for 'extra gravity' (DM) to explain cluster dynamics, it is more than likely necessary for galaxy rotation curve a well.

However, Dark Energy is an entirely different story!!!

Clusters and galaxies are not universe theory specific!

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

I hope all will forgive me for not being a professional cosmologist but I wanted to throw my own two cents in. I tend towards the MOND view myself for the usual reasons. I think at heart that the scientists sense that if MOND is actually the root of the disease that that necessarily means that GR must be removed to cure the patient, and no one wants to be told that both their legs must be cut off to save them.

In my poor amateur view the salient thing that I notice is a steady progressive disparity of expected mass with increase of scale. This seems to suggest to me a regular progressive increase in gravitational force with distance. But how to explain this in a simple way? Newton states that the strength of the force decreases according to 1/d^2. Observation shows that the exponent of 2 must gradually decrease with scale. For galaxies the exponent n seems to be about 1 and decreases further at larger scales. From pure geometry we know the exponent should be 2 but it doesn't cooperate with what we want. There is a hidden assumption here about force propagation that we seem to be ignoring, it is that the force is propagating at a CONSTANT speed. Let us for a moment give this notion up and consider the possibility that gravitation is propagated by gravitons that after they are emitted from the gravitating body constantly accelerate. This changes the force dynamic now because at greater distances the gravitons will impart more energy per solid angle than we expect. Where does the energy come from to accelerate the gravitons? I'm not sure, from the zero point energy background of space or perhaps photons as they travel through space. That might account for progressive redshift with distance.

Regardless of whether I am on the right track or not I feel the basic problem is that the scientists feel they have a good understanding of gravity when in fact they know next to nothing about it. They cannot even say with authority what speed it propagates at other than theoretical conjecture. With such ignorance it is an oddity to me that they lock themselves tenaciously into certain approaches.

I wouldn't say "impossible" for galaxy clusters. MOND reduces the mass discrepancy in clusters from ~10 to ~2-3. There is a possibility that this mass may yet be discovered in baryonic form. But we'll have to see. Another possibility would involve incorrect mass estimates if cluster distances are incorrect or if cluster velocity dispersions are contaminated by factors better discussed in Arp et al.

Even if DM is the ultimate answer to galaxy rotation curves, it must be explained why that DM is behaving in a MOND-like fashion.

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"The scientist who asks the right question reconnoiters a new patch of the unknown, and may, with luck, bring it within the constricted but expanding boundaries of the known."

~Timothy Ferris (The Red Limit) 1982