This post is a recent conversation with David Garofalo for his blog.

Today we talk to Dr. Stacy McGaugh, Chair of the Astronomy Department at Case Western Reserve University.

David: Hi Stacy. You had set out to disprove MOND and instead found evidence to support it. That sounds like the poster child for how science works. Was praise forthcoming?

Stacy: In the late 1980s and into the 1990s, I set out to try to understand low surface brightness galaxies. These are diffuse systems of stars and gas that rotate like the familiar bright spirals, but whose stars are much more spread out. Why? How did these things come to be? Why were they different from brighter galaxies? How could we explain their properties? These were the problems I started out working on that inadvertently set me on a collision course with MOND.

I did not set out to prove or disprove either MOND or dark matter. I was not really even aware of MOND at that time. I had head of it only on a couple of occasions, but I hadn’t payed any attention, and didn’t really know anything about it. Why would I bother? It was already well established that there had to be dark matter.

I worked to develop our understanding of low surface brightness galaxies in the context of dark matter. Their blue colors, low metallicities, high gas fractions, and overall diffuse nature could be explained if they had formed in dark matter halos that are themselves lower than average density: they occupy the low concentration side of the distribution of dark matter halos at a given mass. I found this interpretation quite satisfactory, so gave me no cause to doubt dark matter to that point.

This picture made two genuine predictions that had yet to be tested. First, low surface brightness galaxies should be less strongly clustered than brighter galaxies. Second, having their mass spread over a larger area, they should shift off of the Tully-Fisher relation defined by denser galaxies. The first prediction came true, and for a period I was jubilant that we had made an important new contribution to out understanding of both galaxies and dark matter. The second prediction failed badly: low surface brightness galaxies adhere to the same Tully-Fisher relation that other galaxies follow.

I tried desperately to understand the failure of the second prediction in terms of dark matter. I tried what seemed like a thousand ways to explain this, but ultimately they were all tautological: I could only explain it if I assumed the answer from the start. The adherence of low surface brightness galaxies to the Tully-Fisher relation poses a serious fine-tuning problem: the distribution of dark matter must be adjusted to exactly counterbalance that of the visible matter so as not to leave any residuals. This makes no sense, and anyone who claims it does is not thinking clearly.

It was in this crisis of comprehension in which I became aware that MOND predicted exactly what I was seeing. No fine-tuning was required. Low surface brightness galaxies followed the same Tully-Fisher relation as other galaxies because the modified force law stipulates that they must. It was only at this point (in the mid-’90s) at which I started to take MOND seriously. If it had got this prediction right, what else did it predict?

I was still convinced that the right answer had to be dark matter. There was, after all, so much evidence for it. So this one prediction must be a fluke; surely it would fail the next test. That was not what happened: MOND passed test after test after test, successfully predicting observations both basic and detailed that dark matter theory got wrong or did not even address. It was only after this experience that I realized that what I thought was evidence for dark matter was really just evidence that something was wrong: the data cannot be explained with ordinary gravity without invisible mass. The data – and here I mean ALL the data – were mostly ambiguous: they did not clearly distinguish whether the problem was with mass we couldn’t see or with the underlying equations from which we inferred the need for dark matter.

So to get back to your original question, yes – this is how science should work. I hadn’t set out to test MOND, but I had inadvertently performed exactly the right experiment for that purpose. MOND had its predictions come true where the predictions of other theories did not: both my own theory and those of others who were working in the context of dark matter. We got it wrong while MOND got it right. That led me to change my mind: I had been wrong to be sure the answer had to be dark matter, and to be so quick to dismiss MOND. Admitting this was the most difficult struggle I ever faced in my career.

David: From the perspective of dark matter, how does one understand MOND’s success?

Stacy: One does not.

That the predictions of MOND should come true in a universe dominated by dark matter makes no sense.

Before I became aware of MOND, I spent lots of time trying to come up with dark matter-based explanations for what I was seeing. It didn’t work. Since then, I have continued to search for a viable explanation with dark matter. I have not been successful. Others have claimed such success, but whenever I look at their work, it always seems that what they assert to be a great success is just a specific elaboration of a model I had already considered and rejected as obviously unworkable. The difference boils down to Occam’s razor. If you give dark matter theory enough free parameters, it can be adjusted to “predict” pretty much anything. But the best we can hope to do with dark matter theory is to retroactively explain what MOND successfully predicted in advance. Why should we be impressed by that?

David: Does MOND fail in clusters?

Stacy: Yes and no: there are multiple tests in clusters. MOND passes some and flunks others – as does dark matter.

The most famous test is the baryon fraction. This should be one in MOND – all the mass is normal baryonic matter. With dark matter, it should be the cosmic ratio of normal to dark matter (about 1:5).

MOND fails this test: it explains most of the discrepancy in clusters, but not all of it. The dark matter picture does somewhat better here, as the baryon fraction is close to the cosmic expectation — at least for the richest clusters of galaxies. In smaller clusters and groups of galaxies, the normal matter content falls short of the cosmic value. So both theories suffer a “missing baryon” problem: MOND in rich clusters; dark matter in everything smaller.

Another test is the mass-temperature relation. Both theories predict a relation between the mass of a cluster and the temperature of the gas it contains, but they predict different slopes for this relation. MOND gets the slope right but the amplitude wrong, leading to the missing baryon problem above. Dark matter gets the amplitude right for the most massive clusters, but gets the slope wrong – which leads to it having a missing baryon problem for systems smaller than the largest clusters.

There are other tests. Clusters continue to merge; the collision velocity of merging clusters is predicted to be higher in MOND than with dark matter. For example, the famous bullet cluster, which is often cited as a contradiction to MOND, has a collision speed that is practically impossible with dark matter: there just isn’t enough time for the two components of the bullet to accelerate up to the observed relative speed if they fall together under the influence of normal gravity and the required amount of dark mass. People have argued over the severity of this perplexing problem, but the high collision speed happens quite naturally in MOND as a consequence of its greater effective force of attraction. So, taken at face value, the bullet cluster both confirms and refutes both theories!

I could go on… one expects clusters to form earlier and become more massive in MOND than in dark matter. There are some indications that this is the case – the highest redshift clusters came as a surprise to conventional structure formation theory – but the relative numbers of clusters as a function of mass seems to agree well with current expectations with dark matter. So clusters are a mixed bag.

More generally, there is a widespread myth that MOND fits rotation curves, but gets nothing else right. This is what I expected to find when I started fact checking, but the opposite is true. MOND explains a huge variety of data well. The presumptive superiority of dark matter is just that – a presumption.

David: At a physics colloquium two decades ago, Vera Rubin described how theorists were willing and eager to explain her data to her. At an astronomy colloquium a few years later, you echoed that sentiment in relation to your data on velocity curves. One concludes that theorists are uniquely insightful and generous people. Is there anyone you would like to thank for putting you straight? 
Stacy:  So they perceive themselves to be.

MOND has made many successful a priori predictions. This is the golden standard of the scientific method. If there is another explanation for it, I’d like to know what it is.

As your questions supposes, many theorists have offered such explanations. At most one of them can be correct. I have yet to hear a satisfactory explanation.

David: What are MOND people working on these days? 
Stacy: Any problem that is interesting in extragalactic astronomy is interesting in the context of MOND. Outstanding questions include planes of satellite dwarf galaxies, clusters of galaxies, the formation of large scale structure, and the microwave background. MOND-specific topics include the precise value of the MOND acceleration constant, predicting the velocity dispersions of dwarf galaxies, and the search for the predicted external field effect, which is a unique signature of MOND.

The phrasing of this question raises a sociological issue. I don’t know what a “MOND person” is. Before now, I have only heard it used as a pejorative.

I am a scientist who has worked on many topics. MOND is just one of them. Does that make me a “MOND person”? I have also worked on dark matter, so am I also a “dark matter person”? Are these mutually exclusive?

I have attended conferences where I have heard people say ‘“MOND people” do this’ or ‘“MOND people” fail to do that.’ Never does the speaker of these words specify who they’re talking about: “MOND people” are a nameless Other. In all cases, I am more familiar with the people and the research they pretend to describe, but in no way do I recognize what they’re talking about. It is just a way of saying “Those People” are Bad.

There are many experts on dark matter in the world. I am one of them. There are rather fewer experts on MOND. I am also one of them. Every one of these “MOND people” is also an expert on dark matter. This situation is not reciprocated: many experts on dark matter are shockingly ignorant about MOND. I was once guilty of that myself, but realized that ignorance is not a sound basis on which to base a scientific judgement.

David: Are you tired of getting these types of questions? 
Stacy: Yes and no.

No, in that these are interesting questions about fundamental science. That is always fun to talk about.

Yes, in that I find myself having the same arguments over and over again, usually with scientists who remain trapped in the misconceptions I suffered myself a quarter century ago, but whose minds are closed to ideas that threaten their sacred cows. If dark matter is a real, physical substance, then show me a piece already.

30 thoughts on “Oh… you don’t want to look in there

  1. Are there enough recognizable patterns yet in what the two theories get right, or fail at, in order to test whether there is something complementary about them?


    1. There are plenty of patterns, but I don’t know what you mean by complementary here, much less how to test it. The common attitude seems to be that MOND works in galaxies, but that non-baryonic dark matter is required for the formation of large scale structure. So perhaps they are complimentary in that respect. However, that statement is an oversimplification to the point of being misleading, so I wouldn’t want to base any claims about complimentarity on it.


      1. Maybe galaxy dynamics have little dependence on the age of the universe, while large scale structure development requires it?


    2. The dark matter and MOND ideas are complementary. The former is the hypothesis that there exists hidden mass we can’t detect, while the latter is the hypothesis that gravity is not Newtonian at low accelerations. Both could be correct. Indeed, the previous entry on this blog describes just how such a hybrid model would work:

      This is probably the only way to understand galaxies, galaxy clusters, and large scale structure simultaneously, including early Universe constraints like Big Bang nucleosynthesis and the cosmic microwave background anisotropies, plus a nearly standard expansion rate history. Galaxies would lack dark matter, but it would be relevant on larger scales, especially in galaxy clusters and for cosmology.


  2. Could a much higher density universe. Like 100 times denser, cause the effects that we see with galaxy rotation?


  3. One has to wonder, if the universe is almost as dense outside of galaxies, but nothing is visible, because the galaxy formation process, has not taken hold everywhere.


  4. stacy,

    have you consider non-mond modify gravity models i.e

    A Suggested Alternative to Dark Matter in Galaxies: I. Theoretical Considerations
    Hanna A. Sabat

    But since there are still some problems encountered by the standard dark matter paradigm at the galactic scale, we have resorted to an alternative solution, similar to Milgrom’s Modified Newtonian dynamics (MOND). Here, we have assumed that: (i) either the gravitational constant, G, is a function of distance (scale): G = G(r), or, (ii) the gravitational-to-inertial mass ratio, mg/mi, is a function of distance (scale): f(r).

    ….Furthermore, our model implies the existence of a critical distance at which the MOND effects become significant rather than a critical acceleration. In fact, Milgrom’s MOND converges with our model if the critical acceleration is not a constant but a linear function of the galactic bulge (baryonic) mass


  5. Yes, I’ve considered other theories. Most can be quickly discounted, and (like dark matter) only persist if the can mimic MOND in galaxies. The paper you cite is new enough that I have not looked at it yet, but it is nigh on impossible to properly recover MOND-like behavior from length-scale based theories.

    Liked by 1 person

    1. it is nigh on impossible to properly recover MOND-like behavior from length-scale based theories.

      why is that ?

      MOND-like behavior from length-scale based theories strike me as simpler and possibly explain other issues like galaxies cluster which is what motivated the author


      1. MOND requires that the equations of motion exhibit space-time scale-invariance and that the lagrangian approach a homogeneous function under space-time rescaling in the “Deep MOND Limit”, as shown by Milgrom, Such space-time scale invariance is fundamentally incompatible with dependence on a spatial length-scale.

        Restoration of Newtonian Dynamics as a correspondence limit does require that MONDian scale invariance be a “broken symmetry”, but the symmetry-breaking is characterized by an acceleration scale, not a spatial length scale.

        Dependence on a spatial length scale rather than an acceleration might be “simpler”, but it fails to fit the body of observational data; in particular, it will fail to reproduce the tight and apparently universal “Radial Acceleration Relationship”,, which involves only accelerations, not lengths.

        Liked by 2 people

        1. what about as cited in the paper “(ii) the gravitational-to-inertial mass ratio, mg/mi, is a function of distance (scale): f(r)…. In fact, Milgrom’s MOND converges with our model if the critical acceleration is not a constant but a linear function of the galactic bulge (baryonic) mass”

          since Mond based on the acceleration scale failed for galaxies cluster a single modified gravity law to explain both galaxies rotation and galaxies cluster with out dark matter may require abandoning acceleration scale


          1. Clusters are a problem for MOND, as I’ve discussed here and elsewhere many times. They’re also a problem for conventional dark matter, albeit in different ways. The problem for MOND is a factor of two in mass – just 20% in velocity dispersion, or 40% in X-ray temperature. Big deal – we’d quickly give the conventional picture a pass if it came that close. Things like this happen in astronomy; it would be surprising if there weren’t some tidbit like this that was somewhat off. So yes, while I take this problem seriously, on the scale of astronomical problems, it is not all that bad. Assuming it is real, then yes, one solution might be an adjustment of a single acceleration scale. This was the idea of the eMOND theory of Zhao & Famaey in which the action depends on depth of the potential well as well as acceleration. Maybe the paper you quote provides another way to do this, but offhand it sound wrong. Maybe there’s something to it, but I don’t have time to fact check everything that appears on the arxiv.

            Liked by 1 person

            1. does the sum total mass of standard model neutrinos plus black holes enough to account for this in mond?


              1. In principle, yes.

                We don’t know the mass of black holes in clusters, so this is a logical possibility, but a very open-ended one. It seems like a stretch to me, but all solutions seem like a stretch at this point – for dark matter as well as for MOND! There is adequate uncertainty in the detected baryon content of the universe to solve the cluster problem in MOND with some form of normal but unseen matter (black holes, brown dwarfs, etc.) without violating the constraints from Big Bang Nucleosynthesis.

                For neutrinos, it depends on what the absolute mass of the neutrino turns out to be. It would have to be near the experimental upper limit (~ 1 eV) to have an impact on this problem. The minimum mass from flavor mixing (0.06 eV) doesn’t add up to much. Another interesting thing about neutrinos is that conventional structure formation requires a mass < 0.12 eV, so anything significantly greater than that would break the standard model. A non-negligible neutrino mass actually helps prevent MOND from overproducing structure as well as potentially contributing to the mass budget of clusters.

                Liked by 1 person

          2. Given that galaxy rotation curves are flat, modified gravity explanations for this must necessarily involve gravity transitioning to inverse distance somewhere. With a single galaxy, it is not possible to tell if the critical parameter is distance or acceleration. But suppose it is distance. Then we must have that g = v^2/r = GM/(r*r_0) in the flat region, so v^2 \propto M and the factors of r in the denominator cancel out. Looking at many galaxies, it is clear that the main prediction is a Tully-Fisher relation with a slope of 2, so an empirical fit of Log(M) vs Log(v) should have a slope of 2 as M \propto v^2 in this model. The observed slope is very close to 4, which rules out at over twenty standard deviations the idea of modifying gravity beyond a particular length scale:


            More generally, there is not much of an acceleration discrepancy in the central kpc of the Milky Way, but sometimes very large discrepancies in low surface brightness dwarfs that are also about a kpc in diameter. This problem allowed us to falsify the length-dependent modification to gravity known as Moffat Gravity (MOG):


            However, an acceleration-dependent modification works quite well.

            Liked by 1 person

            1. what about ” (i) either the gravitational constant, G, is a function of distance (scale): G = G(r), or, (ii) the gravitational-to-inertial mass ratio, mg/mi, is a function of distance (scale): f(r).”

              Liked by 1 person

  6. “The minimum mass from flavor mixing (0.06 eV) doesn’t add up to much. ”

    if the mass standard model neutrinos is lower end estimate could you offset this by having more of them and in Mond with no Cdm, tweak Big Bang Nucleosynthesis?


    1. I would consider it more than a tweak to BBN to tolerate more neutrinos. I think the least unreasonable way to do that would be to have a sterile neutrino as Indranil Banik was suggesting in his guest post.

      Liked by 2 people

      1. if dark matter is sterile neutrino in the right quantity based on the Cmb third peak and distribution doesn’t this rule out Mond?

        could gravity ” Slow” or decelerate standard model neutrinos to non-relativistic velocity

        Liked by 1 person

        1. You can have both sterile neutrinos and MOND, I would just prefer not to (Occam’s razor). What would be difficult to sustain is both MOND and cold dark matter. If there is the right amount of CDM in the universe, then it will condense on small scales, contribute to the mass budget in galaxies, then MOND would over-do the kinematics. So one needs to keep any dark matter out of galaxies, if not clusters. Neutrinos, or any form of warm dark matter could in principle play this role, provided the mass isn’t too large (so it becomes too cold). So yes, gravity slows standard model neutrinos over time; whether they remain relativistic now depends on their mass.

          Philosophically, I do not like hybrid solutions. In principle, yes, there can be both MOND and dark matter. But this smacks of Tycho Brahe’s model for planetary motion: the best of both worlds yet completely sterile, informing no further work.

          Liked by 3 people

          1. Given that cosmology and quantum mechanics lie at the two extremes of our observational capacity, isn’t it a little unsettling from a philosophical point of view that the principle of complementarity should pervade one and not the other?

            Liked by 1 person

          2. would 2 long range forces work where the first is newtonian gravity and the second is an unknown very weak attractive force that is only apparent on the scale of dark energy and ao?

            Liked by 1 person

          3. The point against hybrid solutions is well made. “With four parameters I can fit an elephant, and with five I can make him wiggle his trunk.” (as later reported by Freeman Dyson following criticism of his model by Enrico Fermi in 1953, who cited this aphorism as coming from John von Neumann). One wonders what Fermi and von Neumann would have made of the Standard Model of Cosmology, with its need for six parameters to fit the known data.

            On a tangential point, we run the risk of presentism in criticising Tycho Brahe’s model for planetary motion. Based on what was known at the time, one of the principal criticisms of Copernicanism was that the calculated sizes of the stars were not similar to the Sun, but larger than tne entire Solar System. This issue is well covered in Christopher Graney’s book “Setting Aside All Authority” about Riccioli, and comes from the pre-telescopic and early telescopic astronomers failure to appreciate that the apparent sizes of stars were not real but were an artefact of diffraction and scattering in the telescope and the observer’s eye. We have to wait until Huygens and his wave theory of light in 1678 for the first explanation of this.

            Liked by 1 person

            1. Good point – Brahe’s model explained everything, yet didn’t catch on. But, as you point out, some of the data are inevitably wrong or misleading in some way. This is one reason that I think it a mistake to dismiss MOND because it is a bit off in clusters. Even the correct theory will appear to be in conflict with some of the data, just because you can’t trust 100% of the data 100% of the time.

              Liked by 2 people

  7. Hi Stacy, et al, So the creative juices are flowing again and my theory of everything just took another relentless step towards simplicity and math. I thought I would mention it here, because although many might think it is too creative, I just need one person in the field to get it and help it catch fire. Think of it as my holiday gift to you all. Best, Mark
    p.s. the relevance is that it rewrites the LCDM narrative after unifying GR and QM and QFT and …etc. It is all so simple now and there is a tremendous amount of low hanging fruit and recasting required to re-orient all of our brains to the Euclidean frame and immutable point charges. There are also a tremendous number of new discoveries to make. My promise : Once you get over the disbelief and get comfortable with the TOE, you will be so incredibly happy that everything makes perfect sense now. p.s. I am a creative problem solver, ideator, and engineer and score very high on that darned index. I’ve heard it all. Just go study it and think. Also check out many other articles on how this universe works.

    Liked by 1 person

Comments are closed.