You’re sweet as a honey bee

But like a honey bee stings

You’ve gone and left my heart in pain

All you left is our favorite song

The one we danced to all night long

It used to bring sweet memories

Of a tender love that used to be

Now it’s the same old song

But with a different meaning

Since you been gone –The Four Tops

Those of you who’ve been with me since the start of our current series on Dark Matter, including parts I, II, III, and 3.5, know that I’m a big proponent of dark matter. I think, based on everything that we know, that it is the simplest, easiest, and most likely explanation for why — on galactic scales and larger — the Universe behaves as it does.

One of the key observations we have, though, supports another conclusion as well. You see, when we look out at a spiral galaxy, like this:

We can measure how quickly it rotates. We can look at the central part of the galaxy, the intermediate parts, and the outskirts. If there were only stars and gas (i.e., normal matter), we’d expect the outskirts to rotate more slowly than the inner parts, the same way that the outer planets orbit our Sun more slowly than the inner ones.

But it doesn’t do that. The outskirts move at the same speed as the inner portion. The difference between the expected (A) and the observed (B) is easy to see.

Now, there are two possible reasonable explanations for this. The first one is dark matter. In addition to all of the other things that dark matter explains, a diffuse dark matter halo around a galaxy could very easily explain the rotation of these galaxies, and we have very simple models that can accurately explain them.

But the other possibility is that Newton’s laws of gravity get modified for very small accelerations. How small? Below changes in velocity of about one nanometer per second over a time interval of a second. So if you modify Newton’s F = ma to add this minimum acceleration, you can also fix these rotation curves. It works for practically every spiral galaxy.

But then we can take this theory, MOND (MOdified Newtonian Dynamics), and try to apply it to other things.

1.) We can try to apply it to low-surface-brightness galaxies, like above, and see if it gives us a reasonable prediction.

2.) We can try to apply it to galaxies moving around superclusters, and we can see if it gives us a reasonable prediction.

3.) And we can apply it to the structure that the Universe forms, and we can see if we get a reasonable answer out.

As you may have guessed, on counts 1, 2, and 3 above, MOND completely fails! It simply makes a prediction that doesn’t match up with observations at all.

So you’d think that would be the end of the story, right? I mean, MOND has been singing the “we predict galaxy rotation curves” song since 1983, and it’s been failing to predict these other things for just as long.




Because I’ll start listening to the MOND song again, just as soon as it starts explaining these things it currently doesn’t, but not until then. I don’t understand why anyone would think otherwise.


  1. #1 LtStorm
    October 6, 2009

    Okay, I was wrong in my comment on Part 3. It’s MOND that sounds like it’s trying to lead astrophysics down into a cul-de-sac like phlogiston did with chemistry. Just replace George Stahl’s students (but not Stahl him! He was just throwing out ideas!) with Hong Sheng Zhao pushing the theory and ignoring the evidence against it that was quickly piling up.

  2. #2 NewEnglandBob
    October 6, 2009

    What is the “minimum acceleration” based upon? What evidence did they have for saying it is non linear?

  3. #3 Naked Bunny with a Whip
    October 6, 2009

    Didn’t you see the 1978 Superman movie? Jor-El already explained this: each galaxy has its own laws of physics! Sheesh, try to keep up with the literature.

  4. #4 Sam K.
    October 6, 2009

    Ethan, correct me if I’m wrong, but as “simple” as an explanation Dark Matter is, it seems you have to put it in different configurations in different situations to get the desired effect. So, in some sense, I feel like you have an incredible amount of flexibility when you use a D.M. explanation for an observation, i.e. for any observable, you can put D.M. *somewhere* to get the observation you want. This feels kind of week to me. To be a good scientific theory, it has to predict something. What can you predict with it? (as opposed to place some D.M. in some configuration depending on the situation to explain your observation)

  5. #5 Leukippos
    October 6, 2009


    LOL ^ e.

  6. #6 Jason Dick
    October 7, 2009

    My favorite prediction of dark matter, Sam K., is the power spectrum of the CMB. The basic idea is that before the emission of the CMB, the normal matter exists in a plasma state and therefore experiences pressure. But the dark matter doesn’t. This means that when you have a bit of an overdensity in some region, the dark matter will just fall into it. But the normal matter falls in, feels this pressure, and bounces back out.

    We can see the effects of these bounces in the cosmic microwave background by comparing the even and odd peaks of the power spectrum of the CMB. The fact that dark matter doesn’t bounce tends to suppress the even peaks, which means that any even/odd “ringing” of these peaks is very strong evidence of dark matter. And that’s exactly what we see when we look at the CMB.

  7. #7 ScentOfViolets
    October 7, 2009

    What about gravity-modification theories that add an off-brane contribution? Then the dark matter is actually residing elsewhere, but the only contribution it can make is gravitational. Two for the price of one as it were.

  8. #8 Steinn Sigurdsson
    October 7, 2009

    Ethan – this is not a creationism debate.
    MOND may be wrong, but it is not evil.




  9. #9 Mu
    October 7, 2009

    The debate sure has a certain religious fierceness to it, what’s surprising since we’re just talking mathematical models both based on an so far unproven assumption. The dark matter camp needs to find an explanation for the wimps, and the MOND people need to show that gravity is dependent on r other than 1/r^2. And then it still needs to be unified with QM. Lots of road still to travel to be so sure of the right solution.

  10. #10 Ethan Siegel
    October 7, 2009


    I disagree with your analysis of the situation and with your assessment of what the authors have accomplished. I also disagree with your categorization of MOND as wrong but not evil.

    I will probably quote and respond to this comment in a post tomorrow, because it deserves an in-depth treatment.

    But while Leslie Sage may have known what he was doing when he published this (and when his team talked to the press about it), I don’t think I agree with it at all.

  11. #11 Steinn Sigurdsson
    October 7, 2009

    This could get fun.

    Just to satisfy all FTC requirements on blogging:
    Hong Sheng and I were briefly at Cambridge together, worked together on a project, and I somewhere still have a half-written paper we started together.

    Stacy McGaugh, one of the current primary proponents of MOND, is an old friend of mine and we were at Cambridge together for an extended period.
    Apart from Milgrom, most of the people flirting with MOND in astronomy are colleagues, and I am quite familiar with much of their research and where they were coming from when they decided to look at MOND or compare observations with MOND models.

    That doesn’t mean I think MOND is a correct model, but I know quite well why it is still in play with a not-insignificant minority of astronomers.

  12. #12 Ethan Siegel
    October 7, 2009


    I’m going to take some time to put my thoughts together on this, but I’ve gotten to meet a fair number of MOND advocates. Stacy is one of them, as is Arthur Kosowsky and Philip Mannheim.

    But that said, I don’t agree with all of their actions, and I have good reasons for being edgy after reading the Nature paper and the accompanying ‘news’ stories. I’m happy to have a friendly inter-blog discussion with you about this. Come back tomorrow and I’ll have a post up on it.

  13. #13 Pierce R. Butler
    October 7, 2009


    Poking around in the SB archives, I can’t find any SWAB posts addressing this explicitly. Perhaps you could spell this out for us someday?

  14. #14 Thomas Neil Neubert
    October 7, 2009

    This explanation is pretty good because it tells me enough to know why I’d like or NOT the alternative to “dark matter” known as MOND. Whether I agree with the conclusions or not; this is a good critique of MOND. And if I understand this post correctly MOND is the best alternative theory to “dark matter”. What’s the next, alternative to be critiqued?

    Now let me defend “dark matter” theory a little bit. Sam K. says, “To be a good scientific theory, it has to predict something.” Actually I disagree. A good scientific theory need only be descriptive. For example in biology; if I call a newly discovered organism a “bacteria”; then I know a lot of things about that organism. And evolution doesn’t predict the next bacteria or swine flue virus. Even seeing an bacteria in a petri dish, we can’t predict if that bacteria lives in the gut of a rhinoceros or on the leaves of the eucalyptus tree. But knowing that an organism is a bacteria nevertheless says a lot about the organism. Similarly, that “dark matter” theory is descriptive is not one of its faults from my point of view. Actually the fact that it provides a good description is the “dark matter” theory’s most appealing point.

  15. #15 Mu
    October 7, 2009

    Ethan, in one of your earlier posts you mention that the standard model puts an upper limit on baryon matter in the universe – can you explain a bit more why that same limit does not effect wimps? Or, from a different angle, why the standard model has no issues with the amount of energy set free at the bang/inflation being 20 times as much for the dark matter/dark energy model as was needed to form the “classic” universe.

  16. #16 Sam K
    October 8, 2009

    @14: I disagree with your points about evolution and biology. Saying something is a bacteria implies that it has very specific properties. More importantly, you can say that it doesn’t have OTHER properties. This is a falsifiable prediction. Similarly, evolution predicts that we should observe certain things that are later found. Granted, its falsifiability can be a bit of a stretch at times, but it certainly makes at least some falsifiable predictions. Jason Dick’s comment is a very interesting falsifiable prediction because it’s not of the very general form “there exists a distribution of DM that explains what I see.” That’s what I was looking for. My problem was that all the statements about DM that Ethan has described in this (great!) blog post series weren’t really falsifiable. There were of the form mentioned above. If something is not falsifiable, it’s not science, at least how we generally understand it due to Popper.

  17. #17 Thomas Neil Neubert
    October 14, 2009

    Your analysis is well reasoned; there is no reason to further consider MOND. But that doesn’t make “dark matter” part of the bedrock of physics; it just makes “dark matter”theory the most adequate theory at the moment.

    As to why credible publications publish MOND, cold fusion or any number of dubious theories; it is because apparently credible scientists tell us that such and such is so. In the dark comedic documentary the Atomic Cafe; I remember the government scientist explaining to the soldiers who were about to witness a nuclear explosion that, “You don’t have to worry about the radiation; it will just go right through your body.” Then after the explosion (in simulation of a real wartime situation), the soldiers climbed out of their “protective” foxholes and walked toward the “harmless” mushroom cloud. I think scientists are more likely to be naively stupid than pimping the political system.

  18. #18 Fredd
    April 27, 2010

    hi wats ur name my name is tyler i am at school i got to go bye bye

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