Dark matter – that invisible stuff that is supposed to make up some 20% of the Universe – was thought up to explain a puzzling observation. The amount of mass we can see through our telescopes is not enough to keep galaxies from spinning apart. The existence of great quantities of hidden mass would provide the gravitational pull needed to form those galaxies and enable them to rotate in the way that they do.

But not everyone is willing to buy the idea that the Universe is cloaked in “invisible cloth.” An alternate theory, first put forward by Weizmann Institute astrophysicist Prof. Moti Milgrom in 1983, doesn’t require dark matter to explain the phenomenon. Instead, it posits that gravity works differently on the intergalactic scale. With a good tweak to Newton’s formula, the observed Universe falls into place. This is not the violation of a basic law of physics that it might appear: Milgrom points out that gravity works fine in our every-day world, but the formula breaks down at extremes – at the speed of light or in the sub-atomic world of quantum mechanics, for example. So super-galactic scales could be another case in which the rules of gravity simply don’t apply quite as Newton wrote them.

While most are still waiting for the hunt for the mysterious dark matter to yield results, a growing minority of physicists are starting to admit that MOND (modified Newtonian dynamics) could provide a better explanation. Recently, for instance, Prof. Stacy McGaugh of the University of Maryland added fuel to the debate by showing that for galaxies, MOND fits the facts quite reliably – better than dark matter theories.

Even in light of a number of recent studies that lend support to the idea, MOND is considered controversial, and its proponents are often depicted as rebels. Could it be because they keep insisting that the ruling theory, dark matter, has no clothes?

Image: NASA


  1. #1 Left_Wing_Fox
    March 7, 2011

    Sean Carroll and Phil Plait have some solid reasons as to why MOND is controversial (I.e. that it dosen’t fit the evidence as well as Dark Matter.)

  2. #2 Dunc
    March 7, 2011

    Even the strongest MOND proponents still admit you need DM to explain everything other than galactic rotation curves. MOND is not actually an alternative theory to DM, it’s an additional theory which only explains a specific subset of the data.

  3. #3 David
    March 7, 2011

    SB’s own Ethan Siegel has listed the many ways that MOND is either fails to explain or is inconsistent with observations other than galactic rotation curves.


  4. #4 Lazlo's Other
    March 7, 2011

    Posts like this one have no credibility. The hunt for dark matter has yielded results, specifically with gravitational lensing. Dark matter has been observed to cause gravitational lensing where there is no “classic” or “normal” matter. How would MOND explain that, or the microwave background, or galaxy clusters?

    this post has no clothes. If you want to pimp one of your own feel free, but have the intellectual honesty to present the whole picture.

  5. #5 rob
    March 7, 2011

    one word: Bullet Cluster.

    (oaky. two.)

  6. #6 Weizmann Science Writer
    March 8, 2011

    Ok guys. No need to be rude. I understand that feelings run high on the subject, for some reason, but open minds would seem to be in order here. As you well know, the evidence is still circumstantial. (Note the question mark in the headline)

  7. #7 Douglas Watts
    March 8, 2011

    No offense, anonymous science writer, but prior to writing this you must have read the two very recent and lengthy expositories on this subject by Sean Carroll and Ethan Siegel, and should at least used this post to directly address and attack the very clear points of evidence they used to support their positions. Rob, is right. What about the Bullet Cluster? It seems to show that the gravity is not where the matter is; and the matter is not where the gravity is. MOND needs to be able to explain this empirical anomaly as well or better than it is explained by dark matter.

    You say, “Recently, for instance, Prof. Stacy McGaugh of the University of Maryland added fuel to the debate by showing that for galaxies, MOND fits the facts quite reliably – better than dark matter theories.”

    But as you must know, the testing regimen for MOND vs. DM has long ago moved from galaxies to galaxy clusters and the cosmic background radiation. As Sean and Ethan said just week, even MOND enthusiasts admit DM is also required to explain the observed data in these circumstances or MOND just doesn’t work.

    So at minimum it’s not an ‘either/or’ — but this post seems to posit it as such without any evidentiary support for such a dichotomy.

  8. #8 soleil
    March 8, 2011

    Why not look to Earth as a source where Dark Matter can be found in some way? What I think is happening is that we have different concepts for elements of life we already know and maybe those are part of Dark matter or result from it, so when we look to Dark Matter we hope to find something really different to what we know, so maybe we are wrong, why not tie those concepts to a Dark matter process on Earth? @soleilwhale

  9. #9 AThinkingScientist
    March 8, 2011

    I am amazed at the low scientific level of the comments above. Alone resorting to Ethan Siegel’s blog proves a lack of scientific credibility. Comment number 1 points to Sean Carrol’s blog, and there is a long discussion there with quite a few arguments showing why the cold-dark matter hypothesis is, essentially, already ruled out by the astronomical data.

    Apart from this, the overall construction of the LCDM model is unsatisfying, as e.g. even energy conservation is not guaranteed.

    As this useful blog here shows, and more importantly, what Stacy McGaugh’s and other researcher’s recent work has been showing (e.g. Peebels & Nusser 2010, Nature; Kroupa et al. 2010, A&A), is that MOND works overwhelmingly well on galaxy-cluster down to galaxy scales. Whatever one may think, all theories one may try to develop, even if they were based on some extended cold dark-matter notions, _must_ reproduce MOND behaviour on galaxy scales.

    I am not a MOND researcher and have spent most of my successful research career within the LCDM framework, which I still see as a useful calculation tool, but to my mind MOND’ success is too great to ignore. Only fools would not see MOND as a very strong hint at deeper physics at the very fundamental level.

    Milgrom’s discovery of the scale a_0 appears to be one of the major scientific discoveries of the second half of the 20th century, akin to Planck’s introduction of his speculative parameter h (it stands for “Hilfsgroesse”, which is German meaning auxiliary number). Planck nor his contemporaries had any idea what it meant, and its relation to energy quantisation was realised much later.

    So people, _think_ before you follow like sheep.

  10. #10 Paul
    March 8, 2011

    We see massive objects at the bottom of deep gravity wells, so we determine that mass warps spacetime. I’ve wondered if we could’ve just reversed cause and effect here: maybe spacetime is warped of its own accord, and massive objects pool into the wells. Then dark matter is just a gravity well that hasn’t been filled. I think stellar evolution is an argument against the idea, linking gravity with atomic processes, but I haven’t completely convinced myself one way or the other…

  11. #11 I.P. Freeley
    March 9, 2011

    First off Weizmann Science Writer, no one was particularly rude. Pointing out how your post is overwhelmingly week is not rude.

    Second, this post reads just like a creationist track. There’s the construction of a false dichotomy, the cherry picking of one favorable paper while ignoring the mountain of papers against, no links to other relevant posts, and a complete fabrication–”a growing minority of physicists” are jumping on the MOND bandwagon. Name one. Stacy has been doing MOND work for years, he doesn’t count as a new convert.

  12. #12 Dan Echegoyen
    March 9, 2011


    In the continuum of space and time, exists the dichotomy of matter and energy. All things exist as both matter and energy, but are experienced as one or the other.
    As energy, all things exist as wave patterns. Most wave patterns are interferences of simpler wave patterns. The simplest wave forms are those that do not interfere with other waves. These simplest wave forms hold their shape as they propagate. There are three such wave forms.
    The first such wave form is seen in three dimensions as the spherical expansion wave of a bomb blast, and in two dimensions as the circular wave of expansion on the water where a rock was tossed in. The second wave form is seen in three dimensions as the cone of sonic boom following an aircraft traveling faster than sound, and in two dimensions as the V-wake on the water where the boat is traveling faster than the water wave. The third wave form is seen in three dimensions as the propagation torus of a smoke ring and is seen in two dimensions as the double vortexes of an oar stroke on the water.
    The Torus is a particle of discrete exchange, from one point to another. The object exchanges position and momentum. While the spherical wave shows position, and the conic wave shows momentum, the torus shows both at the same time, and has a dynamic finite unbounded reality. The volumes of the cone, sphere, and torus are mathematically related as static objects.
    The Universe is a local density fluctuation. (a wave pulse) On this local density fluctuation wave, lesser wave forms may exist. All simple wave forms are also local density fluctuations, and as such are indeed universes in their own right, where other waves may exist.
    Consider the torus as a universe. Einstein said that gravity is indistinguishable from acceleration. There is both linear acceleration and angular acceleration. Although the torus as a whole travels in a straight line, every local point on the torus travels in a circle and experiences angular acceleration.
    The rubber sheet model of gravity and curved space translates directly to the propagating torus with angular acceleration. Acceleration is downward on the rubber sheet and outward on the torus. The tension field that separates the inside of the torus from the outside holds its shape as a simple two dimensional field of space and time just as the rubber sheet does.
    Experimentally verifiable is that a big fat slow smoke ring generated in a room with very still air will eventually possess a bulge that travels in a circle on the surface of the smoke ring. This bulge, being a gravitational depression, gathers more of the energy of the field toward itself. Finally the bulge gathers enough material to collapse the field and eject a new, smaller smoke ring out in the same direction as the first torus. This collapse is a black hole to the first torus, and a white hole to the second torus, where the axes of space and time in that second torus have reversed.
    While gravity tends to draw depressions together locally on a dynamic torus, even to the point of field collapse, other areas on a torus expand and contract globally as the torus propagates along without regard to local phenomenon on the surface. This is quintessence. The inertia of the torus to propagate is its dark energy. This is a two-dimensional example of the process that we experience in three dimensions.

    From structureofexistence.com by Dan Echegoyen 951-204-0201

  13. #13 Weizmann Science Writer
    March 9, 2011

    I just want to point out that I am exactly what the name “Weizmann science writer” implies: I write about science at the Weizmann Institute. Ergo — far be it from me to try to discredit Ethan Siegal or Sean Carroll, and if I “cherry pick” — that is sort of the point of a blog about the science at a particular institute.

    Having said that, I must say I am quite intrigued by the discussion here, and I have gained new respect for a scientist who has held what is clearly an unpopular position for going on 30 years.

  14. #14 AThinkingScientist
    March 9, 2011

    @I.P.Freely(#11): From this text one can only conclude that I.P.Freely has lost touch with the most recent research results.

    The quite bad blog on cosmology by Ethan Seagal (it is extremely biased and of low information content) and the Sean Carrol one have truly cherry picked results ignoring vast arenas of modern astronomical data. I.P. Freely is apparently following merely wishful thinking rather than hard scientific leads.

    And, yes, I am a researcher who has been working mostly in LCDM, finding it fails after 15 years of hard work, and seeing, at the same time, MOND getting it right. It is blatantly clear that MOND is not the final theory of the universe, but it is even clearer that LCDM is also not. But MOND is at the basis of a new development, while LCDM cannot be improved further.

    I do not want to disclose my name because of people like I.P.Freely who are destroying the avenue of open scientific discourse. I see this as a very major problem for the USA where fundamental theoretical research is all but stagnating – compare with Lee Smolin’s excellent book on “The trouble with Physics”.

    @Weixmann Science Writer (#13): The writer reports correctly on recent research on what amounts to a current contribution on among the most important progress in fundamental research achieved over the past few decades. Thank you! And, respect for Stacy McGaugh for daring to write an excellent and highly significant research paper.

  15. #15 Ben
    March 9, 2011

    @I.P. Freeley: Here is an objective way of backing what the Weizmann Science Writer has writen, if you are willing to do something objective, which by reading your post isnt completely clear to me. Click on the link of the original MOND paper of Milgrom: http://adsabs.harvard.edu/abs/1983ApJ…270..365M … Then click on the link “Citation History”. And tell us if you see something globally growing or not…

  16. #16 Left_Wing_Fox
    March 9, 2011

    I understand that feelings run high on the subject, for some reason, but open minds would seem to be in order here

    Ok, NOW I’m insulted.

    No, I do not have a dog in this fight, I’m an animator with a background in biological sciences, not a cosmologist. I want to tell you, what I, as a regular reader of Scienceblogs, saw when I read this.

    First, I read the posts by Plait and Carroll. While both of them pointed out that MOND works well to explain galactic rotation, both of them pointed out where MOND does not serve as a superior theory to Dark Matter.

    Then I read your post. Instead of actually addressing the issues raised by the other astrophysicists (I.e. That MOND does not explain gravitational lensing anomalies like the Bullet Cluster or match observations of the Microwave background the way Dark Matter does) , you claim the controversy is because of emotional investment in a poor theory.

    When five people pointed out what we believed to be legitimate objections, you seized upon the one sentence in five posts which contained an insult, and admonished us not to be so rude and emotional.

    Perhaps if you actually wrote a post with a clear explanation of why those points were wrong, you might have convinced people. Instead, we get a thread of ad hominem against Carroll and Siegel, admonishments for emotional attachment, and finally an expression of self-martyrdom.

    I’ve seen this behaviour before, and it rarely bodes well for the theory. You shouldn’t be surprised at all that the conversation took the tone it did: YOU SET IT! You chose to complain about tone and ignore the objections. Had you actually attempted to confront the points raised head on, this might have been a constructive debate, rather than the petulant whining it became.

    In the future, try actually responding to arguments, rather than whining about being attacked for holding a minority opinion. Grow a spine.

  17. #17 Lazlo's Other
    March 9, 2011

    Just wow ATS. Inability to post your name due to dark, nefarious forces, an Ad-Hom on Ethan (Ethan’s posts certainly contained more information than the one above), an appeal to your own authority, and a refusal to admit that MOND is quite limited in what it explains, or that there is actually evidence for DM.

    well done.

    As was pointed out above, the bullet cluster collision is a strong indication of the presence of DM, and MOND can’t account for the lensing, unless DM is used – I. Ferreras et al., “Necessity of Dark Matter in Modified Newtonian Dynamics within Galactic Scales”, Phys. Rev. Lett. v.100, p.031302 (2008).

    You yourself admitted the bullet cluster can’t be explained by MOND without also using DM on in the comments section of another blog. MOND doesn’t eliminate DM, it changes it somewhat.

    If you want to be accurate you need to deal with the fact that the above post implies DM is not a viable theory, and that MOND explains more than it actually does. You might also note that in implying this the post misrepresents Stacy’s paper, which doesn’t address DM in any way. The majority of comments here deal with the misrepresentation, instead of dealing with this you erect strawmen.

    As far as open discourse, this strikes me as paranoid. It is the same accusation thrown out there by anti-evolution parties, anti-climate change parties, and even moon hoax proponents. The fact that Stacy can publish and his ideas are noticed and discussed would contradict your assertion. There is other work on MOND out there, easily available.

    If you think MOND is viable, you should encourage those of your interested students to pursue it – a breakthrough could make someone’s career (on Sean Carroll’s blog you stated that you tell them they have to do LCDM). Also note that some scientists (Stacy McGaugh for instance) have worked on both. Telling them to avoid it is cynical (you are steering them into what you consider to be a dead end) and, if you truly think it should be developed further it is self defeating.

    Even the major proponents of MOND accept that DM is required, to state that it is rendered superfluous by MOND is misinformed at best.

    One thing I have noticed is that the DM crowd is willing to point out where MOND does succeed, as well as where it is weak. They explain why they don’t think it will replace DM. The MOND crowd on the other hand seems to focus undue attention on the limited area where it does succeed, ignore where it doesn’t succeed, and ignore the successes of DM.

    The statement that LCDM cant’ be improved further seems to be your opinion. Other notable cosmologists disagree – recent detection in the bullet cluster would indicate more can be learned. Given that we have only indirectly detected DM to this point your assertion appears premature.

    “Only fools would not see MOND as a very strong hint at deeper physics at the very fundamental level.” Funny how the exact same statement could be made for DM.

  18. #18 Weizmann Science Writer
    March 10, 2011

    Sorry for the delay on these last two comments. Not due to my supposed psychological problems but to a connection problem on the site. (Some of you still don’t get it, do you?)

  19. #19 AThinkingScientist
    March 10, 2011

    #17: Not much new here: The comparison to anti-climate people is frequently used… The Bullet Cluster is acutally a major problem for LCDM but fits-in naturally with MOND. There is nothing wrong with having some(hot) dark matter in MOND to explain the lensing, but it may also just be the missing baryons as has been pointed out before. So no real problem for MOND here.

    LCDM: its many parameters are extremely well constrained by now, at least this is the claim by the high-precision cosmologists. So there is not much one can do to improve it. The physics of normal matter is rather well understood too, although LCDM enthusiasts try to distort it to make the model galaxies fit real galaxies. Most of these distortions are unphysical though, sorry. The dynamics of the cold dark matter particles is extremely well constrained. At best one can speculate that there are additional unknown forces that act between dark matter and/or between baryons in order to try to fix the galaxy conundrum.

    But this would then not be the LCDM model any longer, since these forces would presumably also be active in the earliest times and they would then change early structure formation.

    Thus you are stuck! And resorting to Ethan Siegel’s blog is quite unhelpful, because Siegel does not report in a scientifically balanced way.

    As to the students: they need to have an income, and the propagation of the LCDM model beyond the fact that it is merely one attempt to explain what we see is locking-up very major resources.

    I have seen senior scientists jumping up from tables crying “But everyone knows MOND is crap” (in exactly these words!) and I must say that this simply goes too far. When senior professors _only_ teach LCDM as the gospel to advanced students and actively discourage students to look at MOND or alternatives, then we have a system failure. And your contributions do not help to improve the situation.

  20. #20 AThinkingScientist
    March 10, 2011

    #16: This is an unfair and also incorrect representation of what happened.

    First of all, as the WeizmanScienceWriter explained: the issue is to report on research done at the Weizmann institute.

    The blogs by Carrol and Plait did not represent MOND correctly. It is false to write that MOND only explains rotation curves. MOND is a theory explaining the dynamics of normal matter on all classical (as opposed to relativistic) scales. It is a triumphant success on galaxy scales without fine-tuning.

    MOND aleviates very significantly the need for dark matter in galaxy clusters. Perhaps there is some dynamically relevant hot dark matter, but this is no argument against MOND. Especially so, since this hot dark matter can be derived naturally from the standard model of particle physics via neutrino oscillations. MOND has been extended to a relativistic theory with success, so lensing can be studied. But this extension is merely one try and it shows that MOND can be derived from a more general relativistic theory, which, by the way, also contains General Relativity.

    The relativistic extension of MOND then accounts excellently for the Bullet Cluster.

    Here is now my question to the commentators 1-7:

    Why did Siegel, Carrol etc. not explain all of this in a balanced way in their blogs? Why did they instead publicly make false statements on MOND, that it only accounts for a narrow range of data, or that it even fails on clusters? Why are these commentators and those bloggers representing science so wrongly?

    The readers would expect an answer please.

    To commentator 4: The “evidence” for dark matter through lensing comes about because in General Relativity one needs to invoke dark matter to explain the observed lens signal. In another theory of gravitation this is different. For example, in the relativistic extension of MOND significantly less dark matter is needed, while there are other theories where no dark matter at all is needed to explain the lensing signatures.

    Thus, the need for dark matter only arises if you are working in a given gravitational framework, and it may disappear in other frameworks. There is nothing in the books which forces us to live in Einstein’s General Relativity – perhaps gravity is different, perhaps his field equation is not complete, which is a real possibility given that energy conservation is not secured in GR. Yes, GR has passed many tests, and these constraints are obviously also relevant for any extension of GR or any other theory of gravitation.

    Since the standard cosmological model fails to account for galaxies, why should it then be used as an argument for cold dark matter in galaxy clusters? This question is especially valid since another theory, the relativistic extension of MOND, reduces the need for dark matter in galaxy clusters and is a full success on all galaxy scales without fine tuning.

    And why do these commentators then take the liberty to attack the WeizmannScienceWriter?

    The hot discussion here is therefore clearly not the fault of this ScienceWriter, but is driven by the consistently false statements made in public by the named bloggers and the commentators 1-7.

  21. #21 Daniel de Rauglaudre
    March 10, 2011

    With MOND, neutrinos can be possible candidates for the dark matter required to resolve their problem of the Bullet Cluster?

    In the one hand, dark matter alone is simpler than MOND + dark matter.

    In the other hand, MOND + dark matter as neutrinos is perhaps better than dark matter of we-don’t-know-what-it-is-made-of.

  22. #22 anand srivastava
    March 18, 2011

    My problem with DM+GR is not that it does not fit the data.
    My problem is that MOND fits the data without needing DM.
    Since this simple equation works well at the galactic scale everywhere. Actually the fits keep on getting better with more data. There can only be two solutions.
    1) DM does not exist at Galactic scale.
    2) DM position in space is defined by BMs position in space. This is simply untenable if we believe DM to be separate particles. Also BM is not supposed to interact with DM except by gravitational force. This is totally non-sensible

    So the real solution is that DM does not exist at the galactic scale.
    I would have had no problem with DM, if MOND did not work so well at galactic scale.

    The fact that MOND does not work as well at cluster or higher scales makes no difference. It is probably an indication that some form of DM exists on those scales. I don’t even think that MOND can be enhanced to form a theory. TeVeS is just a toy theory that shows how to build one, but I am pretty sure the encompassing physical theory will be discovered from a totally unexpected direction. The recent Verlinde’s theory of Entropic gravity looks interesting.

    I liken MOND to an Empirical law. Any quantum theory of gravity needs to bring out MOND or it is not physical. Since GR in its present form does not predict MOND, it is not physical.

    It is as simple as that. Empirical laws must be explained by all physical theories. If Newtons gravitational theory did not explain Kepler’s laws, it would be as useless (at the solar system scale) as GR is presently (at the galactic scale).

  23. #23 Gerd Petersen
    April 8, 2011

    #5 one word: Bullet Cluster.

    Here the observations apparently confirm the required collisionless property of dark matter. But in the case Abell 520 dark matter behaves completely
    collisionally (http://adsabs.harvard.edu/abs/2007ApJ…668..806M).

    Chameleon matter seems to be the precise term.

  24. #24 Clarky
    May 5, 2011

    As far as gravitational lensing goes, couldn’t this be the effect of an older, smaller galaxy that has been completely consumed by its black hole? Traditional matter could be causing the effect it just isn’t in its original state anymore. (ie, dark) With enough mass in the well wouldn’t MOND be able to explain the warping?

  25. #25 herky stubby
    May 11, 2011

    Easy guys. Let’s keep an open mind about this issue.

  26. #26 Yoron
    May 24, 2011

    I’m not sure on where this discussion will take me. But as I see it, if this modified gravity fits Einsteins principle of equivalence then it sounds possible, if it contradicts it then it will need to be proven beyond doubt. I have no trouble with associating gravity with more couplings than what we know now. To me gravity is what defines ‘space’, that and distance.

  27. #27 Collin
    August 29, 2011

    Perhaps someone here can answer a question that seems to be ignored on other sites:

    How can the success of MOND in the special case of galactic rotation be explained in the context of CDM?

  28. #28 osmanlı iksiri
    October 12, 2011

    It is as simple as that. Empirical laws must be explained by all physical theories. If Newtons gravitational theory did not explain Kepler’s laws, it would be as useless (at the solar system scale) as GR is presently (at the galactic scale).

  29. #29 7.sınıf
    December 14, 2011

    I’m not sure on where this discussion will take me. But as I see it, if this modified gravity fits Einsteins principle of equivalence then it sounds possible, if it contradicts it then it will need to be proven beyond doubt. I have no trouble with associating gravity with more couplings than what we know now. To me gravity is what defines ‘space’, that and distance.

  30. #30 MargeryFERNANDEZ
    December 18, 2011

    People deserve wealthy life time and mortgage loans or bank loan would make it much better. Just because freedom bases on money state.

  31. #31 fiverr
    March 22, 2012

    All i know is that its a good thing it isn’t nearby or we’d all be done for. Ha! this topic is pretty controversial but I still believe in it.