A Case of MOND Over Dark Matter

According to Newton’s Second Law of Dynamics, objects on the farthest edges of galaxies should have lower velocities than objects near the center. But observations confirm that galaxies rotate with a uniform velocity. Some astronomers believe the orbital behavior of galaxies can be explained more accurately with Modified Newtonian Dynamics (MOND) — a modified version of Newton’s Second Law — than by the rival, but more widely accepted, theory of dark matter. The dark matter theory assumes that a halo of dark matter surrounds each galaxy, providing enough matter (and gravity) that all the stars in a galaxy disc orbit with the same velocity. MOND, however uses a different explanation, and a recent study of eight dwarf galaxies that orbit the Milky Way seems to favor the MOND approach over the dark matter theory.

“MOND was first suggested to account for things that we see in the distant universe,” said Garry Angus, of the University of St Andrews. “This is the first detailed study in which we’ve been able to test out the theory on something close to home. The MOND calculations and the observations appear to agree amazingly well.”

Usually the equation F=ma (force = mass X acceleration) solves your basic acceleration problems. But it doesn’t explain the observed rotation of galaxies. MOND suggests that at low values of acceleration, the acceleration of a particle is not linearly proportional to the force. According to Angus, MOND adds a new constant of nature (a0) to physics, besides the speed of light and Planck’s constant. Above the constant, accelerations are exactly as predicted by Newton’s second law (F=ma). Below it, gravity decays with distance from a mass, rather than distance squared. This constant is so small that it goes unnoticed with the large accelerations that we experience in everyday life. For instance, when we drop a ball the gravity is 100 billion times stronger than a0 and the accelerated motion of the Earth round the Sun is 50 million times stronger. However, when objects are accelerating extremely slowly, as we observe in galaxies or clusters of galaxies, then the constant makes a significant difference to the resulting gravitational forces.

When MOND is applied to nearby dwarf galaxies, one effect is that tidal forces from the Milky Way, which have a negligible effect in classical Newtonian Mechanics, can actually make a big difference. This is particularly significant for the dwarfs orbiting close to our Galaxy.

“In these dwarf galaxies, the internal gravity is very weak compared to the gravity of the Milky Way,” said Angus. “MOND suggests that the Milky Way is a bit like a bank that loans out gravity to nearby dwarf galaxies to make them more stable. However, there are conditions on the loan: if the dwarf galaxies start to approach the bank, the loan is gradually reduced or even cancelled and the dwarfs must pay it back. In two galaxies, we’ve seen what could be signs that they’ve come too close too quickly and are unable to repay the loan fast enough. This appears to have caused disruption to their equilibrium.”

Angus used MOND to calculate the ratio of mass to amount of light emitted by the stars in the dwarf galaxies from the observed random velocities of the stars collected independently. He also calculated the orbital paths of the stars in the dwarf galaxies. In all eight cases, the MOND calculations for the orbits were within predictions. For six of the eight galaxies, the calculations were also a good match to expected values for mass-to-light ratios; however for two galaxies, Sextans and Draco, the ratios were very high, which could well suggest tidal effects. The value for Sextans could also be due to poor quality measurements of the galaxy’s luminosity, which Angus said are improving all the time for these ultra dim objects.

“These tidal effects can be tested by updating the 13 year old luminosity of Sextans and making accurate observations of the orbits of Draco and Sextans around the Milky Way. We also need to carry out some detailed simulations to understand the exact mechanisms of the tidal heating,” said Angus.

If Newton’s gravity holds true, the dark matter needed in the dwarf galaxies has constant density in the center which is contrary to theoretical predictions, which suggest density should rise to the center.

“Even without direct detection, the dark matter theory is difficult to prove or refute and although we may not be able to prove whether MOND is correct, by carrying out these kind of tests we can see if it continues to hold up or if it is definitely ruled out,” said Angus.

Original News Source: Royal Astronomy Society’s National Astronomy Meeting

34 Replies to “A Case of MOND Over Dark Matter”

  1. well, the dark matter theory certainly is a lot cooler. where is the paragraphs that says that?

  2. MOND is cooler. That would let us make accurate predictions and calculations without having to guess at how much dark matter may be there. Dark Matter sounds pretty but it is a messy, frustrating concept that begs for something more sophisticated to supplant it.

  3. A good bet would be that there are several, as yet undiscovered and not yet understood principles operating in the univers. That MOND and dark matter/energy seem at odds should not indicate one is correct and the other not. It may be two expressions of the same effect. Before drawing sides, arming ourselves with conflicting data and declaring the others idiots, try to see if the tow can be made to work together. Personally, I think it’s all done with rubber bands and super glue.

  4. How would MOND reconcile the fairly recent observations of dark matter that were made? It came from images of a galactic collision, if I recall correctly…

  5. Can MOND account for the accelerated expansion of the universe? I’m under the impression that is one of dark matter’s stronger selling points. (Aside from altering galaxy shape and rotation)
    Thanks for the interesting article!
    Keep ’em coming.

  6. MOND can not account for Gravitational Lensing (an observed fact) (draw your own conclusions).

    Hence TeVeS and the other variations that have been made to it.

  7. Haven’t they mapped locations of dark matter in empty space using gravitational lensing?

    Also, haven’t some galaxies been discovered which have very little dark matter in them, meaning they rotate just as regular Newtonian gravity predicts? Why isn’t MOND affecting them?

  8. I’ve always considered the accelerated expansion of the universe to be an illusion of our perception. Space-time is not linear. Gravity affects the 4D cone of space-time and light travels through it. I have never liked the Dark Mater/Dark Energy theory.


  9. The galaxy map has missing several of the newly found Milky Way satellites.

  10. I would like that some corrections of the laws of gravity could explain the expansion of the universe( I don’t like the Dark energy concept) but I think the dark matter is there.
    The behavior of three different components of the Bullet Cluster , the stars (visible light) , the hot gases (X-Rays) and the dark matter (gravitational lensing) provides some evidence of the existence of dark matter.

  11. “How would MOND reconcile the fairly recent observations of dark matter that were made? It came from images of a galactic collision, if I recall correctly…”

    The Bullet cluster: First, they are assuming what we see is a collision, second, that our understanding of how galaxies collide is true, and third how dark matter, which may-or-may not be weakly interactive; will behave during such an event.

    It is poor judgement to us an exceptional event, a ‘cluster collision’ to provide ‘definite proof’, as some put it, of the dark matter explanation for galactic gravity anomalies.

  12. Has there been any work to see if MOND could explain the Pioneer Anomally which is occuring closer to home?

  13. The answer to life, the universe and everything:


    What was the question again?

  14. @ Ron Evans:

    Now that’s a good one. I am gonna keep an eye out for that.

  15. Is there any evidence indicating that the observations of the Bullet Cluster -aren’t- the results of a galaxy-cluster collision? It apparently matches theoretical predictions fairly well, from what I’ve read.

    Regarding proof; science doesn’t deal in ‘proof’, it deals in theory, and observation, and *disproof*. Theories can be supported by evidence, but never proven (at least, outside of pure math). This is why a theory that cannot be disproven (a ‘nondisprovable theory’) is not scientific.

  16. Again I say…..”Into the dark matter fray leaps
    Dr Gay….(fray…Gay) nevermind……As an outsider anthropologist I read with glee that my field is not the only one where they eat their young.

  17. I like MOND in some ways because I have such a difficult time accepting the invokation of dark matter and dark energy to explain away inconsistencies in the predominant big bang theory of the universe. The key to proving any theory is that it needs to make predictions which are testable and validated through experimantal measurements. If dark matter does exist, it should not take long to detect it and that would be an enormous breakthrough. The longer we go without detecting and measuring whatever dark matter is, the more appealing alternative theories like MOND begins to sound.

  18. “If they’re right, the cosmological implications are staggering.”
    I think I’m going to leave that as a comment on every article published on this site from now on.

  19. Ron Evans; you asked “Has there been any work to see if MOND could explain the Pioneer Anomaly which is occurring closer to home?”

    Quite a bit of work has been done to investigate this possibility. See, for example (arXiv:astro-ph/0603790 v2 2 Aug 2006)
    “Possible relation between galactic flat rotational curves and the Pioneers’ anomalous acceleration”
    (arXiv:gr-qc/0511026 v3 17 Mar 2006) G”ravitational solution to the Pioneer 10/11 anomaly”

    Peter K; you comment “MOND is cooler. That would let us make accurate predictions and calculations without having to guess at how much dark matter may be there.”

    Your sentiments have been shared by many, however MOND as a stand alone answer has its own problems. In his consideration of galaxy clusters in “THE MASS MISSING PROBLEM IN CLUSTERS: DARK MATTER OR MODIFIED DYNAMICS?” (arXiv:astro-ph/0607142 v1 7 Jul 2006) POINTECOUTEAU concludes “The MOND hypothesis, as a stand alone solution, has proven to be unable to solve the missing mass problem at the clusters scale. An added hot dark component is needed to rescue MOND, massive neutrinos for instance. As the amount of hot DM needed is up to 80% of the clusters mass, this turns the MONDian cosmological framework more into a mixed DM cosmology.”
    Many others have encountered similar problems!

  20. With current experimental evidence, either MOND or Dark Matter can be true theories, with evidence alternatively suppoerting either, depending on circumstances. Generally i dont like to consider mixing theories, but i just realised what would happen if both where true in their own ways.

    Clearly the evidence would then also be partially conflicting, as is right now.

    MOND claims an a0 (F=a*m + a0) at a certain level a0 – but if there was a mix of MOND & Dark Matter, reducing a0 and blending with Dark Matter would possibly also give about the same overall results. Now i am not suggesting this is the case, just considering the possibility this could also be the case.

    Lowering a0 would increase the possibility that it exists due to some quantum mechanical cause, pretty much like Dark Energy is explained by a quantum mechanical cause.

  21. Edit: Just noted that the post above says similar things, nice to see more having same ide. Not that makes it any truer ofc

  22. It has always stuck me that the ‘Dark matter/Dark energy’ designation is simply an intellectual placeholder until a more mature, truly scientific theory can be imagined. It temporarily fills huge gaps in our understanding.

    Even the term “Dark” implies where we are in our understanding of these issues: in the dark.

  23. Once again, dark matter conveniently explains everything that we don’t understand, plus it is invisible so we never have to prove it. Male-pattern baldness? Dark matter above our heads pulling the hairs out. Dark matter was once believed to be created in Chinese factories, but we now know it is produced by a small dwarf living in our stomachs and emitted unnoticed during belching and flatulence. Isn’t modern science wonderful?

  24. Any Hypothesis is merely a placeholder for an idea, if it is a Scientific Hypothesis it should also allow for testing. Admittedly some things can be hard to test properly, but in the end it is only by testing we can find the answers. Dark Matter/Energy is still ‘only’ a Scientific Hypothesis, as is MOND…

  25. I’m with Alex on this one. Dark Matter always feels a bit like “the ether” – too much of a workaround for the problem. I always prefer a decent mathematical model to inventing a new particle or force.

    And it seems that every other edition of New Scientist has a new explanation for gravitational anomalies that lie outside of Newtonian dynamics – how many higher-dimension equations do we need before Dark Matter gets put aside?

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  27. Several discussions here are comparing MOND and dark matter. MOND was proposed by Mordehai Milgrom in 1981 and is in fact just one particular theory of “modified Newtonian dynamics”. If this theory is considered to be wrong for some particular cases (as is sometimes claimed), it does not mean that the whole idea of a “modified Newtonian dynamics” is wrong. In fact, one could equally well invent another kind of MOND. An example of such an alternative theory (from me):
    http://arxiv.org/ftp/arxiv/papers/0712/0712.1110.pdf (more background on my website).

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