# Does MOND make good predictions?

Well, it does according to this preprint for certain scales.

What would be a simple way to explain MOND to a layman?

Does it ignore mainstream physics? How much?

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Related: physics.stackexchange.com/q/6561/2451 and links therein. – Qmechanic Dec 7 '13 at 18:03

## 3 Answers

The basic version of MOND pretty much only attempts to explain the rotation curves by postulating a modification of Newtonian gravity at accelerations smaller than a_0, and does a good job at that. However, many other observational facts (cluster dynamics, stability of spiral galaxies) still require large quantities of dark matter, as acknowledged by proponents of MOND themselves. The Bullet Cluster is the most dramatic evidence that some form of dark matter must exist and does not necessarily trace the baryonic matter distribution.

In regards to its relationship with physics: MOND does not attempt to reconcile with General Relativity, so it cannot do calculations regarding cosmology, gravitational lensing, etc. There is a theory called TeVeS by Bekenstein, which purports to be a relativistic generalization of MOND. I won't pretend I understand it or looked at it in any detail (maybe the value of TeVeS could be a nice question for GR experts). In any case, it has been criticized for yielding unstable solutions for stars, and is still not compatible with the dynamics of the Bullet Cluster.

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Good point about the bullet cluster +1 – user346 Feb 24 '11 at 6:22

Yes. The would appear to be the point of the paper you cite. It has been accepted for PRL apparently so that should lend it some credibility.

MOND - short for MOdified Newtonian Dynamics - is a phenomenological theory that was conceived of by Mordechai Milgrom in order to explain the huge discrepancy between the shapes of galaxy rotation curves as predicted by Newtonian theory and the actual shapes observed. A rotation curve is the plot of the orbital velocity of objects in the galaxy with respect to distance from the center:

The simplest statement of MOND is that it is a theory with a minimum acceleration scale $a_0 \sim 10^{-10} m/s^2$. When this scale is reached at some radius $r_0$ in a galaxy, objects appear to cease to respond to gravitational forces. Alternatively one could say that gravitational forces cannot generate an acceleration lower than $a_0$.

There are many critics of MOND. However, it has had remarkable success over the years. This paper is only the most recent evidence in its favor. MOND makes no claim to explaining the microscopic physics which leads to these effects. Compared to the LCDM model, MOND makes far fewer assumptions (only one, in fact) and thus has the benefit of being simpler.

The challenge for theories of quantum gravity is to either rule out MOND'ian behavior in weak fields or otherwise explain what gives rise to it. So far, none have even tackled the question in part due out of fear of being labeled "fringe" for associating with such rabble as MOND ;]

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Any reason for the down vote, other than "MOND is rubbish"? – user346 Feb 24 '11 at 2:51
Image is not working for some reason ... – user346 Feb 24 '11 at 3:31

It is mandatory that MOND is good at predictions. Specially the before mentioned paper (measuring galaxy rotation) will be a success ;)

It is a phenomenological theory i.e. MOND does not have a model and is accurate because it does data fit, on purpose. As long as galaxies do have a common underlying mechanism MOND will be good.

It is not really physics, it is statistics, accounting, cheating, and it should be forbidden because it can prevent us from progressing. I can say to myself: I do not know the answer, and I will keep looking. Physics, as an institution, can not admit: We dont know. And then Dark matter, MOND, ..., are expressions of our ignorance.
At least the Ptolomaic epicycles had a model, the Geocentric one.
Things will change as soon as you and the comunity can find the correct answer. Try googling "Galáxias e Atractores" and maybe you can find a nice reading.

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– Helder Velez Mar 3 '11 at 22:52