# Does MOND make good predictions?

+ 6 like - 0 dislike
333 views

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?

This post imported from StackExchange Physics at 2014-04-01 17:31 (UCT), posted by SE-user Mark

recategorized Apr 1, 2014

This post imported from StackExchange Physics at 2014-04-01 17:31 (UCT), posted by SE-user Qmechanic

+ 8 like - 0 dislike

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.

This post imported from StackExchange Physics at 2014-04-01 17:31 (UCT), posted by SE-user Platypus Lover
answered Feb 24, 2011 by (80 points)
Good point about the bullet cluster +1

This post imported from StackExchange Physics at 2014-04-01 17:32 (UCT), posted by SE-user user346
+ 3 like - 0 dislike

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 ;]

This post imported from StackExchange Physics at 2014-04-01 17:31 (UCT), posted by SE-user user346
answered Feb 24, 2011 by (1,985 points)
Any reason for the down vote, other than "MOND is rubbish"?

This post imported from StackExchange Physics at 2014-04-01 17:31 (UCT), posted by SE-user user346
Image is not working for some reason ...

This post imported from StackExchange Physics at 2014-04-01 17:31 (UCT), posted by SE-user user346
+ 1 like - 2 dislike

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.

This post imported from StackExchange Physics at 2014-04-01 17:32 (UCT), posted by SE-user Helder Velez
answered Feb 25, 2011 by (-130 points)
a strong critique of MOND, at Cosmic Variance.

This post imported from StackExchange Physics at 2014-04-01 17:32 (UCT), posted by SE-user Helder Velez

 Please use answers only to (at least partly) answer questions. To comment, discuss, or ask for clarification, leave a comment instead. To mask links under text, please type your text, highlight it, and click the "link" button. You can then enter your link URL. Please consult the FAQ for as to how to format your post. This is the answer box; if you want to write a comment instead, please use the 'add comment' button. Live preview (may slow down editor)   Preview Your name to display (optional): Email me at this address if my answer is selected or commented on: Privacy: Your email address will only be used for sending these notifications. Anti-spam verification: If you are a human please identify the position of the character covered by the symbol $\varnothing$ in the following word:p$\hbar$ysicsOv$\varnothing$rflowThen drag the red bullet below over the corresponding character of our banner. When you drop it there, the bullet changes to green (on slow internet connections after a few seconds). To avoid this verification in future, please log in or register.