22 August 2006

MOND Dead?

Recent observations of lensing effects in a collision of two galaxies, at least at first glance, seem to strongly favor dark matter, over a modified gravity theory (background from a 2006 article for lay science enthusiasts here). One of the key papers making this argument is this one.

The gist of the argument against a modified gravity theory is that in this system, unlike almost all other observed systems, the apparent center of the dark matter in the system determined by lensing effects, is separated in space from the center of the regular matter observed. The regular matter is particularly bright, because the collision of two galaxies is heating it up and illuminating it. But, the dark matter appears not to have interacted significantly with the regular matter, and hence the dark matter appears to have cruised through the collision which excited the regular matter. This is illustrated in an image on the second page of the paper.

This is a problem for a modified gravity theory, because in a modified gravity theory, the gravitational effect predicted by General Relativity and Newton's Theory of Gravity, and the modifications (usually as a non-lineral function of distance or some function of gravitational force magnitude) should be centered around the same point.

The directness of the evidence in this collision case, and the relative lack of model dependence involved make it more persausive an argument for dark matter over modified gravity than other recent arguments based on predictions of the cosmic microwave background radiation spectrum.

I'm not entirely satisfied that interference patterns in a modified gravity model couldn't product the observed result, because, while this possibility is briefly considered in the paper above, the analysis isn't very rigorous, creative or searching. Also, it has long been known that some aspect of galactic clusters have not been adequately modeled by MOND even in isolation. Even with a MOND assumption, they seem to have further significant dark matter. So, it isn't that surprising that the theory also doesn't do well with collisions of galactic clusters.

But, I suspect that we will see some papers from MOND supporters addressing the claims in papers like this one, either affirming them or questioning them, with a better analysis of the key issues, in the near future, in the path already established by this one. It also implies that there may be a problem with using straight general relativity theory to produce your lensing based matter distribution, and then checking for a fit to MOND gravity theories only later. But, so far, no one has shown that a MOND theory can produce the observed result either.

This doesn't mean, of coure, that we have a firm understanding of what dark matter is, or how it is distributed.

1 comment:

Anonymous said...

MOND is a toy model at present and can not be considered a theory. At present, MOND does not possess a cosmology. In that vein, it is not any alternative to the standard model or to GR. Any number of observations have led to the intuitive concept of DM and the article appears to offer a proof that the phenomena does exist. But, there is nothing in any theory or the maths that tell us what DM is or why it should exist. Even if we survive that hurdle, the standard model requires dark energy and that has to be identified. GR gives a repulsive model that acts like dark energy but that was no more than Einstein intuitively allowing a repulsive force to balance the universe into a static case. An expanding universe requires a whole lot more of the stuff. Then, MOND will continue to have legs for some time and I believe it should.

paul gibson