Bullet Not Silver?

I learned from New Scientist just now that various researchers working on modified gravity theories are casting doubt on the direct evidence of Dark Matter that was presented by Douglas Clowe and collaborators a few weeks ago. Recall an earlier post on it, here, and Sean Carroll’s post with more detail here.

Some quotes from the New Scientist article, written by Stuart Clark:

“One should not draw premature conclusions about the existence of dark matter without a careful analysis of alternative gravity theories,” writes John Moffat, of the University of Waterloo in Ontario, Canada

There’s more in the paper astro-ph/0608675, where there is an analysis of the gravitational lensing seen in the merging cluster observations. Recall that the it is the lensing that was used as a key part of the story to show the separation of the dark matter from the ordinary matter. Moffat claims that this lensing can be explained in his modified gravity theory, “MOG”. There’s a real calculation suggesting this, apparently, although a full computation is still in progress:

Moffat has worked this out for the Bullet cluster using a one-dimensional model, and is now trying to extend this to two dimensions. If he succeeds, it will contradict Clowe’s direct evidence of dark matter.

(In other words, Moffat has not worked this out for the Bullet cluster.)

In related work,

HongSheng Zhao of the University of St Andrews in the UK and his collaborators applied a theory of modified gravity called TeVeS to the Bullet cluster.

This theory uses at least one extra field, which kicks in to affect the modification when gravity weakens. With collaborators, Zhao has also applied this sort of modified gravity theory (and others) to the lensing signals in merging galaxy cluster dynamics, and seem confident that they are viable. See astro-ph/0606216. They promise a more detailed paper with more computations.

One of the dark matter team, Clowe, says:

“I’m guessing that when they really look at it, they will need much more dark matter”

I must admit that my immediate reaction is that given all the other evidence of dark matter (for example in galaxy formation simulations, and from CMB data and other fits with the other cosmological parameters) this is all now leaving Occam’s razor well behind, but I’ve not read the papers in detail, so I could well be wrong.

One to watch, though.

-cvj

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