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.


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6 Responses to Bullet Not Silver?

  1. Louise says:

    Moffat’s fits many galaxy’s velocity curves to his theory, which is impressive. Corroborating evidence appears to favour “dark matter”.

  2. Doug says:

    Angus and Zhao posted a new paper (astro-ph/0609125) on the 7th, which attempts to model the newer bullet cluster lensing contours using the TeVeS version of MOND. They are able to get a very good looking fit, but have to use a dark matter component which is 2.4 times the baryonic mass of the system. Their claim is that neutrinos with a 2 ev mass could provide this dark matter component, so you wouldn’t need “exotic” dark matter.

  3. Clifford says:

    Oh, I see, so it s a hybrid of MOND and dark matter, where the dark matter is almost garden variety. Does anyone ever test those models (and other MOND varieties) against galaxy formation simulations vs survey data? That is an area where full-component cold dark matter models are supposed to be favoured.



  4. Dissonant says:

    You may want to check out the discussions at Cosmocoffe:


    Regarding structure formation without dark matter, here is a recent paper by Dodelson and Liguori about it:


  5. Clifford says:


    Yes, it is a very interesting discussion indeed. Thanks for the links, which will be useful to all.


  6. Peter says:

    MOG like MOND was developed from the Tully Fisher law which says that luminosity is proportional to the flat orbital velocity raised to a power like 3 or 4.

    The last thing that the highly-trained mathematical theoreticians at the turn of the 19th century, like Maxwell, wanted to believe was that the speed of light was actually constant despite the fact that there were all kinds observational, experimental and theoretical evidence that it was constant. What Einstein did in 1905 was to come up with a way to theoretically account for this observational result.

    My luminosity-based Infralever theory takes the Tully Fisher law at face value. Its development time was much longer than MOND’s or MOG’s.

    With the mass-based theories of Newton and Einstein we have 95 % of the universe we cannot find except theoretically. Is the confidence in the present model so great and the results of the light pressure studies so replete that the Tully Fisher law could not mean what is says it means? Even despite the fact that MOG and MOND, which are based on the Tully Fisher law, do a pretty good job of predicting the rotation curves of galaxies?