Testing Gravity with Galaxies

A gas rich galaxy. Credit: THINGS

A few weeks ago a bunch of news articles came out suggesting that there was some new evidence that ruled out dark matter. It was all based on a paper in the journal Physical Review Letters (PRL) by Stacy McGaugh: "A Novel Test of Modified Newtonian Dynamics with Gas Rich Galaxies", which had appeared as a preprint on the arXiV, and was the subject of a press release: "Gas rich galaxies confirm prediction of modified gravity theory".

The press went a bit crazy about the paper, as was well discussed by Sean Carroll (@seanmcarroll) over at Cosmic Variance: "Dark Matter: Just Fine Thanks". He held this up as an example of the problems of communication between scientists and the science media. I think he has a point. He also presents a nice list of problems with the "alternate theory" held up in the paper: MOND (Modified Newtonian Dynamics).

Ethan Siegel has also written about the problems with the press coverage on his blog: "Good ideas, Bad ideas, MOND, and Dark Matter" illustrated very nicely with lots of pictures.

At about the same time as all this press (it was even on BBC online: Dark Matter Theory Challenged by Gassy Galaxies Result) a request to write a PRL Viewpoint on the article came across my desk.

PRL publishes Viewpoints (articles written by and for scientists) on papers they think are of note. The audience is supposed to be non-specialist physicists, so this was a fun article to write - equations allowed, but not obscure astronomical terms! I roped in my good friend Kristine Spekkens (an Assistant Professor at the Royal Military College in Canada) and we got to work.

The result appeared in PRL yesterday: "Testing Gravity in Gas Rich Galaxies", by Karen Masters and Kristine Spekkens. I was going to write a version for non-scientists for this blog, but having found the two articles by Sean Carroll ("Dark Matter: Just Fine Thanks") and Ethan Siegel ("Good ideas, Bad ideas, MOND, and Dark Matter") while looking for links to the press articles (just to be clear I only read those this morning, after writing the viewpoint) I find they've already done the job for me.

Just like them, our main conclusion is that MOND cannot compete with the standard cosmological model. It is a way of explaining the rotation curves of galaxies without dark matter, and it does that impressively well but it's not a theory of gravity, and it fails at a lot of tests that our standard gravity + dark matter + dark energy model (clunky as it is) passes with flying colours.

I think the most interesting result from McGaugh's paper is the finding that the baryon fraction scales with the rotation velocity of the galaxies (ie. is smaller for lower mass galaxies). This is presented as being an obscure fine tuning for the standard model, but actually it has the right qualitative sense (ie. there are known processes which produce something a bit like that), and can now provide a constraint to the models we have. So let me explain what it means a bit more......

Baryons are the scientific term for normal matter - atoms, electrons, protons etc. that people, stars, planets are made of. We actually have a really good measurement of what the baryon fraction averaged over the whole universe must be (compared to the radiation content). In the standard cosmological model, that fraction is set by our understanding of nuclear physics and the fractions of different light elements which are made right after the Big Bang (known as Big Bang Nucleosynthesis: wikipedia article). The baryon fraction can also be measured in CMB experiments like WMAP.

However it is well known that the baryon fraction in galaxies is lower than the cosmic average. And it makes sense that this would be so. Once the dark matter gets into a galaxy it can only be thrown out by gravity. But baryons do things. They form stars which have stellar winds, and blow up in immensly energetic supernovae. Baryons both flow into and out of dark matter halos, and the balance between those two processes depends on the mass of the halo. Massive halos have stronger gravity and so may be able to hold onto their baryons better - so they should have a higher baryon fraction, just as McGaugh's result shows.

We end our viewpoint by talking about how the next generation of radio telescopes (like SKA) will be able to make a much bigger (and more complete) sample to repeat the test McGaugh proposes. And if the result still holds, we'll have to explain it with our models of how galaxies form. But I seriously doubt it means MOND is the answer.

In fact as part of the research for this paper I also came across the following Physics World article (thanks Chaz!) which presents the best rebuttal for MOND I've seen yet: "New lower limit set for Newton's Law". At it's heart, MOND is a modification of Newton's 2nd law at low acceleration scales (not really a modification of gravity). This article presents the results of a test of Newton's 2nd Law at low acceleration scales well below where MOND proposes a change.  The only problem is that in this experiment the attractive force is the electromagnetic force rather than gravity. But that means that MOND only modifies Newton's 2nd Law for gravity, not for other forces - so that makes it an even stranger theory than it already is!