Dark Matter: Just Fine, Thanks

By Sean Carroll | February 26, 2011 2:07 pm

Astrophysical ambulance-chasers everywhere got a bit excited this week, and why wouldn’t they? Here are some of the headlines we read:

Wow. More evidence against dark matter? I didn’t know about the original evidence.

Sadly (and I mean that — see below) there is no evidence against dark matter here. These items were sparked by a paper and a press release from Maryland astronomer Stacy McGaugh, with the rather more modest titles “A Novel Test of the Modified Newtonian Dynamics with Gas Rich Galaxies” and “Gas rich galaxies confirm prediction of modified gravity theory,” respectively.

I’m the first person to defend journalists against unfair attacks, and we all know that headlines are usually not written by the people who write the actual articles. But we can legitimately point fingers at a flawed system at work here: these articles are a tiny but very clear example of what is wrong wrong wrong about our current model for informing the public about science.

McGaugh’s new paper doesn’t give any evidence at all against dark matter. What it does is to claim that an alternative theory — MOND, which replaces dark matter with a modification of Newtonian dynamics — provides a good fit to a certain class of gas-rich galaxies. That’s an interesting result! Just not the result the headlines would have you believe.

It’s obvious what happens here. Nobody would read an article entitled “Gas rich galaxies confirm prediction of modified gravity theory” — or at least, most editors doubtless feel, fewer people would be interested in that than in evidence that went directly against dark matter. So let’s just spice up the story a bit by highlighting the most dramatic possible conclusion we can imagine drawing, and burying the caveats until the end. Net result: a few more people read the articles than otherwise would have, while many more people just read the headlines and are left with less understanding of modern cosmology than they started with. Scientists and journalists together have a responsibility to do a better job than this at making things clear, not just making things sound exciting.

But let me take this opportunity to lay out the problems with MOND. It’s a very clever idea, to start. In galaxies, dark matter seems to become important only when the force of gravity is not very strong. So maybe Newton’s famous inverse-square law, which tells us how the force of gravity falls off as a function of distance, needs to be modified when gravity is very weak. Miraculously, this simple idea does a really good job at accounting for the dynamics of galaxies, including — as this new result confirms — types of galaxies that weren’t yet observed back in 1983 when Mordehai Milgrom proposed the idea. Whether or not MOND is “true” as a replacement for dark matter, its phenomenological success at accounting for features of galaxies needs to be explained by whatever theory is true.

Which is an important point, because MOND is not true. That’s not an absolute statement; among its other shortcomings, MOND is not completely well-defined, so there’s a surprising amount of wriggle room available in fitting a variety of different observations. But to the vast majority of cosmologists, we have long since passed the point where MOND should be given up as a fundamental replacement for dark matter — it was a good idea that didn’t work. It happens sometimes. That’s not to say that gravity isn’t somehow modified in cosmology — you can always have very subtle effects that have yet to be discovered, and that’s a possibility well worth considering. But dark matter is real; any modification is on top of it, not instead of it.

Let’s look at the record:

  • MOND is ugly. Actually, that’s very generous. More accurately, MOND is not a theory; it’s only a phenomenological rule that’s supposed to apply in a limited regime. The question is, what is the more general theory? Jacob Bekenstein, in an heroic bit of theorizing, came up with his Tensor-Vector-Scalar (TeVeS) theory, which hopefully reduces to MOND in the appropriate limits. Here is the action for general relativity:

    And here is the action for TeVeS:

    Don’t worry about what it all means; the point is that the theory underlying MOND isn’t really simple at all, it’s an ungodly concatenation of random fields interacting in highly-specific but seemingly arbitrary ways. That doesn’t mean it’s not true, but the theory certainly doesn’t win any points for elegance.
  • MOND doesn’t fit clusters. Long ago, rotation curves of galaxies were the strongest evidence in favor of dark matter. Very long ago. We know better now, and a mature theory has a lot more hoops it needs to jump through. The nice thing about MOND is that, despite the ugliness above, when you get down to making predictions for large astrophysical objects, there really isn’t any wriggle room: you fit the data or you don’t. It works for galaxies, but when it comes to clusters — you don’t. Not close. Proponents of MOND understand this, of course, and they’ve come up with a clever workaround. It’s called “dark matter.” That’s right — even MOND’s biggest supporters admit that you need dark matter to explain galaxies. Let’s just emphasize that for those who find all this text kind of tedious:

    Even with MOND, you still need dark matter.

    Some people try to claim that the necessary dark matter could be neutrinos rather than some brand-new particle, and that’s supposed to be morally superior somehow. But there’s no two ways around the conclusion that dark matter is real.

  • MOND doesn’t even fit all galaxies. For almost twenty years now we’ve known that MOND fails for a certain type of galaxies known as “dwarf spheroidals.” These are small (thus the name) and hard to observe, so MONDians have come up with various schemes to explain away particular galaxies. That might even be okay — nobody said fitting the data would always be easy, even in the correct theory — except that it’s precisely this kind of extra work that is being scoffed at in the case of dark matter in these recent news items.
  • Gravity doesn’t always point in the direction of where the ordinary matter is. This is the lesson of the famous Bullet Cluster (and related observations). The evidence from gravitational lensing is absolutely unambiguous: to fit the data, you need to do better than just modifying the strength of Newtonian gravity. Once again, people try to wriggle out of this in TeVeS and other MONDian approaches. However, the way they do it is by imagining that other fields have energy, which warps spacetime, and therefore a gravitational field. We have a useful phrase to describe new fields whose energy warps spacetime: “dark matter.” MOND-like theories don’t replace dark matter so much as they make it much more complicated.
  • MOND doesn’t fit the cosmic microwave background. Saving my favorite for last. One of the coolest things about the temperature anisotropies in the cosmic microwave background is that they are sensitive to the existence of dark matter. In the early universe, dark matter just collapses under the pull of gravity, while ordinary matter also feels pressure, and therefore oscillates. As a result, the two components are out of phase in the even-numbered peaks in the CMB spectrum. In English: dark matter pushes up the first and third peak in the graph below, while suppressing the second and fourth peak. That would be extremely hard to mimic in a theory without dark matter; indeed, this was predicted before the third peak was precisely measured. But now it has been. And…

    See that dotted line? That’s the theory with dark matter, fitting all the data. See the solid line? That’s the MOND (really TeVeS) prediction, definitively inconsistent with the data. Can some clever theorist tweak things so that there’s a MOND version that actually fits? Probably. Or we could just accept what the data are telling us.

Having said all that, I’m glad that some people are still thinking about MOND-like approaches. You can still learn interesting things about galaxies, even if you’re not discovering a new law of nature. And dark matter, to be honest, isn’t established with 100% certainty; it’s really more like 99.9% certainty, and you never know.

What’s less admirable is people (mostly outside the professional community, but not all) hanging onto a theory because they want to believe it, no matter what new information comes along. Personally, I think it would be much cooler if gravity were modified, compared to the idea that it’s just some dumb new particle out there. I’ve put some thought into the prospect myself, which helped lead to some productive research ideas. But ultimately the universe doesn’t care what I prefer. Dark matter is real — gravity could also be modified, but there’s no reasonable doubt about the dark matter. So let’s try to figure it out.


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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .


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