Tuesday, May 15, 2007

The Dark Ring

The title to this post sounds like the title of a bad cross between Star Wars, The Lord of the Rings, and maybe a little of Wagner's Ring Cycle to boot. But, actually, this is the topic of a press conference NASA will be holding later today to announce some new findings about dark matter.

I'm a little interested in what the press conference has to say, although I expect a lot of the announcement to be overblown (like NASA often does with scientific results). But what is the big deal?

Most of you have probably heard about dark matter -- matter that has gravity but doesn't interact with light, the way normal matter does. There are many different lines of evidence for dark matter. So why would it be big news?

One of the problems with dark matter is that we don't know what it is. Some physics theories predict particles with properties similar to dark matter, but until we actually detect dark matter in the laboratory, dark matter remains just a hypothesis.

There is a competing explanation for the observations of "missing matter" called MOND (for "Modified Newtonian Dynamics"). The MOND hypothesis is that there is not missing matter, just that our theory of gravity is wrong. One of the main problems with MOND has been that it has proven very good at "postdiction," or explaining observations once they have been made, but it hasn't made any predictions that can be tested and, if proven wrong, falsify the theory.

This is a crucial point. Any scientific theory must be able to make predictions that, if proven wrong, falsify the theory. Einstein's General Relativity made several predictions, such as the existence of gravity waves and the changing of time itself in gravity, that were crucial to the whole theory but were not tested until decades later. And relativity passed those tests. Until MOND can make a prediction that both differs from dark matter and, if shown to be wrong, falsifies the theory, MOND remains an interesting concept.

MOND adherents claim the same is true of dark matter, but the best dark matter theories do make a prediction that can falsify dark matter -- the existence of dark matter particles with certain properties. In the coming few decades, particle accelerators on earth should be able to detect signatures of these particles. If we don't detect them, dark matter is wrong.

Today's NASA press conference is going to be about the discovery of a "ring" of dark matter in a cluster of galaxies. We see the evidence for this ring because the gravity from the dark matter distorts light from more distant galaxies, bending them into funny shapes. And, when we look with a variety of instruments, we don't see regular matter shaped in the needed ring. So, the ring must be dark matter. Probably.

About a year ago, NASA made a similar press release from another galaxy cluster where the center of the gravitational pull of the galaxy cluster didn't match up with the visible, ordinary matter. There are probably dozens more galaxy clusters where similar observations can be made, so you will probably see many more press releases in the future. The fact remains, though, that until we detect dark matter in the laboratory, some reasonable scientists will doubt the existence of dark matter.

This may be the best legacy of MOND. I have lots of problems with MOND, too many to type out. But it is forcing astronomers to stay on their toes and look for new ways to test gravity and our theories about the makeup of the Universe. And that is a good thing.

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