Dark Matter is the leading theory to explain discrepancies in the dynamics of galaxies (The outer regions spin faster than the laws of physics say they should), because of a large and growing body of observational evidence (We’ve seen it, kinda). But alternative theories persist and still have their supporters. One such theory is known as Modified Newtonian Dynamics (MOND), and it recently scored a win when it was able to predict the motions of stars within dwarf galaxies more accurately than Dark Matter.
When Einstein published his General Theory of Relativity in 1916, the resulting equation for gravitational attraction looked (to a mathematician) very much like the formula Newton devised some 300 years earlier, but with a few key changes: some of the terms of the equation were changed in a way that didn’t affect the answers in a serious way under normal conditions, but changed things dramatically under extremes of gravity or speed. This worked extremely well – nobody has yet managed to set up an experiment in which the results deviate even slightly from Einstein’s predictions. What MOND asks is that, if Newton’s laws were basically correct except at extremely high speed or gravity, perhaps they could also begin to unravel at the low extremes? Such as, for example, the gravity of a star a hundred thousand light years away?
So physicists tinkered with Newton’s equation, so that the force of gravity would start to level off as the distance between objects increase. They used the motions of well-studied galaxies, tweaking the formula till its predictions matched what had already been observed. So far, so good, but matching old records doesn’t prove anything – you can work out any number of equations to fit an existing set of data. For a theory to be taken seriously, it has to make testable predictions, which can then be tested experimentally. Until recently, MOND has consistently been worse at predicting new data than Dark Matter. But that changed a few months ago when Stacy McGaugh, professor of astronomy at Case Western Reserve, and Mordehai Milgrom, the father of MOND and professor of physics at Weizmann Institute in Israel, successfully predicted the motions of stars within ten different dwarf galaxies orbiting the Andromeda Galaxy, several million light years away.
For the first time, MOND has accurately made predictions of stellar motion that could not be made with Dark Matter, and had them confirmed with observational evidence. But Dark Matter has more problems than just this one failure. For one thing, nobody can say for certain what it actually is. According to theory, it should be everywhere, permeating our entire galaxy, but since we can’t see or touch it, we can’t ever collect a sample to study. Theoretical physicists spend a lot of time coming up with mathematically sound ideas about how something so real could be as insubstantial as a ghost, but without observational or experimental evidence, they remain just ideas.
So does all this mean that astrophysicists are ready to abandon the search for Dark Matter and accept MOND as the truth behind the mystery? No. Because despite this one win, MOND has failed in too many other observations to predict a universe that looks like the one we live in. And despite the problems with Dark Matter, we have in fact managed to image it indirectly, by watching how its gravity subtly distorts light passing through it. From this we’ve managed to extrapolate maps of how it is distributed in large galaxies. We’ve even managed to draw maps of colliding galaxies, showing how the insubstantial Dark Matter has been separated from ordinary, dirty, regular matter. Still, it’s interesting that MOND has had this one win. As McGaugh said, “We need to understand why MOND succeeds with these predictions. We don’t even know how to make this prediction with dark matter.” The study was published as a paper in the Astrophysical Journal, titled “Andromeda Dwarfs in Light of MOND. II. Testing Prior Predictions”. It can be read online here: arXiv:1308.5894v1 [astro-ph.CO]