One of the big mysteries remaining in our current understanding of the universe is “dark matter”—apparently a type of matter that doesn’t interact with light (“invisible” might be a better word than “dark”), doesn’t seem to heat up much, but which also seems to comprise about 83% of all the matter of the universe. By its nature, dark matter is very elusive and hasn’t been detected directly yet. The evidence for its existence is really strong—although it was first postulated to explain the strange rotation of spiral galaxies, the best evidence for it is in galaxy clusters and in the structure of the early universe.
Galaxy clusters are a little easier to explain in brief (I may come back to the cosmological evidence later, but in the meantime see the WMAP website); they are large objects containing many galaxies and lots of hot X-ray emitting gas. The picture shows a particular galaxy cluster known as the “Bullet Cluster”, which is the result of two galaxy clusters colliding. The colors aren’t “real”: they’re a way to combine information from several different types of observation, and that’s key to understanding the image. The red is the X-ray observation of the hot gas, showing a clear shock way (the “bullet”) where the gas in the two clusters collided and heated up. The blue is where most of the mass of the clusters resides—not in the gas, but in the region surrounding the galaxies. Since we can calculate the mass of the stars and gas in the galaxies independently, we can see that there’s a lot of mass that doesn’t show up as gas or stars or anything “normal”. In other words, the blue in the picture is the closest we’ve gotten to “seeing” dark matter, even though all we’re actually seeing is the effect of its mass.
This isn’t to say there aren’t real problems with our current dark matter models. (Note my careful phrasing!) One of the perpetual challenges in galactic astronomy has been to relate the light from a galaxy (called its luminosity), which is mostly from stars, to the mass of the galaxy, which is mostly not stars. I’m not an astrophysicist so I’m a little hazy on the details, but this has been an area of intense interest, and it’s not a fully resolved problem. In some types of galaxies at least, the estimate of mass from the luminosity contains fairly large errors. (I welcome correction from my galactic-astronomer friends and readers.)
Enter this article, which is basically a press release for a new paper by astrophysicist Stacy McGaugh. There is a small but vocal group of astrophysicists who think the best way to deal with problems with dark matter is to do away with the need for it entirely, by modifying the equations of gravity. Called modified Newtonian dynamics, or MOND, the changed equations do a pretty good job of fitting the rotational dynamics of spiral galaxies. In the paper cited, McGaugh has successfully also solved the problem of determining the mass-light relationship in a certain type of galaxy using MOND. All well and good, and potentially very interesting.
But here’s where the problem lies: MOND isn’t an actual theory. While Newtonian gravity has been shown to be the limiting behavior of Einstein’s general relativity, the generalizations of MOND to be compatible with relativity are very difficult to understand and quite complicated to work with. (Despite general relativity’s mathematical complexity, it’s conceptually elegant.) If MOND is correct in the same way Newton’s law of gravity is correct, it needs to be shown to be a limiting case of a more general theory, one that satisfies the same experimental tests general relativity has already passed. Ultimately, MOND is a heuristic model that fits some types of galaxies, but it fails for galaxy clusters and for the universe as a whole.
So, for those of you waiting patiently for my main point for non-scientists: real scientific controversy is often of this sort, where just explaining what’s going on to non-specialists can be challenging. I wouldn’t teach MOND in an introductory astronomy or astrophysics course for that reason, no matter how empirically successful it is in the cases where it works well. If McGaugh and his colleagues manage to solve the obvious problems with MOND such that it seems like a viable alternative, then I might consider it, but having given it a fair study I can’t in good conscience present it to my students as something to take seriously. Our students need more background before they can “make a decision” between Newtonian gravity with dark matter, and MOND without dark matter. That’s part of what education must do.