Goodbye, dark matter? (updated)
10 o'clock, October 17, 2005
Update: Cosmic Variance has more. Sean, over there, does a good job explaining (for layfolks like myself) exactly what they were trying to say, mathematically, and what was fishy about it. It’s not as simple as the “just use GR” story one gets from skimming the paper.
In a paper that somehow got mentioned in the CERN Courier and on Slashdot, authors Cooperstock and Tieu have suggested that nonlinear effects in GR could explain flat rotation curves in spiral galaxies (one of the historically important pieces of evidence for dark matter). And in two papers, Kolb, Matarrese, Notari and Riotto and then just Kolb, Matarrese, and Riotto have suggested that nonlinear effects in GR could explain the acceleration of the universe (a key piece of evidence for dark energy). Are these people making sense? Are they crazy? Is this worth thinking about? Have they actually explained away the entire dark sector? (Answers: occasionally, possibly, yes, no.)
Our picture of the universe may have just gotten a lot simpler. The major driver behind theories of non-baryonic (that is, not made out of the protons and neutrons that make up you and me and the rest of the observable universe) dark matter was that it seemed impossible to explain the way galaxies rotate without it — the observed motion only made sense if you assumed that the visible galaxy of normal stuff was surrounded by a massive halo of weird stuff.
Now two astrophysicists at the University of Victoria, Fred Cooperstock and Steven Tieu, have published a paper that makes all the weird stuff go away. According to Messrs. C. & T., the Newtonian approximation works well for stuff like the solar system because the planets’ contribution to the overall gravity of the system is so small compared to the sun’s, but it doesn’t work nearly as well for something like a galaxy where all the elements in the system (stars, in this case) contribute more or less equally. For that, they say, you need general relativity — and if you model a galaxy (which they did — ours and three or four others) with general relativity, explaining galactic rotation just with normal matter becomes pretty straightforward.
On the other hand, Mikolaj Korzynski of Warsaw University says that the Cooperstock-Tieu model requires, in addition to stars and dust and whatnot, a giant, infinitely thin disk cutting through the middle of the galaxy like a black hole in the shape of a cookie sheet, and “should therefore be considered unphysical.”
Still, the approach sounds promising. At least, it raises the bar for dark matter — I suspect more than one astrophysicist out there is now feverishly “doing sums” trying to relativistically replicate the Newtonian dark-matter halo model. Good times!
Egads, David, you're quoting stuff off /astro-ph. Aiiee!
Perhaps due to my teeny amount of self-esteem, I've always been comfortable with the idea that perhaps we see these dark matter halos because our knowledge of gravitation is incomplete. What's more simple and elegant - that we can't see 90% of the universe or that we might just not know everything yet? I find the MOND discussions interesting.
If you ever want to feel queasy about dark matter, try getting up in front of several dozen students and explaining it. Yup.