MOND

I had to give a talk today on modified newtonian dynamics (mond). It was interesting if only to make me realize how there's lots of holes in every theory and how every scientist likes to argue. The basic idea of mond is that people who don't like the idea of a universe filled cold dark matter are trying to come up with ways to explain lots of things by changing how gravity works. "Dark matter" was first proposed was a result of measuring orbits on the outskirts of a galaxy. Things were going a lot faster than they should have been. People suggested that this extra force was coming from unseen (dark) matter. This stuff still hasn't been "observed" in any direct way. In the eighties, perhaps because of all the coke, def-leopard, and big hair, people started hacking with normal, boring, newtonian gravity. They were adding all sorts of modifications to see if they could reproduce something that fit the "extra gravity". The paper i had to review in my talk was one that tweaked the derivation of "GR" and the "einstein equation" to add an extra term. It was pretty neat since it is testable - it makes concrete predictions that are falsifiable. Dark matter doesn't predict curves, it is fit to them. Both seem pretty ad-hoc. There's some cool simulations at the McKenzie, Virgo, or UKAFF supercomputers. This is a plot of a "galactic rotation curve". The x axis is distance from the center (kiloparsec = 3000 Light Years). The y axis is the orbital velocity (km/sec ~ 3Light Years per million years). The normal newtonian dynamics predicts much lower velocities at large distances but by tweaking gravity, you can fit the curve.