Tuesday, January 3, 2017

Vera Rubin and Dark Matter

Somewhat lost amid all of the publicity over the deaths of Carrie Fisher and Debbie Reynolds was the death, on Christmas day, of Vera Rubin. Vera was one of the most influential cosmologists of her generation, and a legitimate contender for the Nobel Prize. I only met her once, at a conference at Irvine in the early 1990s.  (Tom Hanks was there as well -- evidently he was thinking of making a movie about cosmologists but decided we were too boring.  A wise decision).

What did Vera discover?  Her most important work has to do with the "rotation curves" of galaxies. If you observe the way that planets orbit the sun, you notice that their orbital speeds decrease as you look at more and more distant planets. (Mercury moves faster than the Earth, which moves faster than Mars, and so on). In a galaxy, things are more complicated. There isn't one big central mass like the Sun -- instead, you have lots stars doing a complicated dance around each other. But (and here's the key point) far enough away from the center of a galaxy, any object should feel the force of the galaxy as a whole -- it should orbit the whole galaxy in just the same way that the planets orbit the sun.  And that means that stars and gas orbiting far away from the galaxy should move more slowly as you look farther and farther out.

But that's not what Vera Rubin observed at all. Instead, she noticed that objects orbiting a galaxy all tend to move with the same speed as they get farther and farther from the center.  If you graph the speed versus distance, you get a horizontal line -- hence the expression "flat rotation curves." And the best explanation for this is that there is extra matter in the galaxy, in the form of dark matter, that pulls on orbiting objects, making them move faster than expected.

Vera never won the Nobel Prize -- the Prize committee is notoriously conservative, and they prefer ironclad proof of new discoveries. And there's always the possibility that Rubin discovered something other than dark matter. Maybe instead of dark matter filling the galaxy, the theory of gravity itself breaks down on large scales? I wouldn't bet on it, but you never know....


Kathy said...

I first read of Vera Rubin on Discover magazine sometime in the 80s, I think. I do recall the gist of the article, and the title "The Woman Who Spins The Stars."

As to the Nobel Prize, wasn't one awarded for work on Type-A supernovae used to measure the rate of the expansion of the universe, which led to the idea of dark energy? To this day we don't know what dark energy is, or whether it even exists. Much like dark matter.

I'm used to the Nobel foundation or committee overlooking people. See Asimov's essay "The Nobel Prize That Wasn't." I just wish they were more consistent.

Robert Scherrer said...

The Nobel Prize has been awarded for improvements in lighthouse technology (!) and the invention of the lobotomy. The Nobel choices are far from perfect, but the prize has accumulated such enormous prestige over the years that we've come to expect perfection. I think we just need to dial back expectations.

A.A. Kidd said...

Tom Hanks was specifically planning to make an adaptation of Lonely Hearts of the Cosmos in which he would be playing Alan Sandage. He's a well known space buff and science fiction fan in real life; another unfulfilled ambition of his was to star in an adaptation of Heinlein's Stranger in a Strange Land.

Robert Scherrer said...

That's what we were hearing at the conference. I actually bumped into Hanks in the restroom, but had nothing clever to say, so.... But Lonely Hearts of the Cosmos is a great book, if a bit out of date at this point.