Years later, having landed a position with the Carnegie Institute, she returned to the topic of her master's thesis with her collaborator, Kent Ford, reaching the same conclusion. The ensuing controversy convinced Rubin to pursue a less hotly contested area of research for awhile, and she started looking into why spiral galaxies vary in their brightness, assuming it had something to do with their rotation. It turned out to be an especially promising avenue to explore.
She and Ford used a spectrograph to break down the spectrum of light emitted by stars in different parts of select spiral galaxies. She expected to see that the stars were orbiting the center of the galaxy more slowly the further they were from the center, much like the larger outer planets in our solar system take longer to complete an orbit around the Sun than the planets closer in. That's because the visible mass of a given galaxy - and hence, its gravitational pull - is concentrated at the center.
Instead, Rubin and Ford found that the outer stars were orbiting just as quickly as those at the center. The visible matter wasn't sufficient to account for this; the spiral galaxy should be flying apart! Clearly, there had to be some kind of hidden "dark" mass adding to the galaxy's gravitational influence.
And that's when Rubin remembered encountering as a grad student a 1933 paper by Fritz Zwicky analyzing the velocities of galaxies in the Coma cluster. Zwicky's paper concluded that the individual galaxies were moving so quickly that they should be able to escape from the cluster if visible mass was the only thing contributing to the cluster's gravitational pull. Since the cluster hadn't flown apart, he proposed the existence of "dark matter" (from the German Dunkle Materie) - about ten times more abundant than the visible matter - to account for the observational data.
Rubin and Ford had made a direct observation of Zwicky's predicted dark matter, which remains the top candidate for explaining such observations, although Rubin herself has admitted to favoring slightly the competing notion of modified gravity (MOND, or Modified Newtonian Dynamics). She finds MOND "more appealing than a universe filled with a new kind of sub-nuclear particle." Alas, nature doesn't take our aesthetic tastes into account; only time - and multiple experiements, now underway - will tell which hypothesis is correct.
If you asked Rubin, she might tell you that her greatest contribution to science is her four children, all of who have scientific careers: two geologists, an astronomer, and a mathematician. But she's also been justly lauded for her research, having analyzed over 200 galaxies since 1978. She has won election to the National Academy of Sciences and snagged the 1993 National Medal of Science in the process. But the Nobel Prize continues to elude her.
There have been so few women scientists honored by the Nobel Committee over the years: only two have won the physics prize, Marie Curie in 1903 and Maria Goeppert-Mayer in 1963. Rubin is eminently deserving of the honor, and frankly, she's not getting any younger.
Here's hoping she gets another shot at the short list next year. If not - well, I'm guessing Rubin would be fine with that. She's already left an impressive legacy. "Fame is fleeting," she told Discover back in October 1990. "My numbers mean more to me than my name. If astronomers are still using my data years from now, that's my greatest compliment."
Image credits: NASA/HST (top), American Institute of Physics (bottom)