One of the stories coming from the American Astronomical Society meeting involves a topic that we’ve discussed here before: dark matter. Actually, it focuses directly on one of the astronomers closely associated with determining the existence of this mysterious “stuff”: Dr. Vera C. Rubin.
We’ve all heard of dark matter. It’s a weird, “invisible” material that makes up about a quarter of the mass in the universe. Nobody is quite sure what it is, but astronomers are sure it’s out there. The fact that we know even that much is due largely to Dr. Rubin and her efforts at finding it.
The dark matter story begins with a question: why don’t galaxies rotate at the velocity we expect them to? Over many years, Dr. Rubin and her team observed galaxy rotations. They compiled their data into what are called “rotation curves” and noticed that galaxies don’t always rotate the way they were expected to. Why?
Ultimately, the answer was “dark matter”. This is a type of cosmic “stuff” first suggested by Swiss astronomer Dr. Fritz Zwicky as “dunkel materie”. It could constrain the motions of objects in galaxies. It was largely unknown and theoretical then, but its effects are often observed today.
To honor Dr. Rubin’s work, the Large Scale Synoptic Telescope (LSST), currently under construction in Chile, will be renamed the NSF Vera C. Rubin Observatory (VRO). It’s the first national observatory named after a woman. VRO will be heavily involved in the search for dark matter and the even-more-mysterious dark energy.
Tracking Galaxy Motions Leads to Dark Matter
Vera Cooper Rubin began her astronomy career at Vassar college at a time when women weren’t expected to do science. She went to Cornell University and Georgetown Universities, gaining her Ph.D. in 1954. Her thesis suggested that galaxies clumped together in clusters. Today, that’s accepted observational fact. Back then, galaxy clusters were still a theoretical and not-too-popular idea. Still, throughout her career, Dr. Rubin studied galaxies both individually and in clusters, and charted the motions of their stars.
In the 1960s, Rubin began working at the Carnegie Institution of Washington’s Department of Terrestrial Magnetism. Rubin’s work focused directly on galactic and extragalactic dynamics. Those subjects deal with the motions of galaxies both singularly and in clusters. In particular, Dr. Rubin studied the rotation rates of galaxies and the material in them. As I mentioned above, the team promptly discovered a puzzle: the predicted motion of a galaxy’s rotation didn’t match the observed rotation.
To understand why that might seem strange, it’s important to understand that galaxies do rotate. Astronomers expected all the material in a galaxy to rotate at rates dependent on their distance from the center. However, if they do it fast enough, they could fly apart, IF the combined gravitational effect of all their stars was the only thing holding them together. In her team’s observations, the rotation of some galaxies didn’t perform according to expectations. It implied that the mass of their stars wasn’t enough to keep them “together”. Why why didn’t they come apart? Something else had to be holding them together. The difference between the predicted and observed galaxy rotation rates was dubbed the “galaxy rotation problem”.
Dark Matter as a Vera C. Rubin’s Solution
Rubin and others decided that there was some kind of unseen mass in or around the galaxy. It was holding the galactic pieces and parts together. Based on many observations made by Rubin and her colleague Kent Ford, the mystery began to unravel. It turned out that galaxies must have at least ten times as much “invisible” mass as they do visible mass in their stars and nebulae.
Calculations showed that this invisible mass really existed. And, it might be that “dark matter” that Zwicky first suggested in the 1930s. Zwicky himself was scoffed at when he came up with the idea and later on, Rubin and her colleagues faced a lot of the same skepticism. Yet, it made sense when they invoked dark matter as an explanation for the odd rotation curves they calculated.
Dark Matter Focus from Chile
Dr. Vera C. Rubin (who died in 2016) spent much of her later life working on the dark matter problem. For that reason, the renaming of LSST in her honor is appropriate. The Vera Rubin Observatory will begin official operations in 2022. Its telescope will be mated to a state-of-the-art 3200-megapixel camera. Together with other instruments, astronomers will use it to study the universe in search of dark matter. It will also look for evidence of dark energy, study the bodies of the solar system, explore the transient optical sky, and map our home galaxy, the Milky Way. It’s a perfectly fitting tribute to a woman who persisted on research that others felt wasn’t important; not only WAS it important, it led to a new area of astrophysical research.