Vera Rubin: Shedding Light on Dark Matter

Vera sat hunched in the alcove at Kitt Peak observatory, poring over punch cards. The data was the same as it had been at Lowell, at Palomar, and every other telescope she’d peered through in her feverish race to collect the orbital velocities of stars in Andromeda. Although the data was perfectly clear, the problem it posed was puzzling. If the stars at the edges of spiral galaxy were moving as fast as the ones in the center, but the pull of gravity was weaker, how did they keep from flying off? The only possible answer was that Andromeda contained some kind of unseen matter and this invisible stuff was keeping the galaxy together.

Though the idea seemed radical, it wasn’t an entirely new one. In 1933, Swiss astronomer Fritz Zwicky made an amazing discovery that was bound to bring him fame and fortune. While trying to calculate the total mass of the galaxies that make up the Coma Cluster, he found that the mass calculation based on galaxy speed was about ten times higher than the one based on total light output. With this data as proof, he proposed that much of the universe is made of something undetectable, but undeniably real. He dubbed it Dunkle Materie: Dark Matter.

But Zwicky was an insufferable jerk who regularly bad-mouthed his colleagues and other astronomers in general. As a result, his wild theory was poorly received and subsequently shelved until the 1970s, when astronomer Vera Rubin made the same discovery using a high-powered spectrograph. Her findings seemed to provide solid evidence of the controversial theory Zwicky had offered forty years earlier.

To the Stars Through Difficulties

From a young age, Vera felt the invisible tug of the cosmos. When she was ten, her family moved from Philadelphia to Washington, DC. Windows lined one wall of her new bedroom, and she marveled at the stars beyond, captivated by their movement throughout the night.

At school, she took every opportunity to study the stars and even started an astronomy club with some of her classmates. Vera’s father took her to amateur astronomy meetings and helped her build a cardboard telescope, which she used to study the meteors streaking past her windows.

Vera’s parents supported her interest in astronomy, but her teachers were another story. During a college admissions interview, a Swarthmore counselor tried to steer her away from astronomy, suggesting that she paint cosmological objects instead, because art was a more ladylike pursuit. Gender discrimination was a battle she would fight all her life, both for herself and for future generations of starstruck little girls.

With Swarthmore out of the running, she applied to Vassar. Maria Mitchell had taught there, and Vera was inspired to attend an institution that valued women in science. Vera earned her bachelor’s degree in 1948 as that year’s only graduate of astronomy. The same year, she married Robert Rubin, a future physicist.

Redshift, Blueshift

Vera applied to several graduate schools including Princeton, who told her they didn’t accept women in their astronomy program. They wouldn’t even send her a course catalog. She eventually enrolled at Cornell, where Robert was a graduate student.

Soon, she became fascinated with the dynamics of galaxies. Her graduate work mainly focused on identifying heavenly bodies that deviated from the Hubble flow, which is the redshift-based observation of the motion of galaxies due to the expansion of the universe.

Redshift is simply the Doppler effect applied to light-emitting objects. When a fire truck goes wailing past our ears, the frequency of the siren is compressed before it reaches us, so the sound is higher-pitched. The moment it passes by, the sound wave expands, and the pitch goes lower and lower until it’s out of earshot. This shift in frequency is the Doppler effect.

The visible light spectrum does the same trick: distant galaxies moving away from us shift toward the red end of the spectrum, and those moving toward us shift the opposite way, toward the blue end. By measuring redshift, scientists can measure the speeds of galaxies and other objects in deep space.

As the Galaxy Turns

Vera graduated Cornell in 1951 and entered the PhD program at Georgetown about six months later. Her doctoral adviser was George Gamow, an early architect of the Big Bang theory. Here, the gender bias was a real bulldog. Women were simply not allowed in the area of the university that housed Gamow’s office, so Vera took all of her meetings with him the wood-paneled library of the nearby Carnegie Institute.

Vera was still moved by galaxy dynamics. She decided to focus her dissertation on the theory that galaxies clump together rather than being randomly distributed throughout the universe. Another adviser said her paper was good enough to be presented at the next meeting of the American Astronomical Society (AAS)—by him. She said no, she could do it herself.

So with her baby in tow and another due in a few weeks, Vera drove to Pennsylvania through a snowstorm to deliver her talk, titled “Rotation of the Universe”. She didn’t know anyone at the meeting and considered herself a fraud among these ‘professional’ astronomers. Their reaction to her paper was a mostly negative discussion of her ideas, and the experience didn’t do much for Vera’s confidence. Her controversial theory would go unexplored for two decades.

Clocking the Speeds of Stars

For the next eleven years Vera taught math, physics, and astronomy at Georgetown while raising four children. All four eventually became natural science or math PhDs, mostly because Mom always made science look fun.

In 1965 Vera left Georgetown to ask for a job at the Carnegie Institute’s Department of Terrestrial Magnetism (DTM). There was no application to fill out or interview process to sweat through. Instead, she was handed a 2″ x 2″ glass photographic plate containing the spectrum of a star. If she could measure its velocity, she could have the job. She got the job.

At DTM she was introduced to W. Kent Ford, an astronomer and instrument maker who became her lifelong research partner. It was a match made in heaven. Kent had just built an advanced spectrograph, a device that fractures light into its constituent wavelengths. It was lined with the latest in photomultipliers and several times more sensitive than anything else available.

Kent’s spectrograph was the kind of technology that would allow astronomers to examine specific regions of a galaxy instead of taking it at face value. He was eager to test the device’s limitations, and Vera was excited to make the types of observations that might prove her galactic rotation theories.

Analyzing Andromeda

Around this same time, Vera was accepted to Palomar Observatory in California. Palomar had been previously closed to women, so the only restrooms were technically mens’ rooms. Vera took care of that problem in short order. She cut a little skirt out of paper and taped it over the signage of one restroom, claiming it for the ladies.

Vera usually faced gender bias with a combination of humor and straightforwardness, but after the rough treatment at the AAS meeting, she wanted to avoid controversy. Her plan for Palomar was to stay away from hot new topics like quasars and other heavily trod areas. Instead, she decided to aim Kent Ford’s amazing spectrograph at the stars of our neighboring galaxy, Andromeda.

Andromeda is a spiral galaxy made of high concentration of stars swirling around in the center, and many other stars in the outer regions that loosely orbit the swirling cloud. Astronomers long believed that most of a galaxy’s mass was at the heavily populated center. To the naked eye, this seems perfectly reasonable. So does the idea that the stars at the fringe move more slowly than the ones at the center. That’s how our own solar system works, after all: out here on the third rock, it takes ~365 days to orbit the Sun, but Mercury can do it in 88.

If we could lay Andromeda on a table, crouch down, and look at it from the edge, we would see stars coming at us on one side and moving away on the other. With Kent’s spectrograph, Vera could measure the redshift of the stars in the middle with the stars on the outskirts and compare their orbital speeds.

Right away, she noticed that the stars on the outer edges of Andromeda were moving just as fast as the ones in the middle. It didn’t make sense. If they were moving at the same speed, they should just fly away since they’re untethered by the gravitational pull that keeps the center together. Some kind of gravitational matter must be keeping them in place.

Vera and Kent traveled all over the U.S. to different observatories and studied more than sixty regions of Andromeda. She would arrive a few days early and cut out dozens of 2″ x 2″ glass photographic plates in complete darkness, and then bake them for 72 hours to increase their sensitivity. On their scheduled nights, she and Kent took turns at the telescope guiding each star carefully along the guide lines. Then Vera developed the plates and analyzed them with the spectrograph.

They studied other spiral galaxies and always came up with the same data. Vera reasoned that there had to be some kind of invisible matter there with enough gravity to keep it from disintegrating. According to her calculations, there had to be about ten times as much invisible matter as visible matter. The mystery reminded her of Fritz Zwicky’s Dunkle Materie, which she’d learned about in graduate school. Her data seemed to definitively confirm Zwicky’s theory that the universe is mostly made up of unseen matter.

At first, she faced nearly as much disbelief from the scientific community as he had. But the data spoke for itself. There was no denying the presence of some invisible cosmic glue tying the galaxy together. Over time, Vera’s findings would be strengthened by the discovery of the cosmic microwave background and gravitational lensing.

The Real Prize

Vera paved the way for women in astronomy, but she saw scientific inquiry as a human endeavor and argued that we all have equal permission to do science. She was a strong advocate of scientific literacy for everyone from schoolchildren to senators. The fact that she juggled a family and an astronomy career has been an inspiration to women, particularly astrophysicist Sandra Faber.

Vera received several honors throughout her career, including a National Medal of Science and a Gold Medal of the Royal Astronomical Society, but she never won a Nobel Prize. Though she arguably deserved one, it may not have mattered to her because she wasn’t after fame and fortune. To her, “the real prize is finding something new out there.”

Vera died December 25th, 2016 of dementia-related complications. She was 88 years old.