Vera Rubin died last week, and for a brief moment the gaze of the Internet swung her way to notice her remarkable life and work, to say a number of things about them of which perhaps half were true, and then moved back to its usual stalking grounds. That instant of online recognition left a wake of bubbling misconceptions it will be the work of a decade to pop, but perhaps here, in honor of what Rubin did and fought for, we can make a start at sifting out some of the more tantalizing fictions, to get down to the real nature of her revolutionary contributions to astronomy.
The first half of Rubin’s career was governed by restrictions that had no right existing anywhere in twentieth century science, but particularly not in astronomy. By the time she entered college in the mid 1940s, astronomy had blossomed under the work of a dazzling array of women observers and researchers, including Caroline Herschel, Maria Mitchell, Henrietta Swan Leavitt, Annie Jump Cannon, and Cecilia Payne-Gaposchkin. Between them, they had created massive catalogues of the night sky, made astronomy a profession in America, discovered a technique to measure the size of the universe, and determined that stars are composed overwhelmingly of hydrogen, fundamental contributions that, by 1948, no astronomer could ignore.
And yet, she couldn’t get a PhD from Princeton because the program was not open to women. When she married at age nineteen, it was expected as a matter of course that she would follow her husband and make do as best she could with the learning opportunities available near his work, meaning that she had to give up study at Harvard, the preeminent astronomical institution at the time, for Cornell and Georgetown, which were not known for their astronomy departments. (Though to be fair, Rubin’s husband is in a minority of mid-20th Century male spouses I’ve studied who actually made a concerted effort to equitably split household and child-raising duties with their scientific wives.)
The story that’s been repeated several times throughout the week, however, that Rubin moved into measuring orbital velocities of stars only after she was muscled out of her earlier studies by senior researchers who didn’t want a woman in their “territory,” is, by Rubin’s own account, a gross exaggeration. Her early work included determining rotation curves and galactic velocities and studying quasars, but she found that she did not enjoy working in a field crowded with other researchers:
“I didn’t like working on problems that many other people were working on and where I was constantly being besieged with questions about the work. I wanted a problem that I could sit and do at my own pace, where I wouldn’t be bothered… We studied quasars for a year or two and I found it personally very distasteful. I just didn’t like the pressure of other astronomers calling and asking me if I had observed this and if I knew what the redshift was. I didn’t get to a telescope very often and it meant that I either had to give out answers that I was uncertain of, or say I hadn’t done it and somebody else would then go do it. I just decided that wasn’t the way I wanted to do astronomy.” (Bright Galaxies and Dark Matters, p. 157-158)
The constant jostle and pressure of highly-populated research topics was simply not her style, so she found an obscure topic that she could do slowly and thoroughly, mapping the rotational velocities of stars in spiral galaxies using new instrumentation designed by her colleague Kent Ford. It was supposed to be useful, solid work with no surprises but, on the very first day, unexpected data started rolling in. The expectation was that, as with planets rotating around a sun, stars rotating around a galactic center should show decreasing orbital velocities the further from that center they were. At greater distances, the force of gravity should be less, and so, correspondingly, should the velocity decrease.
What Rubin and Ford found, however, was that the velocities flattened out, resolutely refusing to decrease with distance, a result that makes sense only if the rules of gravity work much differently than we believe, or if there is a lot more matter in those galaxies than we can observe through telescopes – dark matter.
Now, to be clear, Vera Rubin did not discover dark matter, and she wasn’t the first person to note anomalies that implied the existence of unobservable matter. At least three different astronomers in the 1930s had made similar observations and one of them, Fritz Zwicky, had hypothesized that the discrepancies he observed were due to an unobservable form of matter which he dubbed “dark matter.” He even came up with the theory of using gravitational lensing to further measure the amount of dark matter in the universe, a technique we use readily today but that was beyond the capacities of his instrumentation to measure.
So, no, Rubin didn’t invent the idea of dark matter or make the first observations that pointed to its existence. What she did do was produce the first modern evidence of it that made the astronomical community sit up and take the notion seriously. Her work was four decades after Zwicky’s, and in the interim astronomy had more or less forgotten about his dark matter gambit. Rubin’s velocity curves from the 1970s made the dark matter hypothesis impossible to ignore any longer, and the past four decades have seen an explosion in speculation about its existence and nature, from dark matter skeptics who believe that the observed phenomena can be explained by alternate means, to passionate advocates like Harvard cosmologist Lisa Randall who recently even used dark matter to explain the extinction of the dinosaurs (and it’s not nearly as crazy as it sounds).
Rubin’s ideas were lent support swiftly by measurements from other fields of astronomy, and in the popular imagination by the steady progress of neutrino physics, which verified the existence of curiously behaved particles that interacted only very rarely with matter. If billions of neutrinos can pass through you and not interact with your atoms, why can’t there be a whole class of matter that will not interact at all with electromagnetic radiation?
And yet, dark matter still possesses a murky status. There are an astounding number of observations that suggest its existence, but there is also a dedicated body of physicists providing alternative explanations for those observations, such as Modified Newtonian Dynamics (or MOND), which Rubin herself eventually came to favor over dark matter as the explanation for her measurements. There are experiments currently underway to provide final and indisputable evidence for dark matter’s existence, but until that data starts rolling in, everybody invested in dark matter is playing a massive waiting game, with either Nobel Prizes waiting at the end or a frustrated but sportsmanlike shrug.
But for Vera Rubin the waiting is over; the clock has run out. The murkiness that characterized how best to interpret her revolutionary results has spilt over into how to define her legacy. We have gotten so wrapped up in whether or not dark matter is a thing that we have overlooked the fact that Rubin’s reputation oughtn’t be dependent upon dark matter’s existence or non-existence. She used new technology to study and analyze hundreds of galaxies to their very edges at a time when most astronomers were only interested in using it to peer deeper into the heart of the universe itself, and in the process produced the data that puts her orbital velocity model beyond question. And that model has created so many puzzles and so many theories and inspired so many people to enter astronomy that it is almost a superfluous question what explanation will win out. Rubin, quite simply, gave astronomy a new lease on life and a new, perplexing set of paradoxes to chew on, right when we thought we had everything nice and figured out.
So, that’s Vera Rubin. She knew how she wanted to do astronomy, and found a way to make that happen, and in the process, created a mystery that drove a half century of research and the technological developments required to support it. Hampered initially by anachronistic restraints (though she did manage to discover the supergalactic plane when she was but 22), she found her way eventually and even credited her odd education with her success in looking for interesting projects away from the spotlight. Unlike the matter she might have uncovered, she shone in the darkness, and will continue to do so as long as we wonder about what might lie in that great vastness we shall never see.
FURTHER READING: There is a collection of Rubin essays and interviews, Bright Galaxies and Dark Matters (1997) that contains some of the only detailed biographical matter you’re going to find about her, along with various articles on dark matter, supernova physics, the historical development of astronomy, and the question of women’s place in physics, all written with an engaging zest for finding things out. For the state of dark matter research today, Lisa Randall’s Dark Matter and the Dinosaurs (2015) is an informative read that takes it as a given that the evidence for dark matter is so substantial as to be entirely conclusive. Online, I’d recommend Sabine Hossenfelder’s excellent blog BackReaction, the dark matter material from which can be found here!