Vera Rubin didn’t discover dark matter.
Rubin died last weekend, at the age of 88. Headlines have repeatedly identified her as having “discovered” dark matter or having “proved” the existence of dark matter. Even the Carnegie Institution's press release announcing her death—she had worked as a staff astronomer at Carnegie’s Department of Terrestrial Magnetism in Washington, D.C., for half a century before her recent retirement—said that she “confirmed the existence of dark matter.” Rubin would have said she did no such thing. I know, because she did say that, to me, on several occasions.
One could make the argument that the correct formulation of her achievement is that she discovered evidence for the existence of dark matter, and while Rubin likely would have acquiesced to that construction, she would have found it incomplete, perhaps even misleading. She would have said that while she discovered evidence for the existence of dark matter, you shouldn’t infer from that statement that dark matter actually exists.
The distinction wasn’t merely a matter of semantics. It was, to her, a matter of philosophy, of integrity—a matter of how science works.
In the late 1960s she and her frequent collaborator W. Kent Ford, Jr., began studying the gas and dust in our neighboring Andromeda Galaxy. They were in effect testing a new instrument designed by Ford that provided a level of precision previously unavailable. What they found surprised them: The gas and dust swirling at the outer edges of the galaxy were rotating just as fast as the gas and dust near the center of the galaxy. If the galaxy really were spinning at that rate, it should be shredding in every direction, but clearly it’s stable. Rubin and Ford published their results in 1970, but one anomalous detection does not a compelling argument make.
Throughout the 1970s, however, they and other astronomers found the same pattern again and again, in galaxy after galaxy, until theorists had little choice but to reach a consensus: Galaxies are embedded within a vastly much larger, stabilizing halo of matter we can’t detect in any range of the electromagnetic spectrum—that is, matter that’s “dark.” Theorists even identified the properties of what the hypothetical matter might be, and experimenters began designing instruments that in principle would be able to detect the particle or collection of particles.
In 1980 Rubin predicted the discovery of dark matter within ten years. Ten years later, the British astronomer (and future Astronomer Royal) Martin Rees predicted the discovery of dark matter within ten years. Eleven years later, in his book Our Cosmic Habitat, Rees wrote, “I think there is a good chance of achieving this goal within ten years.” Five years later, at an American Institute of Physics symposium, Rees doubled down on that prediction: five more years, he vowed. Rubin, who happened to be in the audience, stood up.
“I know of earlier predictions,” she said.
Rubin told me about that last exchange shortly after it occurred, in 2006. She told me about it again over dinner in January 2011. By then, another five years had passed. And now another five years have passed.
She delighted in that anecdote, I think, because it illustrated an important point about science—one that she herself often made when I interviewed her as part of my research or, later, when we talked just because we enjoyed the conversation. She’d done her job; she’d made observations. Now it was up to others to do their jobs: interpret the data, predict further results, make the discovery.
Or not make the discovery.
It was the “or not” that intrigued her. Maybe the discovery of dark matter was not possible. Maybe dark matter doesn’t exist. Maybe what she detected in the 1960s and 1970s was evidence that gravity doesn't work on large scales in the manner that Newton taught us. Some theorists and observers have been pursuing that possibility since the early 1980s, though the community generally has seen their work as somewhat contrarian.
Not Rubin. The longer that dark matter went undetected, she said, the more likely she thought the solution to the mystery would be a modification to our understanding of gravity.
“You do?” I said to her, when she told me that this is how she feels.
“Wow,” I said.
“I do,” she repeated. Then she said, “Why not?” and she shrugged.
That shrug should be her legacy as much as her observations. She found something odd about the universe, something that one way or another challenges physics in a fundamental way. That’s pretty impressive. But shrugging off the most common interpretation of her life’s work—that she discovered evidence for the existence of dark matter—in the absence of corroborating data? That’s pretty impressive, too.