Vera Rubin works with a telescope at Lowell Observatory in Arizona in 1965. (Courtesy Carnegie Institution of Washington)
Click for a full size image
Vera Rubin Made “Dark Matter” a Household Phrase
Years later, the astronomer recalled her “incredible delight” in the discovery for which she was most famous.
Astronomer Vera Rubin, best known for finding evidence of dark matter, in 1977, died Sunday at 88. Rubin, a staff scientist emeritus at the Carnegie Institution in Washington, D.C., studied the large-scale motions of galaxies and galaxy clusters. Here’s how she remembered her discovery of the dark stuff in galaxies for science writer M. Mitchell Waldrop in a 1993 Air & Space feature, “The Case of the Missing Matter”:
“I remember being startled even that first night,” says Vera Rubin. On November 14, 1977, she and colleague Kent Ford were down at the big Mayall telescope at Kitt Peak National Observatory in Arizona, testing a brand-new technique to study the motions of stars within spiral galaxies. And Rubin, who has the air of a feisty, energetic grandmother (which she is), couldn’t wait to see how things were turning out. So every now and again she left Ford at the helm of the telescope and took a freshly exposed photographic plate downstairs to the observatory’s darkroom, where she developed it on the spot.
What each plate recorded (they hoped) was the spectrum of each galaxy, from the brightest point in its center to the faintest edges of its spiral arms. This was something no one had ever done before. But if Rubin and Ford could do it, with the aid of a new image intensifier Ford had developed, they could use some basic physics to calculate the velocity of the stars in each region. Parts of the galaxy whose emissions were “redshifted” toward longer wavelengths would be moving away from us, whereas regions whose emissions were blueshifted toward shorter wavelengths would be moving toward us. When the telescope’s spectrograph was properly aligned, the resulting spectrum of the target galaxy would be essentially a graph, a plot showing how the velocity of stars varied from one side of the galaxy to the other.
And that’s exactly what Rubin saw in plate after plate as she held them up to the darkroom light. “I remember this incredible delight,” she says. “We’d done it!”
Yet even in the midst of her excitement, Rubin remembers being taken aback. Without thinking much about it, she had always assumed that the velocity recorded in these graphs would decrease as it was measured farther and farther from the galaxy’s center. A spiral galaxy is shaped like a fried egg, with most of the stars concentrated in a bright central bulge. The stars out in the spiral arms presumably orbit around this massive center in the same way that planets orbit the sun. In accordance with Newton’s law of gravity, the distant planets always orbit more slowly than the inner planets; likewise, the outer stars in any given galaxy ought to move more slowly than the inner ones.
Except that they didn’t. For galaxy after galaxy, Rubin’s freshly developed plates showed that the velocity curves flattened out. The outer stars were moving just as rapidly as the inner stars.
In the rush of completing the night’s observations, Rubin admits, “I wasn’t wise enough to just look at the plates in the darkroom and realize immediately what was going on.” It wasn’t until she and Ford were going over their results several days later that the full implications of those flat velocity curves hit home: the total mass in each galaxy—the stuff that holds visible stars in orbit through gravity—is not concentrated in that bright central bulge. In defiance of all photographic evidence, which shows that the number of visible stars in a spiral galaxy always falls off rapidly beyond a certain distance from the center, the total mass of the galaxy continues to increase for some distance at an undiminished rate. Somehow, Rubin and Ford realized, there is an immense amount of mass out there that isn’t in the form of stars, or anything else they could see. Somehow, each spiral galaxy was surrounded by a kind of nimbus, a ghostly halo of “dark matter.”
“By the time we’d done 10 of these curves, we said, ‘We’ve just got to publish,’ ” says Rubin. Their paper, released in 1978, attracted the attention of astronomers everywhere. Invisible matter? There had been occasional hints of unexplained gravitational forces before, as far back as the 1930s. But the idea had always seemed remote—until now.
“Observationally, it was very, very satisfying,” says Rubin. “Anybody could look at these curves and see what they said.” Subsequent observations by Rubin, Ford, and many other astronomers only strengthened the conclusion: In every spiral galaxy they looked at, the stars were orbiting too fast. Furthermore, in every cluster of galaxies they looked at, individual galaxies were moving too fast. In neither case could the visible stars exert enough gravitational force to hold things together. To make up the difference, the dark matter had to outweigh the visible stars by at least a hundred times—and perhaps a great deal more than that.
By 1980 the idea of dark matter had completely overtaken the astronomical community. And in the decade since then, the implications have become even more staggering. It’s now abundantly clear that galaxies are not the mighty dreadnoughts of the cosmos they appear to be. They are more like flecks of foam upon an immense dark sea, going wherever the underlying currents take them. Indeed, the ebb and flow of dark matter is almost certainly what created galaxies in the first place. The huge gravitational field of dark matter may even determine the fate of the universe itself, deciding whether it expands forever or ultimately collapses. And if this dark cosmic ectoplasm is really made of something that transcends matter as we know it—as many scientists now suspect—then it may well point the way toward a new understanding of the most basic laws of physics.
So. What is this stuff?
“I really thought we’d know more by now,” sighs Rubin…. “In 1980 I really thought that in 10 years we’d have it. It’s a big disappointment to realize that the answer is as far away as ever.”