When Vera Rubin was 14, she fashioned a home-made telescope out of a cardboard tube and began scanning the night sky. Her early enthusiasm paid off in 1950 when, as a 22-year-old mother of a month-old baby, she hit the headlines of the Washington Post with a provocative theory that the universe is rotating. Her later observations of orbiting stars on the outskirts of galaxies helped spark a remarkable realization: the vast majority of matter in the universe is invisible, or 'dark'. Now 73, she continues her cosmic exploration at the Carnegie Institution of Washington, D.C. She shared some recent insights with associate editor Josie Glausiusz.
How did you first realize the universe is filled with invisible matter? In a spiral galaxy like our own, a couple hundred billion stars orbit around the center. Spirals appear to thin out toward the edges, so the gravitational pull on the outer stars should be weaker out there. Everyone therefore assumed that objects near the edges orbit more slowly. But that is not what I found: Even stars at the periphery are orbiting at high velocities. There has to be a lot of mass to make the stars orbit so rapidly, but we can't see it. We call this invisible mass dark matter.
Do you know what dark matter consists of? No. We absolutely do not. I said in 1980 that we'd know what dark matter was in ten years, that the particle physicists would tell us. And although at the present time we think that most of the matter in the universe is dark, there is a lot of it only because the universe is so enormously large. Particle physicists have to do in their laboratories what the universe is doing on a very large scale, so they are continually building more powerful and larger detectors. However, there would be so few of these particles in the distances and energies they're involved with, that they're looking for something that's much rarer than a needle in a haystack.
Could a flaw in our ideas about gravity explain your findings? There is one alternative to dark matter, and that is the assumption that Newton's laws don't hold over distances as great as galaxies. But we know that Newton's laws hold over a very large domain. And virtually one hundred percent of the physics and astronomy community believes that there is matter in the universe that does not radiate. Having said that, we really don't understand what gravity is or why it's such a weak force. You can pick up a paper clip, and you're overcoming the gravitational force of the Earth. Well, that's astounding!
How could the study of dark matter help us to understand the size, shape and ultimate fate of the universe? If there is enough dark matter in the Universe then it is possible that the Universe would ultimately stop expanding and collapse. We think at the present time that there is not enough dark matter plus light to halt the expansion. And it looks as if there is a cosmological constant, a hidden energy whose effect is to make the expansion actually speed up.
What is the current focus of your work? Still galaxies, but slightly more peculiar ones. About ten years ago, I discovered a disk galaxy in the Virgo cluster in which half the stars go clockwise and half the stars go counterclockwise—intermingled in the same disk. That's possible because stars are so far apart. But we also started studying polar ring galaxies, that have rings of stars and gas over their poles. I've gotten interested in these kinds of objects partly because it's very difficult to understand how they form.
What lies in wait for our own galaxy, the Milky Way? We and Andromeda, the closest large galaxy, are actually approaching each other. In about two billion years we will pass by or through each other. Or it could be a dead-on hit. But the gravitational attraction should be enough that we'll start orbiting closer and closer and closer, and probably by five billion years we will have merged.
Do you think there could be life elsewhere in the universe? I think it would be impossible for there not to be. We know that the laws of physics are virtually the same all over the universe. That was essentially learned by Cecilia Payne-Gaposchkin in her 1925 thesis, which no one would believe. She discovered that stars were mostly hydrogen, and that they were all pretty much the same. There are 200 billion stars in our galaxy and there are billions of galaxies, so there's been plenty of chance for planets. And even though improbable things happen, I find it impossible to believe that our solar system is unique.
What do you think of Steven Weinberg's contention that "the more the universe seems comprehensible, the more it also seems pointless"? I don't really know what he means by "pointless." For those of us living in the universe, it's a very interesting experience, one I'm glad I have. Certainly, the universe was not designed for us. But I don't know. It's spring today, and there are lots of beautiful flowers in Washington, and I love walking to work and seeing them all. I don't know that that's all pointless. It may not have a universal aim, but it is very nice to be here.
What is the biggest challenge that you faced as a woman scientist? Child care. I have four children, and my husband and I were devoted to them all, and it was always a problem. I worked for almost all of my early career as a part-time person so that I could be home at 3 o'clock, and that was after they were all in school. It was almost overwhelming. I did a lot of my work at home. When I was home, the kids didn't really care what I was doing, and so I sat at the dining room table and worked.
What advice would you now give women pursuing careers in science? I guess I would say that if they really want to do it, to just go ahead, try not to let anything discourage them, try not to quit, but to recognize that academia is still not kind to women. In the United States industry has been hiring women. But the numbers of women in academia is still pathetic. I think academia is still really quite a male society, and it's easier for people to hire people who are like themselves, and have the same outlook on everything, and do things essentially the same way.
All four of your children have doctorates in the sciences. To what do you attribute their love of science? They saw their parents having so much fun that they thought that would be a good thing to do. In fact our fourth was quite a talented musician, and we tried telling him that we enjoyed having a musician in the family, that we didn't want him to think he had to be a scientist because all his siblings were. And he said, no, music is too hard! He could be a scientist and still do music, but he couldn't be a musician and do science.
If you could visit another galaxy, which would you choose, and why? That's easy. I would choose to go to Andromeda, so I could look back at our galaxy and see what it looked like. I expect I would see a beautiful spiral with a central bright bulge, and dust lanes outlining the spiral arms. Actually the trip there would be lots of fun, too, because you'd move through our galaxy to get there, so you'd see things close up. I'd make the person taking me go closer to the center, but maybe not the exact center, because then we may get swept into a black hole. But nearby to see what dense cores of galaxies are like. Maybe we'd discover other bits of galaxies. And if we could tell where the dark matter was, maybe we could tell how far it went, and whether it actually went as far as Andromeda.
