SAUL PERLMUTTER: His Supernova Cosmology Project revealed that the universes expansion is accelerating, a result that is still upending theories. About the same time, another group came to the same conclusion.
Gabriela Hasbun

One of the biggest scientific findings in recent years is the discovery that the universe is not only expanding, but it is also accelerating in its expansion. Under the influence of a mysterious dark energy, the universe will eventually thin out to nothingness and die a cold death. For the Insights story, "Dark Forces at Work," appearing in the May 2008 Scientific American, David Appell talked with Saul Perlmutter of the University of California, Berkeley, and one of the leaders of the group that came to the astonishing conclusion. Here is an edited excerpt of that interview.

In finding that the universe is on a path to runaway expansion, you had to find type Ia supernovae, which can act as distance markers. How did you get involved with supernova searching?
I was at the University of California at Berkeley for graduate school. One of the heroes here at Berkeley is Luis Alvarez. The tradition that he started is looking for interesting science no matter where it is and then finding tools to do those things. For example, he invented one of the first steady cams.

One of his protégés was my professor, Richard Muller. There was a project to do a superautomated supernova search that Luis Alvarez had suggested to Rich. They had just done one of the first adaptive-optics experiments.

So, as a grad student, you began developing the techniques for a robotic supernova search. But you also found that they could be used for other kinds of targets.
I had gotten interested in a theory that had been put forward by Rich Muller and Marc Davis and Piet Hut—namely, that you could explain periodic mass extinctions on the earth by a possible companion star around the sun. They called this star Nemesis. We realized that with the most common kind of star in the sky, the red dwarfs, you wouldn't know if it were orbiting around our sun. You could assume it was a much farther away red giant, just because it would have very little motion with respect to us, because it's bound to us. We realized that we could look in the catalogue of red stars for one that had this motion that you expect and that we could use the same telescope we were using for the robotic supernova project. So I developed the astrometric techniques to make this precise measurement of the parallax motion.

Have you done any work on Nemesis since?
No, it was one of the things I was doing, but my main project was working on the robotic nearby-supernova search. I had developed some new software. We were running the search and finding  supernovae, cataloging and keeping track of everything so you could compare it to everything we found.

Are you more of a theory guy, or are you more of a gearhead?
I'm really an experimental type. I really enjoy trying to catch the universe in the act of doing something very surprising and perhaps figuring out what it's doing by making measurements.

You were the team leader of the Supernova Cosmology Project in the 1990s, which consisted of a dozen members or so. How did you delegate responsibilities to your team members?
You're trying to figure out what are the critical paths to get to the result you want. And sometimes you think it's faster doing it yourself, and other things come along and you'd like to do them, but I'm trying to get this one done and somebody else comes along in your group and they start doing it. So it's much more organic in the sense that you find that you can't do everything. The best world would be one in which everyone you find to work with is better than you, because then you're doing better work than you could ever do.

What were the downsides to working on the project?
The worst thing about my research life was that I was always worried about something that had to happen in the next 24 hours, or sometimes, the next two hours. It was a terrible way to lead an ordinary life.