When you started the project, you were expecting to find that the universe was decelerating in its expansion, but the supernova data did not hew to that hypothesis. Was there ever a eureka moment where everyone said, "Maybe we have to consider acceleration"?
There were a number of things that primed that for us. Supernova 1992bi had error bars that were down into the range of acceleration. I asked, what do we do with that part of it? That would be a universe with negative mass. And someone said, there is that old cosmological constant we could throw in. So we put it in there formally as a way to express the answer. We didn't hate the cosmological constant, but everyone accepted that it would be set to zero. With any reasonably large deviation from zero, the universe would look nothing like it looks today.
Indeed, Albert Einstein called the cosmological constant his greatest mistake. It was a theoretical patch job to keep the universe stationary instead of collapsing on itself because of gravity. But the redshift measurements you took of supernovae—which is related to an assumption about the cosmological constant and the critical mass of the universe—led you to draw conclusions about the fate of the universe.
The idea that you could ask something so deep just by making this measurement—I was shocked that people weren't tripping over themselves to do it. You could find out if the universe was going to last forever, and you could find out if the universe was infinite or curved in on itself. What would be more enticing as an experiment to do?
How long did the data analysis take?
Analyzing all the stuff takes forever with a huge amounts of cross-checks. The last thing you do is plot it on the Hubble diagram [a plot that connects an object's distance and redshift], but even then we know that we still have six things left to check.
First we got seven supernovae on the Hubble diagram, but it was suggestive of an "omega mass = 1' universe.
In other words, the mass density of the universe was such that the universe is "flat": it would not expand forever, but it would not collapse back on itself, either.
Then we got data from the Hubble Space Telescope, and that made a huge difference, because it could peer much farther, to much higher redshift. That was the first time you started to see the data pull away from a flat universe, and it was suggestive.
The fact that we were not getting an omega mass of 1 started to imply to people that maybe there was an omega lambda [that is, a cosmological constant]. That was the first time we started to have that from our own data. This was the beginning of 1997.
Eventually we had 42 supernovae. The error bars were still big enough, and there's so may things you have to cross-check, you just don't take seriously something like this [acceleration result] until you check everything that could be a mistake. We knew a cosmological constant would be an amazing discovery, if true. But this is not one you want to get wrong.
So you reached the conclusion about a runaway universe gradually? There was no sudden realization?
If you want to say was there a eureka moment, it was the moment we finally decided this was true, and we started to go out and give talks about it. It was the most bizarre eureka moment you could imagine: E…U...R…E...K…A, spread out over nine months. In this case you've seen that data for almost a year, and it's not a surprise to you anymore, but you're a little bit surprised that you're starting to believe it.
For me, the closest thing to a eureka moment was when I gave a talk at Santa Cruz, and I showed the result and tried to make it clear. After the talk, the famous cosmologist Joel Primack got up and said, "Before anyone asks any questions, I want to make clear to the audience what the significance of the cosmological constant is. This is a shock, a huge deal." The cosmological constant just wasn't in people's brains at that point, and it was then that I realized it was a shock. That was late November to early December of 1997.