Saul Perlmutter: 'Science is about figuring out your mistakes'

Nobel prize-winning astrophysicist Saul Perlmutter never thought "Eureka!" when, in 1997, his observations of exploding stars called supernovae suggested that the expansion of the universe, begun by the big bang, was accelerating. In fact, explains Perlmutter, he thought he and his team must have made a mistake. For a start, they had embarked on the project a decade earlier, expecting to measure the rate at which the universe's expansion was slowing. To find the opposite seemed crazy.

Yet after months of painstaking checking, what they saw simply wouldn't go away. "Finally we came to the conclusion – this is what we have, and we're going to have to publish and present it," says Perlmutter.

The result suggests a mysterious force, since called dark energy, driving the acceleration. The discovery, unveiled in 1998, shook cosmology to its foundations and won Perlmutter, a professor of physics at University of California, Berkeley, one-half of the 2011 Nobel prize for physics. It also gave him a premium campus parking spot for life (Berkeley awards one to all its Nobel laureates) and a street named in his honour at the Lawrence Berkeley National Laboratory, where his office overlooks San Francisco Bay. "Perlmutter Road" was officially unveiled last year.

The other half of the prize (a quarter each) went to two members of a rival team, who beat Perlmutter's in being first to publish their results. "It actually felt really appropriate that we ended up sharing it," says Perlmutter, adding that, between them, the two teams spanned almost everyone who had worked on supernovae in the recent past and whose input along the way had been crucial.

The discovery has led him to think deeply about the nature of science. "Science isn't a matter of trying to prove something – it is a matter of trying to figure out how you are wrong and trying to find your mistakes," he says.

Perlmutter, 53, grew up the middle of three children in a culturally Jewish family in Philadelphia. Both his father and mother were academics and the household was alive with intellectual conversations. As a child, Perlmutter wasn't a stargazer and didn't even have a telescope, but he liked to think deeply about how the world worked. "I always thought you need to get to the bottom of things," he says.

That instinct made it tough to decide whether to study philosophy or physics at university, but he eventually chose the latter, taking his undergraduate degree at Harvard and then heading to Berkeley for a PhD. There, Perlmutter joined the group of innovative astrophysicist Richard Muller and blossomed. focusing on tackling the most fundamental of questions.

Perlmutter wrote his PhD thesis on the possibility that a second "killer" sun orbiting ours every 26 million years is responsible for periodic mass extinctions, including that which wiped out the dinosaurs. "This is still not yet completely disconfirmed," he says.

Another of the group's projects led Perlmutter to his Nobel-winning discovery. Building on recent technological developments in digital imaging (CCD detectors), and using a small telescope at the university's observatory, he helped build a robotic system to take pictures of hundreds of nearby galaxies over several nights. Automatically subtracting the images from different nights revealed temporary bright spots – supernovae.

It was a colleague who suggested they use the same technique to hunt for older supernovae in distant galaxies, in particular a newly identified type (type 1A), which explode with about the same brightness every time. That would allow testing an idea first suggested in the 1930s: if you could find a series of standard supernovae of different ages (fainter means older) and measure how much the universe had stretched over that time (from the wavelength, or colour, of their emitted light), it would be possible to calculate the presumed slowdown of the universe's expansion. That might reveal if and when the universe would stop altogether and collapse into a big crunch. With the end of the millennium approaching, predicting the end of the universe appealed to Perlmutter, who thought the idea "kind of fun".

In 1987, he and colleagues set out to automatically scan thousands of distant galaxies to hunt for type 1A supernovae. Catching rare, random, short-lived flashes at exactly the right moment (their peak brightness) would be challenging enough, but the project soon hit an even bigger problem. No one was willing to schedule them valuable time on any of the big telescopes needed to study something so far away. The supernovae couldn't be predicted and the astronomical "fad" of the day was to stick spectrographs, not imagers, on telescopes.

Perlmutter's team eventually got a break: a telescope in the Australian outback offered 12-and-a-half nights of observing time in exchange for being able to keep the novel wide-field camera they together would build. The results were promising. "It got the whole ball rolling," says Perlmutter, though it would be a few more years before the team arrived at a method of finding the supernovae that persuaded other observatories worldwide, including the Hubble space telescope, to allow them to schedule projects.

Finally, based on 42 supernovae, they reached their surprise discovery. Its implications – "a whole series of big questions" – have been occupying Perlmutter ever since. For a start, there's the question of what is the dark energy driving the universe's acceleration? "To this day, we still don't have a good explanation," he says.

The discovery also adds to the mystery about the universe's fate. Will it expand faster and faster, getting emptier and colder for ever, or will the dark energy decay and the universe collapse as originally considered?

All those questions and the theories proposed in response have convinced Perlmutter and others that they need to gather more precise data. For the past decade, he has worked on the design of a new space telescope to study the properties of dark energy. The project has had setbacks, winning funding only to be scaled back and put on hold, but Perlmutter is now cautiously optimistic again thanks to an unusual intervention. America's spy satellite agency recently donated some parts to Nasa that could be used for a space telescope called the Wide-field Infrared Survey Telescope (WFIRST), which incorporates Perlmutter's project. He is excited by the prospect of what it might uncover. The payoff for these long-term projects "is you hope you find something really surprising about fundamental things".