Schmidt and Riess were on a competing team that arrived at the same conclusions as Perlmutter and his colleagues, about the same time in 1998.
The two teams calculated the universe's rate of expansion by measuring the light emitted by dozens of exploding stars called type Ia supernovas. They gauged the distance of these stars by the brightness of their light. The acceleration of the universe was determined by changes in wavelengths of this emitted light as the space between Earth and the stars continued to stretch.
At a news conference in Berkeley, Perlmutter said that while the basic concept of his research could be explained in a few words, the scope of his subject was indescribably vast.
"As soon as you consider any of these things, your mind is boggled," he said. "I think you have to enjoy having your mind boggled."
The research was the purest of basic science, of the kind with no immediate practical application, yet Perlmutter noted that many such discoveries do end up proving useful in everyday life. He said he was driven by the quest for knowledge.
"This is part of what it means to be a human being," Perlmutter said.
He went to public kindergarten in Philadelphia, where he was always eager to raise his hand, recalled his parents, Dan and Felice Perlmutter, who now live in Narberth. His teacher was concerned she could not give him the proper amount of attention and told his parents he might be better off at private school, if they could manage.
"She felt very guilty," recalled Felice Perlmutter.
So Saul Perlmutter switched to Greene Street Friends School for the elementary grades, followed by Germantown Friends for grades 7 through 12.
His younger sister, Tova Perlmutter, who also attended Germantown Friends, said it was a good fit.
"He was just always exploring and learning and pushing," recalled his sister, who is executive director of the Sugar Law Center, a public-interest organization in Detroit.
But he has one failing: "He talks really, really, really fast," she said. "When he was in high school, I actually used to interpret for him to my parents."
John Tuton, Perlmutter's eighth-grade English teacher and a retired psychologist, said: "He had a wonderfully gentle sense of inquiry and, I would just say, wonderment."
Perlmutter was so influenced by his Quaker education that he and his wife, anthropologist Laura Nelson, included a 20-minute period of silence in their wedding ceremony, during which anyone could rise and speak if they felt so moved.
Academic pursuits were in the family. Perlmutter's father was a chemical engineering professor at the University of Pennsylvania, who has a new book out this year, The Challenges of Climate Change: Which Way Now? His mother, now retired from Temple University, is a much-published author in the field of social work.
From Philadelphia, their son went on to Harvard and then to the University of California, Berkeley, for graduate school. His quest to measure the expansion of the universe began in 1988; Riess and Schmidt joined the chase in 1994.
The discovery that resulted has profoundly changed our view of the cosmos, said Princeton University physics professor Paul Steinhardt, who is in the same field.
The continued expansion of the universe does not mean the distance between Earth and nearby planets will grow, he said. Galaxies such as the Milky Way have "clumped" together due to gravity and will remain so.
It is the blackness between that will continue to stretch and cool, so that in many billions of years, our galaxy will be on its own, he said.
"It'll be like an island universe," Steinhardt said.
Before the Nobel-winning scientists could measure the light from the type Ia supernovas, they had to find them, a hard task given their rarity.
The scientists tackled this problem by taking images of large swaths of the night sky, waiting a few weeks, and then doing it again to see if any new bright spots had appeared.
The scientists learned of their awards via early-morning phone calls from Sweden, though Perlmutter's parents said their son first heard the news from the Swedish media. They had been mentioned as possible Nobel winners for the last few years.
They head to Sweden in December to accept their awards. But their quest for knowledge is not over. Said Perlmutter: "There's so much more left to discover."
Contact staff writer Tom Avril
at 215-854-2430 or firstname.lastname@example.org.