Three American astrophysicists were awarded the 2011 Nobel Prize in physics Tuesday for showing that the universe is flying apart at an ever-accelerating pace, a discovery that overturned decades of assumptions in the field of cosmology.

Discovered nearly simultaneously by rival teams in 1998, the finding all but settled one of the biggest questions in science — the ultimate fate of the universe. Billions of years from now, as galaxies continue to pull apart from one another, the ever-enlarging universe will turn black and cold.

In its award citation, the Royal Swedish Academy of Sciences wrote that “the discovery that this expansion is accelerating is astounding. If the expansion will continue to speed up the universe will end in ice.”

In the 1990s, two teams raced to measure the expansion of the universe by observing dozens of stars exploding in distant galaxies.

By measuring how fast the stars, called Type 1a supernovae, were racing away from Earth, the teams reached the same astonishing conclusion: The universe was flying apart with ever-quickening speed.

One of the teams was led by Saul Perlmutter, 52, who heads the Supernova Cosmology Project at the Lawrence Berkeley National Laboratory and the University of California at Berkeley. He will receive half of the $1.4 million prize.

The other half will go to two members of the second team, Brian Schmidt and Adam Riess.

Schmidt, 44, heads the High-z Supernova Search Team at Australian National University in Weston Creek, Australia. He holds dual U.S.-Australian citizenship.

Riess, 41, is an astronomy professor at Johns Hopkins University and a scientist at the Space Telescope Science Institute in Baltimore.

Riess said intense competition “lent a sense of urgency” to the hunt for distant supernovae. By the time he and Schmidt entered the race, Perlmutter’s team had a five-year head start.

“There was some friction between the two groups. They felt like we were usurpers,” said Robert Kirshner, a Harvard University astrophysicist and graduate adviser to Riess and Schmidt when the High-z team began its hunt. “But young people like Brian [Schmidt] with supple minds could catch up very quickly. So we did catch up.”

By late 1997, the team had enough data from a telescope in Chile and the Hubble Space Telescope to calculate the mass of the universe — and how fast, or slow, it was expanding.

The answer confused Riess. “I remember thinking, ‘Ugh, I’ve made a terrible mistake. And I have to find this mistake.’ And then spent weeks looking for it.”

Riess soon heard that Perlmutter’s team members — some of whom Riess knew from games of “mud football” on the Berkeley campus — had come to the same astounding conclusion.

“And when I did find out that they were seeing the same thing, it went from ‘Oh, this must be a terrible mistake’ to ‘Oh, this must be the right answer,’ ” Riess said.

At a news conference Tuesday, Perlmutter said: “I thought it was the kind of project any scientist would love. You’re getting to ask a very philosophical question about the fate of the universe.”

When the teams began their work, scientists thought the expansion of the universe was slowing. Some proposed that it might even reverse, retract and collapse upon itself in a “big crunch.”

Perlmutter recalls thinking at the time, “How much is it slowing down?”

Instead, the universe’s inflation was speeding up.

The discovery pushed cosmologists to an uncomfortable conclusion: Some mysterious anti-gravitational force must be pushing the universe apart. Scientists dubbed this force “dark energy” and are racing with new instruments to discover what it might be.

The discovery resolved a conundrum that stumped Albert Einstein in 1917. At the time, scientists did not know that the universe was expanding. But a static, non-expanding universe did not fit Einstein’s general theory of relativity. So Einstein introduced a factor he called the “cosmological constant,” a hypothetical, repulsive, universal force.

Einstein later discarded this force, calling it the “biggest blunder of his life,” according to his friend, physicist George Gamow.

When Schmidt and Riess published their discovery in Astronomical Journal in November 1998, they pointed to Einstein’s cosmological constant as an explanation — flipping Einstein’s blunder into an observed reality.

Perlmutter’s team published its results in the Astrophysical Journal in June 1999.

Riess learned of the award in a predawn phone call. “I was home, and I was trying to sleep, and my 10-month-old was kind of yipping and yapping,” he said. “The phone rang, it was 5:30, and it was Swedish-sounding people, and I knew they weren’t from Ikea.”

Riess was born in the District and grew up in Warren, N.J. He attended public schools and was tapped as a high school junior to attend a program for the 100 most promising science students in the state. He first took physics the following year as a senior.

“And it was hard, but I knew I liked it,” he said. “I liked its sort of relentless pursuit of the most basic knowledge, sort of the building blocks of the knowledge that we have.”

He earned a bachelor’s degree in physics at the Massachusetts Institute of Technology and a doctorate in astrophysics at Harvard. He then went to the University of California at Berkeley as a Miller fellow; that is where he did the work that earned the award.

Riess has taught at Johns Hopkins since 2006. He won a Mac­Arthur Foundation “genius grant” in 2008. Early this year, he won the Einstein Medal, presented annually to a scholar who furthers the work of Einstein.

Riess teaches an introductory astrophysics course for non-majors at Hopkins. He hopes the prize will not upend his teaching and research in Baltimore.

“I’m really hoping I can keep doing what I’m doing,” he said. “I’ve always enjoyed telling the story of science to people who don’t know much about it, and I really enjoy the sort of pedagogical process for people who don’t have a lot of background. I like to explain how we came to understand the universe. We keep discovering.”