This colorized Chandra image of a supernova 1a remnant shows X-rays produced by high-energy particles (blue) and multimillion degree gas (red/green.) Photo by NASA/CXC/Rutgers/J.Hughes et al.
Three U.S. physicists won the Nobel Prize in physics Tuesday for discovering, by studying supernovae, that the universe is not just expanding, but speeding up.
One half of the prize was awarded to Saul Perlmutter of the Lawrence Berkeley National Laboratory, and the other half was jointly awarded to Brian Schmidt of Australian National University and Adam Riess of Johns Hopkins University and Space Telescope Science Institute. The teams presented research in 1998 that mapped the universe by locating and measuring the most far flung supernovae. Their finding, that the most distant supernovae appeared to be moving faster, was at first, astonishing.
The discovery boils down to measuring the history of cosmic expansion, said Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics, who was also involved in the research. “What they found is that the universe wasn’t slowing down due to gravity, but speeding up,” he said. “That meant there was something else that isn’t so obvious that’s making the universe accelerate. That’s what we call dark energy. The observation was acceleration. The inference was that dark energy is doing it.”
Universal expansion depends on a tug of war between gravity trying to slow things down and dark energy trying to speed things up. While there are still many questions about what exactly dark energy is, the commonly held theory is that it makes up most — about three-quarters — of the energy in the universe. Stars and galaxies constitute only a small fraction — about 5 percent.
To map the changes in the universe, the teams studied a specific kind of supernovae called Type 1a Supernovae. These supernovae are caused when compact stars called white dwarfs, as heavy as the sun but as small as the Earth, spectacularly explode like a bomb, emitting as much light as a whole galaxy as they release all of their nuclear energy at once.
They are useful to study because their light curves — their rising and falling brightness over time — are nearly the same, and they are bright enough to be seen at tremendous distances.
The two research teams found more than 50 distant supernovae whose light was weaker than expected. The discovery came as a complete surprise even to the laureates themselves, according to the Royal Swedish Academy of Sciences.
“By the end of 1997, we had this data that were staring us in the face that said that the universe is accelerating,” Kirshner said. But it took courage to go out on a limb with a theory that second guessed Einstein’s theory. “The idea that we were seeing something that had to do with the cosmological constant was repulsive to us,” he said. But it helped that Perlmutter’s team had a data set “that showed this same wacky result.”
Supernovae peak for about a month. The scientists had to measure the changing brightness and the color of the supernova during this time in order to get an accurate distance. “Supernovae are like fish,” Kirshner said. “After you catch them, you have to use them within a couple of days.”
Schmidt spoke to the Nobel committee from Australia just after the announcement. “I feel weak at the knees, very excited and somewhat amazed by the situation,” he said. “It’s been a pretty exciting last half hour.”
The Nobel Prize for medicine came out on Monday. The award for chemistry will be handed out on Wednesday.