Scientist who helped discover the expansion of the universe is accelerating
Breakthrough Prize winner continues investigating fundamental physics of the world
February 3, 2015
In the late 1980s, astrophysicist Saul Perlmutter and his colleagues set out to determine how much the expansion of the universe was slowing. At the time, the prevailing belief among scientists was that gravity would be slowing the expansion, perhaps enough to ultimately switch to a contracting universe that would cause the galaxies to draw even closer together.
Of course, the world learned in 1998 that this was not the case. As it turned out, the expansion of the universe was, in fact, not slowing at all, but speeding up.
"That means there is something else going on besides gravity," says Perlmutter, director of the Supernova Cosmology Project at Lawrence Berkeley National Laboratory, who shared the 2011 Nobel Prize in physics for the work. "We thought we understood the physics, but this was a real surprise."
The "something else" is an enduring mystery that continues to fascinate, and elude, scientists today, Perlmutter among them. Astrophysicists refer to it as "dark energy."
"We call it 'dark' because we don't know what it is," he says. "But it possibly means that as much as 70 percent of the universe could be made out of this previously unknown energy."
Moreover, researchers don't know why the universe is speeding up. "But it leaves the possibility that if whatever is speeding it up goes away, then it will start to slow down again," he says. "There is still a lot in play, and we are still trying to learn what it is."
It's an exciting prospect, since research into the ongoing puzzle of dark energy could provide "a new understanding of the fundamental physics of the world," Perlmutter says. "We have no idea what the consequences will be if we learn what dark energy is. But history has shown us that these kinds of steps forward in our fundamental understanding make us a more capable civilization.
"Moreover, learning how this world is put together, in a way, is a deep, almost poetic experience," he adds.
Perlmutter, also a professor of physics at the University of California, Berkeley, is a recent recipient of the 2015 Breakthrough Prize in Fundamental Physics, sharing the $3 million award with his Supernova Cosmology Project team, and with Brian P. Schmidt, an astrophysicist at the Australian National University Mount Stromlo Observatory and Research School, and Adam Riess, an astrophysicist at The Johns Hopkins University and the Space Telescope Science Institute, and the High-Z Supernova Search team that they led.
The three, who also shared the 2011 Nobel, received the Breakthrough Prize together with their teams for their work providing evidence that the expansion of the universe is accelerating.
For Perlmutter, the research leading to this discovery began in 1987, with a project under the auspices of the newly created Center for Particle Astrophysics, a National Science Foundation science and technology center based at Berkeley. Perlmutter, a postdoctoral fellow at the time, designed the study with Carl Pennypacker, also a researcher in the group which was then under the direction of physics professor Richard Muller, a 1978 NSF Alan T. Waterman award winner.
"When the project began in 1987, the standard picture of cosmology was that the universe was expanding, but everyone assumed it would slow down because gravity would attract everything to everything else," Perlmutter says. "We wanted to find out: How dense is the universe? How much is it slowing down?"
The scientists decided to try to measure the state of the universe by looking several billion years in the past using a new understanding in the field about a specific type of supernova, or exploding star, called Type Ia, that explodes in a similar way every time. "Since they brighten to essentially the same brightness every time, and then fade away, we can tell how far away they are by measuring how bright they appear to us," he says.
Since light always travels at 186,000 miles per second, researchers can then use the distance measurement to calculate how long ago these supernovae exploded. Also, while the light is traveling--and the universe is expanding--the light waves traveling from the exploding supernova stretch along with everything else. As these wavelengths stretch, they look redder and redder, a phenomenon in astronomy known as "redshift."
"When the supernova explodes, it sends out mostly blue light," Perlmutter explains. "That blue light means a short wave length of light. The more it stretches, the more it starts to turn red. And that tells us the amount the universe stretched between the time of the explosion, and today."
Taken together, the brightness and colors of the supernovae provide compelling evidence of an accelerating expanding universe. The degree of their brightness reveals how far back in time the star exploded, and the extent of redshift indicates how much the universe has expanded during that time. So a series of measurements each taken for a supernova exploding at a different time throughout history--7, 4 and 2 billion years ago--revealed that the stretching of the universe was increasing, and that it wasn't slowing down at all.
The difficulty initially was finding these supernovae in time, since they are rare and random, and reserving a stint at some of the largest and most advanced telescopes in the world, not to mention hoping for good weather.
Ultimately, they found a way to make discovering Type Ia supernovae more predictable.
"Instead of watching one galaxy, we figured out how to use novel wide-field cameras on the big telescopes to watch thousands of galaxies," he says. "You take a bunch of images one night, then go away, then come back two and half weeks later and take another bunch of images. Now you have two almost identical images of the galaxies, but with a time gap just long enough to allow a new supernova to appear."
"Everything had to happen like clockwork, and anytime the night was cloudy, you'd have to scramble to cover that time another night someplace else," he says.
The researchers developed a special computer program "to hunt through thousands of specks of light to find a new speck that wasn't there before," that is, looking for a new supernova. Then, using a spectrograph, they analyzed the light waves to determine whether the supernova was a Type Ia, the type they needed to study. Finally, they ran a series of observations following the supernova, obtaining images four to six more times as it brightened, then faded, which told them how bright it was at its peak.
"When we started the project, I thought we were just going out and doing a simple measurement of the brightness of exploding stars, and finding out whether the universe was going to end," he says. "It turned out that what we discovered was a huge surprise. We have been comparing it to throwing an apple up in the air, and finding that it doesn't fall back to earth, but instead blasts off into outer space, mysteriously moving faster and faster."-- Marlene Cimons, National Science Foundation
University of California-Berkeley