Quotes of the Day

The research that leads to a Nobel Prize in physics can sometimes be a little obscure. In 1990, for example, three scientists got the nod "for their pioneering investigations concerning deep inelastic scattering of electrons on protons and bound neutrons." Got that? The next year, the prize went to a scientist "for discovering that methods developed for studying order phenomena in simple systems can be generalized to more complex forms of matter."

But sometimes, you just can't help saying, "Wow!" and maybe: "What took the Nobel folks so long?" That's what Adam Riess's friends kept asking him — and this morning at 5:30, the phone finally rang. "I knew that was the famous time," says the Space Telescope Science Institute astronomer. "The voice sounded Swedish, and I was pretty sure it wasn't Ikea." Sure enough, Riess and two other astrophysicists had just been awarded the 2011 Nobel Prize for the astonishing discovery a little over a decade ago that the universe is expanding faster and faster as time goes on. The most likely reason: a mysterious cosmic force known as dark energy.

Riess and his collaborator Brian Schmidt, of the Australian National University, had no intention of discovering dark anything when they launched the High-z Supernova Search in the mid-1990s. Neither did Saul Perlmutter, of the Lawrence Berkeley Laboratory, when he and his colleagues started the competing Supernova Cosmology Project. Both teams knew the universe has been expanding ever since the Big Bang. The question they wanted to answer: Is the gravity of 100 billion galaxies, all pulling on one another, slowing the expansion down? And if so, by how much?

To get the answer, the scientists looked to supernovas — exploding stars so bright they can be seen all the way across the universe. The farthest of these cosmic bombs detonated when the universe was still young. It's taken billions of years for their light to reach us, so that light is a snapshot of what the expansion looked like back then. The closer ones are a snapshot of more recent conditions. Once the astronomers found them, they clocked the supernovas' speed — and thus, the expansion speed of the universe at different eras — by measuring subtle features in the wavelength of their light.

What they found, to their astonishment, was that the universe wasn't slowing down at all. It was speeding up. "We spent at least a year struggling to understand what we were seeing," Perlmutter told TIME. In the end, improbable as it seemed, both teams concluded independently that there must be some unseen, unknown force pushing the cosmos apart. Their joint discovery was named Breakthrough of the Year for 1998 in the journal Science.

What's even crazier is that just such a force was predicted, nearly a century ago, by Albert Einstein himself. When he put together the equations of general relativity back in 1916, Einstein applied them to the universe as a whole. To his consternation, they predicted that if the universe wasn't expanding, it should be collapsing. It seemed obvious that it wasn't expanding — but since it wasn't collapsing either, something must be propping it up. Einstein called the something the cosmological constant and added it to his theory with some distaste, because the work had been so mathematically beautiful without it.

Then, a decade or so later, Edwin Hubble discovered that the universe is expanding after all. Einstein abandoned the cosmological constant with relief, declaring that its invention had been "my greatest blunder." He was talking about his failure to trust his elegant equations in the first place — but it's also true that Einstein would probably have bagged himself another Nobel for predicting the expanding universe.

The idea of a cosmological constant didn't go away entirely. "It was something theorists would pull out in desperation," says Riess, "when they couldn't make the age of the universe come out right." As University of Chicago astrophysicist Michael Turner once put it: "The cosmological constant is an idea that's come and gone ... and come ... and gone."

But the accelerating universe brought the idea of some kind of such invisible force back with a vengeance. Even so, the Nobel committee took its time, and despite what Riess's friends say, he thinks the lag was perfectly justified. Even though the odds were against both teams being wrong somehow, it was impossible to rule that double error out either. Maybe supernovas were different in the past, for example, which made it look like the universe was accelerating. It was important for other groups to confirm the acceleration using different techniques. One particularly strong clue that Riess and the others were right: the Hubble Space Telescope showed that the universe actually did slow down early on, then sped up as the dark energy kicked in. With enough confirmations in hand, everyone could finally relax.

Everyone, that is, except the theorists. Just because dark energy is the leading explanation for the accelerating universe, it doesn't mean anyone actually understands what it is. Asked whether he thinks scientists are any closer to figuring that out, Riess answers bluntly, "I do not." But, he adds, "They're getting more creative about it, and maybe that will be the key." If they finally do crack the mystery, another Nobel is pretty much guaranteed.

• Michael D. Lemonick
• What Albert Einstein called his "greatest blunder" turns out to have been an insight