Oct. 4, 2011 at 8:01 PM ET
Most of the research recognized by a Nobel Prize has to do with solutions, but this year's physics prize highlights a problem that's been bugging scientists for more than a decade. And there may be more such problems to chew on in the years ahead.
"The way science makes progress is through an interplay between theory and observation," Sean M. Carroll, a theoretical physicist at the California Institute of Technology, told me today. But when it comes down to theory vs. observation, "observations always win," he said.
As an example, take the research that won today's Nobel Prize for physics: When the three physicists who won the award started charting the brightness of distant supernovae, they expected to find out how much the expansion of the universe was slowing down, in accord with the accepted theories for cosmic evolution. Instead, they were surprised to find that the expansion rate was speeding up.
"We thought this would be an interesting experiment to do, but we didn't know it would be this interesting," one of today's Nobel laureates, Johns Hopkins University astrophysicist Adam Riess, told journalists during a teleconference.
Physicists didn't have a good explanation in 1998 for why the cosmos should go against gravity's pull and fly apart at a faster and faster rate. And they still don't. Their best guess is that our universe has a built-in, outward-pushing feature known as dark energy, which appears in Albert Einstein's equations for relativity as a cosmological constant.
"Dark energy still looks like the right answer — the best guess, I should say," Riess said. Einstein's cosmological constant appears to account for the effect to within 10 percent accuracy, he said. But physicists are in the dark about the mechanism. It's as if you're watching a car speeding down the road, faster and faster. Riess said you might hypothesize that there's such a thing as a gas pedal, and that pressing on it was causing the speedup. But there's not yet any way to say for sure. And there's no guarantee that the speedup will continue. There might still be a let-up on the cosmic accelerator, "in which case all bets are off," Riess said.
So is this Nobel premature? Riess said it was important to note that the prize was "awarded for seeing or discovering that the universe is accelerating," rather than for explaining why.
How to crack the mystery
There are lots of experiments in the works to expand upon the discovery made by Riess and his fellow Nobel laureates, Saul Perlmutter of the University of California at Berkeley and Brian Schmidt of the Australian National University in Canberra. Just today, the European Space Agency gave its go-ahead for the 2019 launch of the $650 million Euclid space telescope, which is designed to study dark energy's effects on the large-scale structure of the universe. NASA's $1.6 billion Wide-Field Infrared Survey Telescope, or WFIRST, would also target the mystery surrounding dark energy.
But Riess suspects that the mystery can't be solved by observations alone. "We won't really resolve it until some brilliant person, the next Einstein-like person, is able to get the idea of what's going on," he said.
So he issued a plea to the theorists: "Keep working," he said. "We need your help. ... It's a very juicy problem, it's hard, and you'll win a Nobel Prize if you figure it out. In fact, I'll give you mine."
Carroll, the theorist, was sympathetic to Riess' plea. But he wasn't overly encouraging.
"You don't need to tell us that this is a big one," Carroll said. "Many of us have tried. I've tried. I've written many papers about it. But it's hard."
There are plenty of possibilities, to be sure. The acceleration could be caused by vacuum energy that doesn't vary over time, but is just a feature of empty space. It could be a slowly varying quality of the cosmos known among physicists as "quintessence." It could be some unanticipated twist in the nature of gravity, or a byproduct of multidimensional spheres of existence.
"I've spent my time on this, and I'm increasingly willing to predict that the answer is a boring one," Carroll said. Maybe the best that scientists can ever say is that this is just the way our universe works.
More deep, dark questions
For now, dark energy is just one item on a growing list of puzzling questions for big-thinking physicists — questions that also include:
Faster-than-light neutrinos would be far more troublesome for scientists than the speeding-up universe. As strange as the Nobel-winning supernova observations appear to be, Carroll said they actually "explain a whole bunch of things that people had been worrying about for a long while," including apparent discrepancies in measurements of the universe's age.
"Unlike the 'accelerating universe,' ... the faster-than-light neutrinos would create a whole bunch of problems to worry about," Carroll said. For example, would the phenomenon allow for backward time travel and reverse causality? Could a neutrino go back in time and "kill its grandfather"?
In a posting to the Cosmic Variance blog, Carroll floats some ideas that could get theorists out of a time-traveling jam, but it wouldn't be pretty. "If neutrinos are moving faster than light, the question is, how can we adapt special relativity to a framework which allows for this?" he said.
Riess, the experimenter, offered some advice for Carroll and his fellow theorists, based on his experience with the surprising supernova observations.
"As a lot of my colleagues say when they hear about a strange result, they go, 'Oh, that's wrong,' and usually 'How do you know?' then, 'Well, most things that are weird turn out to be wrong.' And that's true," Riess said. "But you don't want to completely close your ears and eyes to seeing weird things, because a lot of the most interesting things we see at some point were the weird things."
Tune in to 'Virtually Speaking Science'
Carroll and I will be talking about the accelerating universe, faster-than-light neutrinos and other weird and interesting things on Wednesday at 9 p.m. ET (6 p.m. PT) on "Virtually Speaking Science," an online talk show that I host on the first Wednesday of the month. You can listen to the hourlong show via BlogTalkRadio, or be a part of the audience at the Stella Nova auditorium in the virtual world known as Second Life. (Here's the SLURL for your teleporting pleasure.) You can ask questions during the show via Second Life chat or BlogTalkRadio's call-in number.
If you can't make it in real time, don't worry: The show will be archived at BlogTalkRadio as an audio podcast for on-demand listening. Many thanks to the Meta Institute for Computational Astrophysics for providing the Second Life venue.
More podcasts from 'Virtually Speaking Science':
More about the Nobel-winning research:
Carroll will also be a featured speaker for the New Horizons in Science symposium, presented Oct. 16-18 at Northern Arizona University in Flagstaff by the Council for the Advancement of Science Writing as part of ScienceWriters2011. I'm a member of the CASW board.
Connect with the Cosmic Log community by "liking" the log's Facebook page or following @b0yle on Twitter. You can also add me to your Google+ circle, and check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.