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What's dark energy? In this illustration, the mysterious repulsive force is represented as a smooth purple grid that overwhelms the effects of gravity (represented by a lumpy green grid).
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.
Caltech's Sean Carroll of Caltech describes dark energy and the accelerating universe.for "Minute Physics."
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:
- What's dark matter made out of? Observations from the past decade suggest that dark energy accounts for 74 percent of the universe's mass-energy content, and that another 22 percent consists of similarly mysterious stuff known as dark matter. So far, dark matter has been detected only through its gravitational effect, but physicists have come to assume that it takes the form of exotic subatomic particles that interact only weakly with the 4 percent of the universe we can see. Researchers had been hoping they'd see the signature of those exotic particles at the Large Hadron Collider, but so far there's been no sign.
- Where's the Higgs boson? Researchers are also looking for the Higgs boson, the last fundamental particle whose existence is predicted by the Standard Model of particle physics. Fermilab's Tevatron collider had been in the hunt until its shutdown last week, and if there's no confirmed detection hiding within the Tevatron data yet to be analyzed, it'll be up to the LHC to spot the Higgs, which is thought to be responsible for creating the mass of some subatomic particles and has been nicknamed the "God Particle." Again, there's been no sign so far, but physicists say they should know within the next year or so whether the Higgs exists. If there's no such thing, theorists might have to rewrite one of the scientific world's most successful theories.
- Why does the universe seem fine-tuned? A good number of physicists have noted that if the fundamental constants of physics had been tweaked slightly differently, life as we know it — perhaps even the universe as we know it — could not have endured for long, if at all. If you ascribe the workings of the cosmos to God, this doesn't present a problem. But this apparent "fine-tuning" poses a challenge if you're trying to explain why the universe is just so. One possibility would be to say there's a plenitude of universes out there, and we just happen to be in a universe that works pretty well. Or maybe the universe is governed by a "feedback loop" that operates forward and backward in time. Or maybe it's some sort of weird quantum phenomenon, as Stephen Hawking has proposed. As Keanu Reeves might say: "Whoa..."
- Why does time run only one way? Speaking of time's direction, Carroll's favorite conundrum has to do with why we experience time in only one direction, moving from the past into the future. In his book "From Eternity to Here," Carroll makes the case that the arrow of time moves in the same direction as entropy, from low entropy at the time of the big bang to higher entropy today, and even higher entropy tomorrow. "The question is, why was entropy low near the big bang?" Carroll said. "I'm still very much up in the air as to the answer to that question." As he studies that question, Carroll is delving into other puzzles ranging from the origin of life to the debate about free will vs. determinism. "You don't have to get into those age-old questions," Carroll admitted. "My own impulse is to enjoy those questions and get into this."
- Was Einstein wrong about the speed of light? This is one of the most recent unsettled questions for modern physics. For more than a century, the overwhelming evidence has been that Einstein's special relativity theory was correct in claiming that nothing could move faster than the speed of light in a vacuum. That's now been called into question by observations suggesting that some neutrinos achieved faster-than-light speeds during a 450-mile trip between two underground labs in Europe. Carroll said the observations are "very, very unlikely to be right," but if they are verified, that would force a radical reinterpretation of Einstein's theories.
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':
- Rand Simberg on the private-enterprise vision for spaceflight
- Martin Hoffert on the future of energy policy
- George Djorgovski on science in virtual worlds
- Alan Stern on suborbital research and NASA's mission to Pluto
- Col. 'Coyote' Smith on the outlook for space solar power
- Tim Pickens on rocket ventures and the Google Lunar X Prize
More about the Nobel-winning research:
- Cosmic Variance: Dark energy FAQ
- Inside Science: The story behind the accelerating universe
- Life's Little Mysteries: What's dark energy? | How find was made
- Search for dark energy on msnbc.com
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.
