In nearly a decade since it was discovered, a mysterious cosmic feature dubbed "dark energy" has lain like a downed redwood across the path of scientists trying to reach the holy grail of physics – a fundamental theory of matter and its basic forces.
Unlike gravity, which binds galaxies together and gathers them into large clusters, dark energy would drive them apart. For several billion years, gravity acted as a brake, slowing a universe ballooning since it formed some 13.7 billion years ago in what cosmologists call the "big bang." These days, however, dark energy appears to be taking over and speeding that expansion.
So far, no one has devised a widely accepted reason why dark energy exists. Nor has anyone figured out why it acts as a repellent. Yet cosmologists now calculate that dark energy is 74 percent of the universe's inventory of matter and energy.
"It's almost unfair that the universe is teasing us in this way. It gives us this dramatic clue, then shuts up," says Sean Carroll, a cosmologist at the California Institute of Technology in Pasadena. "We want to understand this dramatic fact much better. But in order to do that, we need to get more information about it."
Late last week, a National Research Council (NRC) panel became the latest group to answer that call. It recommended that the National Aeronautics and Space Administration and the US Department of Energy underwrite a mission to take dark energy's measure. After examining a range of possible NASA missions grouped under the heading "Beyond Einstein," the panel concluded that efforts to probe dark energy had the right mix of critical science questions, available technologies, and reasonable cost.
If NASA proceeds as the NRC recommends, it would join attempts by other researchers using ground-based telescopes to find the missing pieces that would help solve the dark-energy puzzle.
Dark energy was discovered in 1998 by two groups working independently. They found that the universe was expanding faster than it should be, given the density of matter and energy the universe was estimated to contain.
Researchers spent the first six months or so after the discovery trying to answer the question, "Is this right?" says Adam Riess, an astronomer at Johns Hopkins University in Baltimore and a member of one of two teams. Among other things, their observations led them to estimate that the vast majority of the universe's inventory of matter and energy was this befuddling dark energy.
Since then, Dr. Riess continues, researchers have used various approaches to confirm the discovery and to pin down the point in the universe's history where, over very large distances, gravity began to yield to dark energy.
Four years ago, for instance, scientists using ground-based telescopes and a satellite to study the microwave "hiss" left over from the big bang gave the discovery a major boost. The satellite – the Wilkinson Microwave Anisotropy Probe – measured hot spots and cold spots in this background radiation, which represents the universe when it was 300,000
years old. The hot spots/cold spots correspond to different densities of matter across the sky. That pattern matched with the large-scale structure of the universe astronomers see today. It also confirmed that dark energy accounted for 74 percent of the universe's total "matter-energy density."
Another approach uses light from exploding stars, or supernovae, at different distances as a kind of cosmic speedometer to track the universe's expansion rate at different points in its history. One particular kind of supernova, a type 1A, provides the information. Its pattern of brightening and dimming acts as a fingerprint, allowing scientists to identify it. The pattern also tells astronomers how intrinsically bright it is.
Researchers then compare the supernova's intrinsic brightness with its reduced brightness as viewed from Earth. This yields an estimate of the distance to the supernova. Then they turn to apparent changes in the light's color as a measure of the universe's expansion rate. The redder the light appears, the faster the supernova is speeding away – and the faster the universe is expanding at that distance.
Three years ago, Riess and colleagues used the Hubble Space Telescope to discover 16 supernovae, including six of the most distant ever seen. They added those to 170 previously analyzed supernovae, then looked for trends in deceleration and acceleration. They found that for the first 7.7 billion to 8.7 billion years of its life, the universe was expanding, but at an ever-slowing pace – just as the reigning theory of the universe's birth and evolution predicted.
After that, however, the expansion rate began to quicken. So if the rate of increase in dark energy's influence remains constant, anything beyond about 10 billion light-years away becomes an intergalactic "Shane," riding into the sunset, never to be seen again. It vanishes because space beyond that distance would be expanding faster than the speed of light. If the impact of dark energy rises with time, the ultimate end could come with "the big rip," tearing apart everything from galaxies to atoms.
Back in the real world, though, astronomers are happy with the challenges dark energy poses today.
"After about 10 years it's clear [dark energy] is not going away.... We have to really figure out what this is," Riess says. The past decade also has shown that "dark energy lives at the crossroads of two of our best theories of physics: quantum mechanics and general relativity."
A successful marriage of quantum theory and gravity is the last major hurdle in demonstrating that the basic four forces of nature – gravity, electromagnetism, and weak and strong forces that operate at the subatomic level – are manifestations of a single force that dominated the universe in the first few fractions of a second after the big bang. With dark energy, "nature is giving us a hint of how it does quantum gravity," Riess says.
More observation is needed. The type of project the NRC recommends includes three candidates, all of them variations on a space-based telescope.
For example, a team led by Saul Perlmutter at the Lawrence Berkeley National Laboratory, proposes an orbiting observatory to view 2,000 supernovae a year. The hope is to turn the current jerky flip-book sequence of how dark energy's influence changes with time into a smoother reconstruction to help determine the constancy of dark energy's influence. It would also analyze how the distribution of matter changes with time, using another Einstein phenomenon: gravity's ability to bend light when it passes by massive objects such as stars or galaxies.
"This tells you about the fight between dark energy and gravity" over time, says Dr. Perlmutter, who led the other team credited with discovering dark energy. If the supernova data and the light-bending data tell the same story about the expansion history of the universe, "then what you're seeing is something like a dark energy." If not, "then what you're seeing is some modification of Einstein's theory of gravity."