If the universe were ice cream, it would be vanilla. That's the take-home message from researchers working with the European Space Agency's orbiting Planck observatory, who today released the most precise measurements yet of the afterglow of the big bang—the so-called cosmic microwave background (CMB) radiation. The new data from Planck confirm cosmologists' standard model of how the universe sprang into existence and what it's made of. That may disappoint scientists who were hoping for new puzzles that would lead to a deeper understanding.
"We're always hoping to find new things," says Glenn Starkman, a theoretical physicist at Case Western Reserve University in Cleveland, Ohio, who does not work on Planck. "But we're finding that our model is really, really good—maybe disappointingly good." Max Tegmark, a cosmologist at the Massachusetts Institute of Technology (MIT) in Cambridge, agrees. "The sensational news is that there is no sensational news," he says.
Cosmology's standard model goes a bit like this. The universe sprang into existence instantaneously in the big bang as a hot, dense soup of matter and energy. Then, in the first 10-30 seconds, space itself expanded much faster than light speed. That growth spurt, known as inflation, had two main effects. First, it smoothed the universe out and rendered it geometrically "flat" on the largest scales. At the same time, it greatly magnified tiny quantum fluctuations in the density of hot matter and energy. These density fluctuations then left tiny variations in the temperature of the CMB across the sky and, much later, seeded the formation of the galaxies. Like other CMB missions before it, Planck, which was launched in 2009, studied these variations.
From studies of the CMB and other measurements, cosmologists have also deduced the composition of the universe. Planck refines those measurements, particularly those of NASA's Wilkinson Microwave Anisotropy Probe (WMAP), which collected data from 2001 to 2010. According to Planck, the universe consists of 4.9% ordinary matter, 26.8% mysterious dark matter that has revealed itself through only its gravity, and 68.3% weird, space-stretching dark energy. Those numbers amount to roughly 3% more dark energy and 3% less dark matter than WMAP's result. The Planck team also pegs the age of the universe at 13.8 billion years, 100 million years older than WMAP found.
All in all, the results fit the expectation of the standard model of cosmology almost perfectly, reported George Efstathiou, a cosmologist at the University of Cambridge in the United Kingdom and a Planck team member, at a press briefing in Paris. "If I were an inflationary cosmologist, I would be very happy and thinking about a Nobel," Efstathiou said. Inflation was invented by Alan Guth, a physicist and cosmologist who is now at MIT.
More important, however, is what Efstathiou didn't say. Scientists had a short list of things that they had hoped Planck would find. For example, its measurements could have shown that a weird type of particle called a sterile neutrino existed, that the variations in the CMB weren't random in a particular way, that the original distribution of density fluctuations didn't jibe with the simplest model of inflation, or that space isn't really flat. But Efstathiou mentioned no such evidence. "These changes are not favored by the Planck data," he said. "The data don't want any of these things."
Still, there is some hope for new puzzles, Efstathiou said. Researchers break the mottled CMB down into superimposed patterns of larger and smaller spots, much as a musical chord can be broken down into individual notes. And at larger, angular scales there appear to be some anomalies, Efstathiou says. For example, the northern half of the sky appears to have slightly stronger large-scale variations than the southern half.
Such anomalies had been seen before in the WMAP data. The fact that they exist in the Planck data too means they must be real, Starkman says. "If you had made a mistake with WMAP, you wouldn't expect Planck to see it," he says. Tegmark agrees and says that the large-scale anomalies could provide clues to how inflation began. "Maybe the universe is trying to tell us something," he says. "I think we should start to take this more seriously now."
It's not clear that the anomalies mean anything, however. The CMB is the product of a random process, so the anomalies could simply be statistical flukes. Alas, with the CMB and the big bang, researchers can't redo the experiment to see if the effect goes away.