As predictably as the heroine's death in an opera, the biggest claim in cosmology in years has finally officially unraveled. Last March, cosmologists working with a specialized telescope at the South Pole called BICEP2 claimed direct evidence that in the first fraction of a second after the big bang, the universe underwent a bizarre exponential growth spurt called inflation. The signs came in their study of the big bang’s afterglow, the cosmic microwave background (CMB). But now, in a joint analysis with cosmologists working with the European Space Agency's (ESA's) Planck spacecraft, BICEP researchers take back that claim and report no such signs of inflation, according to a press release issued by ESA.
Like Mimi in Giacomo Puccini's opera La Bohème, the BICEP claim seemed doomed from early in the drama. "I would have been surprised if it had turned out otherwise," says Suzanne Staggs, an observational cosmologist at Princeton University. In September, the Planck team released data that suggested the BICEP signal was largely, if not entirely, an artifact of dust in our galaxy, which emits microwaves of its own. The joint analysis sought to resolve the conflicting results. And it rules out the BICEP team's blockbuster claim.
What the BICEP researchers saw were swirls in a patch of the southern sky. The CMB—a kind of electromagnetic backdrop for everything else in the visible universe—is polarized, like light reflected off a lake. According to cosmologists' standard model, the enormous stretching of inflation would have set off ripples in space and time called gravitational waves, which would imprint telltale pinwheel-like patterns—called B modes—in the polarization. And to much fanfare, the BICEP team claimed to see such "primordial B modes."
However, swirls can spiral up from other sources. In particular, radiating dust in our galaxy can produce them, and to see the CMB signal properly, researchers must first strip away this "foreground" contribution. Ordinarily, experimenters do that by taking data at multiple microwave frequencies. However, BICEP2 took data at only one frequency to try to maximize sensitivity and relied on preliminary data from Planck—taken from a slide presented in a talk—to estimate that foreground contamination. The BICEP team believed it was small. But in May, other cosmologists suggested that BICEP researchers may have misinterpreted the Planck data and underestimated the dust contribution. And in September, Planck's final data suggested that BICEP’s patch of sky was as dusty as an old pillow.
Now the joint analysis, which also includes data from BICEP2's successor at the South Pole, the Keck Array, yields no definite sign of primordial B modes. If they exist in the data, they can be no more than half the signal BICEP claimed. That limit is in line with what Planck researchers had earlier deduced indirectly by studying tiny variations in the temperature of the CMB across the sky.
In spite of the sad end to this particular tale, Max Tegmark, a cosmologist at the Massachusetts Institute of Technology in Cambridge, says he’s optimistic about the chances of spotting primordial B modes relatively soon. "I'm in the minority," he says, "and I think there is something in there that we can see—if not in this data, then in the next couple of years." However, Princeton's Staggs says the incident underscores that the foreground emissions are likely to be sizable and tricky. If it's there, the big bang signal is more likely to emerge slowly through several experiments than suddenly in one definitive discovery, she says.
"These signals are very faint, and there are lots of things that can go wrong, not just foregrounds," says Charles Bennett, a cosmologist at Johns Hopkins University in Baltimore, Maryland. Nevertheless, multiple teams are striving to detect B modes. "I don't think the BICEP2 experience has slowed anybody down," Bennett says.