The biggest news in physics this year—actually, this decade—was the discovery in March of what looked like gravitational waves from shortly after the big bang. Soon after the announcement, critics argued that the signal might instead be caused by contaminating dust in our galaxy, and physicists have been awaiting further data to help settle the debate. Now, newly released dust measurements from the Planck satellite appear discouraging for the gravitational-wave believers.
The original finding, from the BICEP2 experiment at the South Pole, measured what’s called B-mode polarization—a curling pattern in the orientation of light waves—in the cosmic microwave background (CMB) radiation. This radiation pervades all of space and dates from just 380,000 years after the birth of the universe. B-mode polarization could be caused by gravitational waves, or ripples in spacetime, just after the big bang, which would have stretched and compressed space, altering the light waves of the CMB. Such polarization could also be caused, however, by light scattering off dust in our own galaxy—a much more mundane explanation.
BICEP2 examined a small region of the sky using highly sensitive instruments. Its measured B-mode signal was so large, the researchers originally concluded that dust could not account for it. It turns out, however, that they may have underestimated how much dust is present in the area of the sky they observed.
The Plank satellite, which gathered data from 2009 to 2013, observed the CMB over the entire sky, albeit with less sensitivity. A new study from the Planck team analyzes how much polarized light from dust is present in all directions, and finds that it is much more prevalent than the BICEP2 team had assumed. “We show that even in the faintest dust-emitting regions there are no ‘clean’ windows where primordial CMB B-mode polarization could be measured without subtraction of dust emission,” the Planck scientists wrote in a paper posted to the preprint site arXiv. In fact, Planck’s data show that the effect from dust could completely account for the polarization BICEP2 measured, without any of the signal being caused by gravitational waves. Since making the original announcement, the BICEP2 team has “taken a step back regarding the cosmological interpretations,” says Stanford University physicist Chao-Lin Kuo, a co-leader of the experiment. “On the other hand, this paper does not rule out a substantial contribution from gravitational waves.”
Nonetheless, the news is disheartening to many physicists. Such waves are predicted by the theory of inflation, which posits that the universe expanded explosively during its early years. A measurement of primordial gravitational waves would offer smoking-gun evidence for inflation. Furthermore, as cosmologist Lawrence Krauss of Arizona State University in Tempe wrote in the October Scientific American, it “would allow us to test ideas about how the universe came to be that hitherto scientists have only been able to speculate about.”
The authors of the latest Planck paper stress that their analysis does not rule out gravitational waves. Further study is necessary to know if dust is behind BICEP2’s signal, they say, and the team plans to collaborate with the BICEP2 scientists to learn more. “It’s not completely definitive—but it’s pretty powerful,” physicist Sean Carroll of Caltech wrote on his blog. “BICEP2 did indeed observe the signal that they said they observed; but the smart money right now is betting that the signal didn’t come from the early universe.” To know for sure, more experimental data will probably be necessary. “In the long run we need to wait for BICEP2 level of sensitivity at multiple frequencies, at which point it will be easy to tell dust from primordial stuff,” physicist Shaun Hotchkiss of the University of Sussex wrote in a blog post.
Many fundamental questions about the birth of the universe will hinge on the outcome of this issue. Debates about how best to announce major scientific findings will likewise be affected. The BICEP2 scientists shared their discovery via an ecstatic press conference without first putting their analysis through rigorous peer review. That decision may have backfired, suggests University of Sussex physicist Peter Coles. “I think the public needs to understand more about how science functions as a process, often very messily,” he wrote on his blog, “but how much of this mess should be out in the open?”