BICEP2: dust in its eyes? (Image: Steffen Richter/Bicep2)

It was hailed as the discovery of the century. But now the researchers who earlier this year reported the first detection of primordial gravitational waves – ripples in space time hailing from the early universe – say they are not so sure after all.

“Has my confidence gone down? Yes,” says Clement Pryke of the University of Minnesota, co-leader of the team that reported the original result.

In March, the team, which uses a telescope called BICEP2 based at the South Pole, announced their discovery at the Harvard Center for Astrophysics in Cambridge, Massachusetts. They also posted their results online.


Today, the first peer-reviewed version of their results appears in the journal Physical Review Letters – and it backtracks on the certainty of the original announcement.

Star dust

Back in March, the BICEP2 team reported a twisted pattern in the sky, which they attributed to primordial gravitational waves, wrinkles in the fabric of the universe that could have been produced when the baby universe went through an enormous growth spurt. If correct, this would confirm the theory of inflation, which says that the universe expanded exponentially in the first slivers of a second after the big bang – many believe that it continues to expand into an ever-growing multiverse.

Doubts about the announcement soon emerged. The BICEP2 team identified the waves based on how they twisted, or polarised, the photons in the cosmic microwave background (CMB), the earliest light emitted in the universe around 380,000 years after the big bang. Other objects, such as the ashes of exploding stars or dust within our galaxy, can polarise light as well.

The paper published today is significant because it is the first time the researchers themselves have dialled back on their original claims.

In the initial version of their paper, the BICEP2 researchers used a snapshot of a map from the Planck satellite, which gathers data on the CMB, that was presented at a conference in 2013. This helped them to calibrate parts of their analysis of a potential false signal from dust but led to some worry that the BICEP2 team had underestimated the amount of polarisation caused by dust in the Planck map.

“We don’t have a good handle on what the size of that dust signal is,” says BICEP2 member Colin Bischoff of Harvard University. “We still maintain that our data favour a cosmological origin of the signal over a dust origin, but it’s not as strong.”

Contamination high

The peer reviewed version of the paper omits one of the calibration models based on Planck data. In a footnote, the team says: “We have concluded the information used for the DDM2 model has unquantifiable uncertainty. We look forward to performing a cross-correlation analysis against the Planck 353 gigahertz polarised maps in a future publication.”

Another note on the paper, added after submission, refers to more recent Planck data that also points to a possible false signal. “While these papers do not offer definitive information on the level of dust contamination in our field, they do suggest that it may well be higher than any of the models considered,” write the team.

“It seems that real data from Planck is indicating that these dust models are under-estimates,” says Pryke. “The prior knowledge of dust at these latitudes in our field of view has gone up and so the confidence in the gravitational wave component has gone down.”

Ultimately, science moves forward not by peer review, but by replication. At least eight experiments, including BICEP3 and the Keck Array, which are both in Antarctica, POLARBEAR in Chile, and data from the Planck satellite due to be released in October, are already working toward that goal.

“When Planck does have a full data release of their polarised data, that will be really interesting,” says Bischoff. “Doing a full analysis of BICEP2 and Planck will be really powerful to help identify, really nail down what the signal is.”

Pryke is philosophical about the final outcome: “Data trumps models and that’s the way it is.”