Cosmic swirls that were hailed earlier this year as evidence for primordial gravitational waves – ripples in spacetime dating back to the early universe – may turn out to have been caused by dust. But several top physicists are standing by the decision to announce the result back in March, before it had been peer-reviewed.
“I think it’s important to give it at the same time to the scientific community as to the general public,” said Rolf Dieter-Heuer, the director general of Europe’s particle physics Laboratory, CERN. “We did the same two years ago when we announced the discovery of the Higgs boson.” Not a member of the BICEP2 team that made the original claim, Dieter-Heuer was speaking to journalists at the International Conference on High Energy Physics (ICHEP) in Valencia, Spain.
When the BICEP2 team first reported that their telescope in the South Pole had detected twists in the polarization of relic light from the Big Bang, known as the cosmic microwave background, the results seemed to confirm that the baby Universe underwent a period of rapid expansion, a theory known as inflation. But since then, the finding has been scrutinised and challenged by physicists who raised the possibility that grains of dust in the Milky Way – rather than gravitational waves – created the swirling polarization pattern. That culminated in the team last month scaling back their claims when publishing in the peer-reviewed journal Physical Review Letters1
It is something that the European Space Agency’s Planck telescope has the power to answer, said Enrique Martinez, a physicist at the Institute of Physics of Cantabria and a member of the Planck collaboration. He too was speaking at ICHEP, in a huge auditorium packed with delegates who had gathered in the hopes of some resolution on the BICEP2 result. But his talk failed to satisfy the expectant crowd.
Instead he promised that Planck would release results for the part of the sky relevant to the findings within a month, followed by its full dataset in October. He also confirmed that the Planck and BICEP2 teams were in the final stages of forming an agreement to collaborate on a joint, but separate, analysis.
Was BICEP2’s March announcement premature? Speaking alongside Dieter-Heuer, Alan Guth, the cosmologist who first proposed the inflation concept in 1980, said the team’s decision to speak to the press, which they did at the same time as posting a paper to the pre-print server ArXiv, was a natural one to take. “The press wants to know, the referring process is slow, and meanwhile the scientific community would probably find out anyway,” he said.
However, there should have been more caveats and cautionary remarks, he said: “It was almost presented if there was no way the experiment could not be interpreted as they interpreted it to be. When others came to look at it, that seemed no longer to be the case.”
In a talk in which he represented the BICEP2 collaboration, Roger O’Brient, a physicist at Caltech’s Jet Propulsion Laboratory, stressed the effort his team had taken to distinguish the signal from the dust or other causes of polarization. “We actually thought through this for about a year and a half before we published,” he said.
Afterwards, O’Brient told Nature that releasing the paper publicly had actually been good for science, as it had spurred others, sometimes from surprising areas of the scientific community, to examine the results. “It’s not clear to me that the peer review process on its own in a vacuum would have necessarily caught issues.”
Guth is not giving up on BICEP2. During another talk at the conference, he said there was “reasonable hope” the signal would turn out to be the long sought after gravitational waves. However he also stressed that if the signal turns to dust, it would not cause any problems for his theory – other than from a public relations perspective. “If it turns out to be all dust, that’s not a mark against inflation,” he said.