Rumours swirl over credibility of big bang ripple find

bicep2Has the recent discovery of gravitational waves been reduced to dust? Not so fast.

The news that ripples in space-time, called gravitational waves, had been spotted stunned the physics community earlier this year. This week, rumours began swirling that the scientists who reported the find have now admitted to making a mistake. The team missed a key detail in its analysis of galactic dust, the rumours suggest, making it more likely that the signal came from a source other than gravitational waves.

But the team’s response to this claim is unequivocal: “We’ve done no such thing,” says principal investigator John Kovac at Harvard University. The validity of the discovery won’t be known until another group either supports or opposes their finding, which could happen later this year.

In March, the BICEP2 collaboration announced that it had seen an imprint on ancient cosmic light that it says was created by gravitational waves. Those waves are thought to be products of inflation, a period of rapid growth during the first sliver of a second after the big bang. The finding was hailed as a smoking gun for the theory of inflation – and as evidence that theories of a multiverse may be true.

Misread map

Almost as soon as the buzz surrounding the discovery died down, doubts began cropping up. BICEP2’s signal was based on the alignment – or polarisation – of the first light emitted in the universe, a mere 380,000 years after the big bang. But other things could mimic the signal created by this light, such as the ashes of exploding stars or dust in our own galaxy.

The BICEP2 team strongly ruled out some of these alternative explanations, but there are others it can’t rule out yet. The first possible tiebreaker is expected to come from the European Space Agency’s Planck satellite, which is set to release its own polarisation maps of the entire sky in October.

On 12 May, however, a rumour emerged on the physics blog Résonaances that the BICEP2 team has already admitted defeat. The blogger, particle physicist Adam Falkowski at CERN, says he has heard through the scientific grapevine that the BICEP2 collaboration misinterpreted a preliminary Planck map in its analysis. That map, presented at a conference, showed many possible sources of polarised light. The team reportedly used the map assuming that it only charted dust.

“The rumour is that the BICEP team has now admitted to the mistake,” wrote Falkowski.

Kovac says no one has admitted anything. “We tried to do a careful job in the paper of addressing what public information there was, and also being upfront about the uncertainties. We are quite comfortable with the approach we have taken.”

Dusty data trails

Even when Planck does release its complete map, the teams won’t be able to compare data side by side. The instruments on the Planck telescope produce noise that interferes with the signal, so the scope can’t tell exactly how much dust is in any isolated patch of sky, says Shaun Hotchkiss at the University of Helsinki in Finland. The BICEP2 team deliberately chose to look at a region that is relatively free of dust. The region had to be left off a preview map released by Planck last week because the small signal from the dust was difficult to distinguish from instrument noise, meaning Planck could not get an accurate polarisation pattern there.

The complete map from Planck will be able to tell how much dust there is in an average patch of sky, rather than specific areas. “A full-sky map from Planck where the dust polarisation fraction is quite large would be dangerous for BICEP,” says Hotchkiss.

Confirmation could also come from several other observatories able to spot ancient gravitational waves, such as the South Pole Telescope, the POLARBEAR experiment in Chile or the Keck Array – an experiment made up of five BICEP2-style telescopes that is led by the same team.

“That experiment has been observing this patch of sky for the past two years,” says Kovac. “We are working hard at analysing that new data, and we look forward to cross-correlating those data sets with others, including the South Pole Telescope and Planck maps when they become available. That’s where we are putting our focus.”

Lisa Grossman –


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