Dust erases evidence for gravity wave detection

An elusive signal from the dawn of the cosmos is officially still elusive.

Galactic dust accounts for much of the signal that researchers originally interpreted as ripples in spacetime imprinted on the universe’s first light, a new analysis confirms. The study, conducted by the BICEP2 team that claimed the discovery and scientists with the Planck space telescope, nullifies a result that would have provided the first direct evidence of cosmological inflation, a brief moment after the Big Bang when the universe rapidly ballooned in size.

The new analysis, announced by the European Space Agency on January 30, does not mean that the theory of inflation is wrong or that these primordial ripples, called gravitational waves, don’t exist. In fact, it’s possible that the signature of inflation is subtly embedded in BICEP2’s data. But after properly factoring in the pesky influence of dust, Planck and BICEP2 researchers agree that there is not enough evidence to back up the original extraordinary claim.

“It’s perfectly plausible that there are primordial gravitational waves,” says Raphael Flauger, a cosmologist at Carnegie Mellon University in Pittsburgh. “But experiments right now are just not accurate enough to determine this.”

The joint analysis provides the latest and most definitive word on a finding that has drawn increasing skepticism since it was announced last March. At the time, BICEP2 researchers proclaimed that their 26-centimeter telescope in Antarctica had detected swirling patterns in the alignment of light waves, known as the cosmic microwave background or CMB, that were emitted 380,000 years after the Big Bang (SN: 4/5/14, p. 6). Those patterns, the researchers said, were imprinted when the fabric of space rapidly stretched during the era of inflation.

Unfortunately for the BICEP2 team, our Milky Way galaxy plays a cruel trick that stamps light with a similar pattern. Shards of carbon and silicon in the Milky Way emit light imprinted with a swirl that is indistinguishable from the signature of primordial gravitational waves. Despite BICEP2 scientists’ insistence that they had accounted for the contribution of this dust, other researchers published multiple independent analyses questioning the discovery (SN: 6/28/14, p. 20).

Seeking to solidify its result, the BICEP2 team joined scientists from Planck, which is mapping the CMB with unprecedented sensitivity, to take a careful look at the slice of sky measured by BICEP2 and another scope called the Keck Array. While BICEP2 measures light at a single frequency, Planck captures a wider spectrum, allowing scientists to isolate the influence of dust, which preferentially emits light at particular frequencies. The detailed survey revealed that BICEP2 underestimated the effect of galactic dust. Once the contribution from dust is removed, the remaining signal is too small to be considered a discovery.

Joanna Dunkley, an astrophysicist at the University of Oxford and coauthor of the new analysis, says the BICEP2 researchers didn’t initially appreciate the uncertainty of dust’s contribution to their measurement. The ballpark figures they used, she adds, “were taken a little too much as gospel truth.”

Despite the dusty disappointment, the new study leaves plenty of wiggle room for future experiments to uncover evidence of inflation. BICEP2’s original measurement of a variable called r, which compares gravitational wave and matter density deviations in the CMB, was considerably higher than most theories predict. The new combined Planck and BICEP2 result pushes down r to a maximum value that is more in line with the simplest inflationary theories, Flauger says. That means any primordial gravitational waves will be harder to detect, but it also gives hope that the signal will eventually shine through as an army of telescopes, including the recently upgraded BICEP3 and Keck, dissect the CMB. Other experiments, including the South Pole Telescope, ACTPol and SPIDER, are also scanning the skies. “We’re still excited about looking for this,” Dunkley says.