Headline-grabbing observations of primordial gravity waves now face cloudy prospects, suggests a newly released satellite map of cosmic dust.
In March, the South Pole-based BICEP2 physics team reported seeing the surprisingly strong signature of gravitational waves in maps of the cosmic microwave background (CMB). Filling the sky, the CMB is thought to be residual heat left over from the aftermath of the big bang. (See “Big Bang’s ‘Smoking Gun’ Confirms Early Universe’s Exponential Growth.”)
The BICEP2 team reported a strong signal of gravitational waves—curled ripples in the fabric of space and time—seen in temperature fluctuations across a small section of the CMB. Detection of these ripples seemed to support a simple kind of cosmological “inflation” that rapidly expanded the universe in the first instant of the big bang, some 13.8 billion years ago.
But the European Space Agency’s Planck satellite mission, working in cooperation with the BICEP2 team, has now released a sky map of galactic dust that might have clouded the gravitational wave result. The dust map suggests that gravitational wave signal reported earlier this year might instead have been an illusion, triggered by hot galactic dust emitting microwaves and fooling the BICEP2 team with their signal.
“The evidence for these gravitational waves is shrinking, going away, vanishing,” says physicist Neil Turok of Canada’s Perimeter Institute for Theoretical Physics in Waterloo, Ontario, a proponent of alternatives to cosmic inflation theory who had criticized the BICEP2 result earlier. “The BICEP2 team is hoping for some evidence for gravitational waves to remain. In my view that is wishful thinking,” he says.
Initially, the BICEP2 team had argued that the portion of the southern sky they had observed from Antarctica was far from the center of the galaxy and was likely a low-dust environment. But the Planck map shows galactic dust everywhere: “We show that even in the faintest dust-emitting regions there are no ‘clean windows’ in the sky,” the researchers write in the study, slated for publication in the journal Astronomy & Astrophysics.
“Our galaxy is a rather grubby place to live. Even the windows are filthy,” says physicist Peter Coles of the United Kingdom’s University of Sussex on his In the Dark blog. “Although it is consistent with being entirely dust, the Planck analysis does not entirely kill off the idea that there might be a primordial contribution to the BICEP2 measurement, which could be of similar amplitude to the dust signal,” he adds.
The finding comes after sometimes harsh criticism of the BICEP2 results this summer, which came after the team published their results in Physical Review Letters and concluded with a note describing increased uncertainty over cosmic dust. (See “Grand Cosmological Claim Crumbles?”)
Essentially, the BICEP2 team had reported that roughly 20 percent of the fluctuations seen in the CMB derived from gravitational waves. But the Planck results suggest that it is less than 10 percent. Turok has bet physicist Stephen Hawking $200 that it is less than 5 percent. “I am halfway to winning,” he now says.
In emailed comments sent to reporters from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, BICEP2 team leader John Kovac of Harvard said that the team is working with the Planck satellite team to produce better maps of cosmic dust. That effort should help determine the true strength of the gravitational wave signals. “We are focused on these new, more powerful analyses and data sets, which should give a much more constraining answer within the next several months,” Kovac says.
About ten experiments, aside from BICEP2, aim to settle whether gravitational waves are responsible for the fluctuations in the CMB, Turok notes. Those analyses should determine whether the waves cause as little as one percent of the fluctuations, and should answer the question over the next three to five years.
“We will have our answer, and I think I’m going to win my bet,” Turok adds.
Many physicists may agree. In an online straw poll conducted by Coles this summer, about 80 percent of the respondents thought the BICEP2 measurement was just dust.
“Whatever that answer is, we will do our best to report the evidence in an unbiased way and put it out in front of the whole scientific community, as we did last March,” Kovac added in a note sent to National Geographic. “That is the best way to advance the science.”