Scrutiny of BICEP2’s evidence of inflation suggests signals could come from Milky Way dust
J.-P. Barnard/Planck Collaboration, R. Flauger
In March, cosmologists announced with much fanfare the first direct detection that our universe, in its earliest moments, underwent an unimaginably rapid expansion known as inflation (SN: 4/5/14, p. 6). The researchers had seen twirling patterns in the alignment, or polarization, of the first light released into space just 380,000 years after the Big Bang, what’s known as the cosmic microwave background. Those twirls, the researchers said, could come only from gravitational waves — ripples in the fabric of space — that spread out in the wake of inflation. If the results held up, this was the type of discovery that wins people a trip to Stockholm.
The key phrase: “if the results held up.” After the initial celebrations, other cosmologists quickly got to work scrutinizing the team’s dramatic conclusions. Concerns have come and gone, but in recent weeks debate has grown about something seemingly mundane: dust. Did the researchers not properly account for the dust in our own galaxy?
Interstellar dust can be a nuisance when trying to study the universe. Olivier Doré, an astrophysicist at Caltech, likens the dust to fog, which can confuse a viewer’s interpretation of distant objects beyond it.
Dust not only blocks the view of stars and galaxies but it also polarizes light. Any light that reflects off dust grains gets realigned in the direction that the dust grains are pointing. The trouble is that dust and gravitational waves can polarize light in similar ways. And that raises concerns for some cosmologists.
“The measurement is exquisite, it’s really an amazing technical achievement,” says Lloyd Knox, a cosmologist at University of California, Davis, of the polarization data. But, he adds, “their case that it’s not something in our own galaxy is not very strong.”
BICEP2, the Antarctic-based telescope used in the March discovery, detects how strongly microwave light is polarized, but it can’t tell the distance of the light source. Since astronomers don’t know much about how strongly galactic dust polarizes light, researchers involved in the Background Imaging of Cosmic Extragalactic Polarization, or BICEP, experiment relied on whatever information they could get their hands on. That included slides from a talk given last year by a researcher from another experiment called the Planck collaboration. The Planck satellite mapped the entire sky in microwave light and allowed cosmologists to make the most precise determinations yet of the age and composition of the universe. It also recorded polarization data at many more frequencies of light than BICEP2 did. The complete results have yet to be released, but one of the Planck presentation slides showed a preliminary polarization map of the sky. BICEP researchers extracted numbers from the slide as one way of estimating dust contamination in their own data.
But Raphael Flauger, a physicist at the Princeton Institute for Advanced Study, last week suggested in a presentation at Princeton that the BICEP team misinterpreted the slide. Flauger’s analysis — which relies on scraping data from other Planck slides — found that BICEP researchers may have underestimated how strongly dust polarized the light.
Unfortunately, it’s hard to know just how much the dust affects the results. “At that lower end of my estimation, it looks like everything may be okay,” Flauger says. “At the upper end, the dust could make up the whole signal.”
Princeton physicist Lyman Page attended Flauger’s presentation and offered commentary at the end. He pointed out that the best evidence on both sides of the debate comes from digitizing maps from PowerPoint slides, none of which were intended to be used in this way. “This is a really peculiar situation,” he said. “This is not sound methodology.”
Jamie Bock, a Caltech astrophysicist on the BICEP team, points out that BICEP’s analysis does not depend solely on the preliminary Planck data. The researchers used six theoretical models of how the dust affects polarization, mostly drawn from older data from other satellites.
The team also has data from the first-generation telescope, BICEP1, Bock says. It observed the same patch of sky as BICEP2 did, but at a slightly lower frequency. The BICEP1 data support the assertion that dust contamination in this part of the sky is very low. “We stand by our data,” he says.
But the real test will come when Planck publicly releases all of its polarization data, says Bock, who helped design the detectors for both Planck and BICEP2. Planck measured the polarization of the entire sky at seven light frequencies, whereas BICEP2 observed only at one. Having data at multiple frequencies can help astronomers distinguish what is dust and what is not. Those data are still being analyzed and should be made public later this year.
The Planck team released its first map of polarization data, which used a single frequency that is very sensitive to dust, this month (SN Online: 5/9/14). The map can’t directly help with the dust controversy — the area of the sky mapped by BICEP2 is masked in the early map because the emission is very weak and researchers are still analyzing it. But Flauger argues that the map indirectly supports his assertion that the amount of polarization from dust is higher than BICEP assumed because the map shows stronger polarization, in general, across the entire sky than BICEP researchers used in their calculations.
“Patience is the key word here,” Doré says. “It’s very important and everyone wants to get it right. There’s no shortcut; it just takes time.”
Even Flauger admires BICEP2’s measurement, calling it “really amazing.” But, he adds, “Because it’s such an important result, you want to make sure it’s true.”
This is not so much a squabble, but the discovery process in action. “Everyone understands this is how science is supposed to work,” he adds. For cosmologists, finding evidence of inflation may be the biggest discovery in the last 20 years. “People understand that it will be checked. I think [the BICEP researchers] want you to check it. Everyone wants to be sure, including them, of course.”
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BICEP2 Collaboration. BICEP2 I: Detection of B-mode polarization at degree angular scales. arXiv:1403.3985. Posted March 19, 2014.