Science

Gravitational wave evidence disappears into dust

There may still be evidence of cosmic inflation, but it will be hard to spot.

John Timmer – Sep 22, 2014 5:40 pm | 71

The Planck cosmic microwave background observatory and its map of temperature fluctuations on the sky. Credit: ESA

Earlier this year, researchers who used a telescope based at the South Pole called BICEP announced that they obtained evidence for gravity waves caused by the Big Bang itself. The results would provide direct evidence that a model of the Universe’s origin called inflation had left its mark on the present-day Universe.

But in reporting on the results, our own Matthew Francis suggested that the discovery was not as definitive as it might be, writing “the story of BICEP2, inflation, and primordial gravitational radiation is just beginning.” And since then, it became clear that there was a complicating factor—dusty material in our own galaxy—and that the BICEP team’s way of controlling for it left a little something to be desired (it involved using processed data obtained from a PDF used in a conference presentation).

Yesterday, the team that put the PDF together in the first place released its own analysis. And they’ve determined that BICEP was probably staring at dust, rather than the earliest moments of the Universe.

Signs of the big blow up

All of the results hinge on inflation, a theory that nicely explains how the Universe can be both incredibly even—matter is rather spread out through its expanse—yet sufficiently lumpy that we have large-scale structures like galaxy clusters. Inflation suggests that, very early in its history, the Universe expanded at a furious pace. This expansion took small quantum fluctuations and blew them up into the large scale structures we see today.

By looking at the Cosmic Microwave Background, which provides a window into the Big Bang, we’ve found evidence that’s all consistent with inflation. But we haven’t found any evidence of a signal that’s been predicted specifically to be produced during inflation. BICEP was built to search for one.

The prediction is based on the fact that inflation should produce gravity waves that would ripple through the fabric of the Universe. These gravity waves would leave their mark on the photons of the cosmic microwave background, imparting a subtle bias to their polarization. Unfortunately, other things also affect the polarization of the cosmic microwave background photons, so BICEP was built to stare at what was thought to be a relatively interference-free part of the sky for long enough that a signal emerged.

And according to the paper released early this year, it had.

Dust in the (interstellar) wind

But even though the patch of sky imaged by BICEP was relatively free of interference, there’s a known source of microwave photons that’s pretty much everywhere: interstellar dust. It’s more concentrated in the plane of our galaxy, but it still surrounds the Earth. Because of the way that asymmetrical dust particles interact with the Milky Way’s magnetic fields, the photons can end up polarized as well. So a key part of the BICEP paper was estimating the amount of dust present in the area that was being observed.

To estimate the amount of dust in their field of view, the BICEP team obtained data from the European Space Agency’s Planck mission, which had also imaged the cosmic microwave background. Or, rather, they obtained data from a PowerPoint slide made by members of the Planck team that was saved as PDF after having been presented at a conference. Clearly, having access to the raw underlying data would have been a better way of going about things, but the BICEP and Planck teams were in competition, so that didn’t happen.

In the manuscript that was released over the weekend, the Planck team performed its own analysis of the dust present in every direction—including the one that BICEP was staring at. To do so, they also had to get rid of confounding signals. The paper handles them one after another: point sources of microwaves, emissions from carbon monoxide, and so on. While carbon monoxide emits at the same wavelengths as dust, it also emits elsewhere in the spectrum. So the Planck team built a whole-sky map of carbon monoxide signals and subtracted that from data from the observatory.

With this and other issues accounted for, there’s one obvious conclusion: dust has gotten everywhere. “We show that even in the faintest dust-emitting regions there are no ‘clean’ windows in the sky,” the authors write. This includes the region observed by BICEP. In fact, one of the potentially important results of this work is that the authors identify several areas of the sky where dust emissions would be half those of the region imaged by BICEP.

More importantly for inflation, once the dust was mapped, the authors went back and looked at whether the polarization present could stand out above the signal from the dust in the Planck data. Within experimental error, the two were indistinguishable: “This value is comparable in magnitude to the BICEP2 measurements”

But the authors didn’t dismiss the BICEP results completely. Instead, they suggest it’s time the teams shifted from competition to cooperation and figure out a way to merge the BICEP and Planck data to perform a single analysis. For now, the story will continue.

The arXiv. Abstract number: 1409.5738 (About the arXiv).

Listing image: ESA

John Timmer Senior Science Editor

John is Ars Technica's science editor. He has a Bachelor of Arts in Biochemistry from Columbia University, and a Ph.D. in Molecular and Cell Biology from the University of California, Berkeley. When physically separated from his keyboard, he tends to seek out a bicycle, or a scenic location for communing with his hiking boots.

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