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The Rise and Fall of a Would-Be Gravitational Wave Observation

The deep sky is a more complex place than BICEP cosmologists had assumed.
​The Planck spacecraft's view of the BICEP field. Image: ESA

​The cosmologists behind a sketchy but widely publicized 2014 claim touting an indirect gravitational wave observation have officially disowned their results, surprising no one at all. After months spent working with researchers from the European Space Agency's Planck team on a new, revised analysis, the BICEP group ​officially concludes that, no, their observations indicate nothing conclusive or even all that promising. It's disappointing mainly because the initial results garnered the hype they did.

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Gravitational waves should be almost unfathomably faint within the dust and radiation of space—as a force, gravity is almost unfathomably weak. The electromagnetic force, for example, is around 10^33 (a 10 with 33 zeros behind it) times stronger than gravity. So, to see gravitational waves, we need to think big and look to the most powerful cosmological entities out there—binary systems of neutron stars or black holes.

Or we might look to the Big Bang itself, when everything of everything was all bunched up momentarily. Here, gravitational waves, as carriers of energy, might've left their imprint on the cosmic microwave background, which is the curtain of thermal radiation leftover from the hypothesized inflationary period just after the Big Bang that just kind of hangs over everything in the universe.

This is where cosmologists at the BICEP experiment—which is centered around a sky-scanning telescope array at the South Pole—thought they saw gravitational waves, as faint ripples in ancient radiation. The sky has a memory, at least in terms of heat.

The original BICEP announcement, offered last March in a presentation at the Harvard-Smithsonian Center for Astrophysics, ​was celebrated almost across the board:​ "The biggest scientific discovery of the year"​"'definitely'" worthy of a Nobel prize"​"this gravitational-wave evidence is stronger than many of us, including me, had expected"​"did the Earth just move for cosmology?"

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The first strong note of skepticism (that I could find) didn't come until weeks later, when a trio of cosmologists ​gave a possible alternative explanation: "radio loops," remnants of ancient supernova explosions that hang in the primordial sky alongside the CMB. The BICEP results didn't rule that possibility out, it seemed.

Image: ESA and the Planck Collaboration/D. Ducros

In May of last year, a rumor emerged that the BICEP team had made an incorrect calculation in its efforts to isolate the background radiation from the muddying effects of the foreground. Then, in September, ​the Planck group released its own results, which explained away the BICEP team's gravitational waves as dust, another radiation-emitting medium found in the deep sky. This was especially damning, not just for the BICEP results but for potential future results—the cosmic foreground will be tricky to filter out.

"Searching for this unique record of the very early Universe is as difficult as it is exciting, since this subtle signal is hidden in the polarisation of the CMB, which itself only represents only a feeble few percent of the total light," said Jan Tauber, ESA's project scientist for Planck, in a statement.

The essential problem with the BICEP analysis has to do with where the cosmologists chose to look. They had made something of an educated guess about which patch of sky would have the smallest amount of interference. And it turns out they guessed wrong, hugely underestimating the contributions from other sources.

Now, researchers can say with some certainty that the expected gravitational wave signal will be no more than half of the total background signal, which doesn't rule out the possibility of future observations.

"The new upper limit on the signal due to gravitational waves agrees well with the upper limit that we obtained earlier with Planck using the temperature fluctuations of the CMB," said Brendan Crill, a member of both the Planck and BICEP2 teams. "The gravitational wave signal could still be there, and the search is definitely on."