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Physicists Say a Dust-Up Over Big Bang Gravity Waves Has Fizzled Out

A detailed analysis of microwave polarization has thrown a final splash of cold water on claims about the detection of Big Bang gravity waves.
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A detailed analysis of microwave polarization from the Big Bang's aftermath has thrown what's likely to be the final splash of cold water on last year's claims that gravitational waves were detected from a burst of cosmic inflation, physicists say.

"Unfortunately, we have not been able to confirm that the signal is an imprint of cosmic inflation," Jean-Loup Puget, principal investigator for the High Frequency Instrument on the European Space Agency's Planck spacecraft, said in a news release issued Friday.

The report brings a months-long saga of discovery and doubt to a conclusion.

The saga began last March, when physicists from the BICEP2 experiment at the South Pole reported seeing the signature of primordial gravitational waves in polarized patterns of microwave radiation. Had the results held up, they would have been a surprisingly strong confirmation of inflationary Big Bang theory and might have earned someone a Nobel Prize.

Flash Interactive: Beyond the Big Bang

Inflationary theory proposes that the universe went through a dramatic blow-up in size during the instant following the Big Bang. That would account for the observation that the universe's structure is relatively smooth, with small fluctuations in temperature that became "spread out" during the inflationary burst.

Theoretically, the blow-up should have produced a blast of gravitational waves, and the BICEP2 scientists thought they saw the signature of those waves in the microwave background radiation observed in the skies over the South Pole.

But follow-up results from the Planck space mission suggested that the polarization could have been caused instead by higher-than-anticipated levels of galactic dust, and that there might not be any excess signal pointing to primordial gravitational waves.

To make sure, the Planck and BICEP2 teams decided to combine their data for a final analysis.

The cat's out of the bag

On Thursday, a French-language update from members of the Planck team reported that the joint Planck/BICEP2 analysis was complete, and had drawn upon observations from the Keck Array as well. "There remains an excess, but it is far too low to be considered a detection and could be the result of simple variations associated with experimental noise," the report said.

That update was quickly taken off the Web, but the cat was out of the bag. On Friday, ESA revealed more of the details in a report aimed at a lay audience. The National Science Foundation followed with a similar news release.

ESA and NSF said the analysis found a large contribution to the polarization results from galactic dust, and a much smaller contribution from another source — deflections in the light waves that were induced by the cosmic web of massive structures that permeate the universe. This effect is known as gravitational lensing.

Once all those effects were accounted for, the evidence for primordial gravitational waves was "no longer robust," Puget said.

The full study with all the technical details, titled "A Joint Analysis of BICEP2/Keck Array and Planck Data," was submitted for publication in Physical Review Letters, ESA and NSF said. A preprint version of the paper was made available via the website on Friday.

Inflation theory still lives

The latest twist doesn't mean that inflationary theory is disproven. If that were the case, the physics world would be thrown into a tizzy. However, the findings do set a new upper limit on the amount of gravitational waves caused by inflation — which could boost some variants of the theory while knocking down others.

Last week, British physicist Stephen Hawking said he was working on a "new theory of the origin of the universe" that predicts primordial gravitational waves should pervade the entire universe. Presumably, the latest findings will have an effect on his work.

"The gravitational wave signal could still be there, and the search is definitely on," Brendan Crill, a physicist at NASA's Jet Propulsion Laboratory who is a member of the Planck and BICEP2 teams, said in Friday's update. Enhanced observations of the cosmic microwave background radiation are expected to come from the Keck Array as well as a new BICEP3 telescope.

Caltech theoretical physicist Sean Carroll told NBC News that "the bottom line is simply that the current data don't say much about inflation, one way or another."

"The original BICEP2 detection seems to have been incorrect, but it was at a surprisingly high amplitude, so having it go away just sends us back to where we were a year ago, which is fine," he said in an email. "I am not at all sure that we will someday see evidence for inflation, but I'm certainly open to the possibility."

Update for 3:05 p.m. ET Jan. 30: Physicist Matt Strassler, a visiting scholar at Harvard, told NBC News that he hoped someday to see evidence either for or against inflation, but emphasized that "I don't think I should presuppose what we might discover."

"It's also worth mentioning that it is fairly common for the first claim of a discovery to not hold up under further scrutiny," Strassler said in an email. "That's part of why we always want to see confirmation from a different experiment before we accept such claims. It's disappointing this discovery claim was premature, but it's not particularly unusual. Doing science at the forefront of knowledge is hard."

Update for 5:10 p.m. ET Jan. 30: Two more knowledgeable outside observers have weighed in. Here's a comment from Marc Kamionkowski, an astrophysicist from Johns Hopkins University who predicted years ago how the imprint of gravitational waves could be found:

"What we thought was a gravitational-wave signal is dust. There's still room, though, for inflationary gravitational waves to be detected in the future, and a number of strong experimental groups are trying to do so. I remain hopeful that we'll see something more definitive within the next decade, if not the next few years."

And here's the word from Fermilab physicist Don Lincoln, who will be my guest next Wednesday on the "Virtually Speaking Science" online talk show:

"The fact that additional analysis makes BICEP's original measurement less significant should not be viewed as a failure of science. Indeed. I think it should be viewed as a strong affirmation of the scientific method. An initial and exciting measurement was confronted by more data, which strengthened the experimenters’ understanding of their result. I am sure that scientists on BICEP, Planck and all astronomers remain committed to getting at the truth and will continue to look at the polarization patterns in the cosmic microwave background as a powerful way to support or refute inflation."

Update for 7:45 p.m. ET Jan. 30: Robert Garisto, editor of Physical Review Letters, let folks know via Twitter that a preprint version of the paper was available and noted that the Keck Array data made a significant contribution to the analysis: