Year in review: Dust obscures possible gravitational wave discovery

Ancient ripples in the fabric of the space square off against soot in the galaxy

Planck's measurement of dust in Milky Way

NOT-SO-CLEAR SKIES  The signal that BICEP2 researchers interpreted as gravitational waves may be due to interstellar dust. The Planck satellite mapped dust in the entire sky. The map shows the sky above the plane of the Milky Way (left) and below the galaxy’s plane (right). Planck found areas heavily contaminated by dust (red) and regions that are relatively clean (blue). The black box shows where BICEP2 searched for gravitational waves.

Planck collaboration


Gravitational waves from the Big Bang captured worldwide attention in 2014. But then interstellar dust clouds stole
the show.

Detection of such waves — ripples in the fabric of space — would be direct evidence for the theory of cosmological inflation, a brief epoch immediately after the Big Bang when the visible universe abruptly swelled to at least 1075 times its initial volume.

In March, astrophysicists thought they had captured their elusive gravitational wave quarry. Researchers with the BICEP2 project reported swirling patterns in the alignment of electromagnetic waves in the cosmic microwave background, or CMB, the primordial light released into the universe about 380,000 years after the Big Bang (SN: 4/5/14, p. 6). Those patterns supposedly reflected the influence of gravitational waves launched during the epoch of inflation. 

In the wake of the announcement, as cosmologists popped champagne bottles and spoke of Nobel Prizes, others struggled to make sense of the data. BICEP2’s signal was much stronger than anyone had expected.

Astronomers knew that interstellar dust, sootlike grains of carbon and silicon, could produce a signal mimicking gravitational waves. The BICEP2 scientists had estimated the effect of dust on their data in reaching their conclusions. But when other researchers took a closer look at BICEP2’s dust calculations, the results were discouraging, suggesting that dust, not gravitational waves, might account for the whole signal (SN: 6/28/14, p. 20).

“That’s when people got really depressed,” says astrophysicist Katherine Mack of the University of Melbourne in Australia.

BICEP2’s discovery was further dimmed in September by measurements from the Planck satellite, a mission designed to probe the CMB in unprecedented detail. Planck’s dust data were consistent with the possibility that BICEP2’s supposed gravitational wave signal was entirely due to dust. Cosmology’s biggest result in years may turn out to be nothing more than Milky Way soot (SN: 10/18/14, p. 7).

“We’re kind of bummed,” says Mack, who is not involved with BICEP2 or Planck. A confirmed detection of primordial gravitational waves “would have been really nice.”

But the saga is not over. Planck and BICEP2 researchers plan to publish a joint analysis of their data, which could clarify how difficult it will be for future projects to find gravitational waves. And half a dozen other experiments are planning to release results in the coming year that could show whether inflationary gravitational waves are really there. Planck data do not rule out the existence of the waves; cosmologists may just need to dig a little deeper.

Meanwhile, astronomers want to know what they can learn from this episode and how it affects science communication. Some accused the BICEP2 team of doing “science by press release,” announcing results before submitting them to a scientific journal. Others argue that the researchers were right to shout from the rooftops — it was a huge discovery and the added attention triggered a much wider peer review.

Mack thinks the lesson for researchers is to be candid about how science works. Astronomers took an initial result, discussed it, got more data and came to a consensus. The BICEP2 measurement was exquisite, she says, and the team worked with the best information it had. But the researchers needed to stress that the results were preliminary. Science is always evolving. “When we present results as monolithic answers,” she says, “that’s when people distrust science.”

Christopher Crockett is an Associate News Editor. He was formerly the astronomy writer from 2014 to 2017, and he has a Ph.D. in astronomy from the University of California, Los Angeles.

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