At a celebratory press conference in mid-March, a group of physicists gathered at the Harvard-Smithsonian Center for Astrophysics in Cambridge to announce strong evidence supporting a more than three-decade-old theory about the dramatic expansion during the birth of our universe — the bang of the “Big Bang.”
After months of back-and- forth among scientists, the paper presenting those results was published on Thursday in the journal Physical Review Letters, but with an important caveat: the telltale signal that astronomers unveiled in March could have just been starlight scattering off of dust.
The Harvard-led team still thinks the measurement it took with a South Pole telescope was the curled polarization pattern of ancient light from the earliest moments of the universe — considered the smoking-gun evidence that the universe underwent a rapid, violent inflationary period of exponential expansion.
But they admit they can’t rule out the possibility that they might have measured polarization that was instead emitted by a more mundane source, galactic dust.
“Since we submitted this paper new information on polarized dust emission has become available,” wrote the authors, led by John Kovac, a Harvard astronomer.
They acknowledge the dust levels may be higher than they accounted for in their analysis and noted that “more data are clearly required to resolve the situation.”
After the festive March event, which was attended by two theoretical physicists who forged the so-called inflation theory, other scientists began debating the results at conferences, online, and in publications, providing an unusual public window into how science works. The dust issue was raised by several physicists during these discussions.
“I think the issue is still unresolved . . . the issue being whether what they measured comes from gravity waves in the early universe or from the dust,”
Scientists are eagerly awaiting a forthcoming analysis of observations made from the European Planck satellite, which could be released in a few weeks and should resolve the questions about dust.
Either way, scientists said, the measurement by the Harvard-led BICEP2 team was a remarkable feat.
Astrophysicist Matias Zaldarriaga of the Institute for Advanced Study in New Jersey said that, like most other people in the field, he was extremely excited when he first heard about the BICEP2 results. He eagerly accepted an invitation to a conference at the California Institute of Technology focused on the finding. But over the subsequent month, he said, he and a colleague began to wonder whether dust in our own galaxy that absorbs the light of stars and then re-emits it might be an alternative explanation.
As he did calculations, he became convinced that he couldn’t rule out dust. He debated whether he should cancel attending the meeting.
“It could be possible all the signal was coming from dust. Not that one could be sure, but that was a possibility. That was a big letdown for me,” Zaldarriaga said.
He reluctantly gave his talk, bringing up the problem. “It was a little bit of a bummer.”
Other physicists reached similar conclusions. A paper posted by physicists from the University of California Berkeley on Arxiv, a prepublication website, cast further doubt.
“This result does not automatically mean that BICEP2 has no evidence for primordial gravitational waves in its data,” the Berkeley authors wrote. But, they added, “it is thus too early to celebrate the BICEP2 results as a definitive proof of inflation.”
The bottom line is that no one can yet be absolutely certain what, exactly, the Harvard-led team measured.
“One should clearly understand that we are talking about the cutting edge of scientific exploration,” said Andrei Linde, a theoretical physicist from Stanford University who helped devise the theory of inflation.
“Whatever we learn will be hugely important for the further development of cosmology, and we are going to know the final answer before too long.”