A team of astronomers that last year reported evidence for gravitational waves from the early Universe has now withdrawn the claim. A joint analysis of data recorded by the team’s BICEP2 telescope at the South Pole and by the European spacecraft Planck has revealed that the signal can be entirely attributed to dust in the Milky Way rather than having a more ancient, cosmic origin.
So, that’s that then. As many will point out, this doesn’t mean the idea of observing primordial gravitational waves is dead, but it does remove what it would have implied (evidence for cosmic inflation, bubble universes, etc) back into speculative science, at least for now. As someone who replied to my tweet said: science at work.
Most astrophysicists continue to believe that cosmic inflation is real, but of course, most astrophysicists before 1998 believed that the expansion of the universe was slowing, that it was only a matter of determining by how much it was slowing. The discovery of dark energy caused a massive reset in those beliefs. It awaits to be seen whether the beliefs about inflation will eventually require a similar reset.
Think of the Big Bang, and you probably imagine a moment in time when matter, energy and space itself all burst into existence at once. Yet many astrophysicists now believe that the “Big Bang” was actually two distinct events: first the inaugural instant of space and time, and second the generation of most of the “stuff” that populates that space. So, which really deserves to be called the Big Bang?
This article calls attention to a dispute I’ve noticed lately. Some physicists insist that cosmic inflation happened before the Big Bang, others that the Big Bang was the moment of everything began. Of course, if you accept the theory of eternal inflation, then there was no beginning, and the period of inflation ending is the point of the Big Bang.
Given how imprecise the term “Big Bang” actually is, how inappropriate of a term it is for describing what it applies to anyway, the debate strikes me a somewhat meaningless. (No, as the article describes, I don’t have a catchier name than “Big Bang”. That still doesn’t make it accurate.)
Personally, I’ve always thought of the Big Bang starting from the earliest moments of our universe that we can know anything about, and lasting until the cosmic microwave background was generated (after all, the CMB was often called the “afterglow” of the big bang), but I realize that isn’t how most cosmologists think about it. Of course, an argument could be made that we’re still living within the Big Bang, since everything is still expanding and cooling, although dark energy complicates that assertion.
The debate does serve one purpose however. It illuminates the stages of the early universe, which given how difficult these concepts are to describe, is actually a good thing.
The astronomers who this spring announced that they had evidence of primordial gravitational waves jumped the gun because they did not take into proper account a confounding effect of galactic dust, two new analyses suggest. Although further observations may yet find the signal to emerge from the noise, independent experts now say they no longer believe that the original data constituted significant evidence.
Researchers said in March that they had found a faint twisting pattern in the polarization of the cosmic microwave background (CMB), the Big Bang’s afterglow, using a South Pole-based radio telescope called BICEP2. This pattern, they said, was evidence for primordial gravitational waves, ripples in the fabric of space-time generated in the early Universe (see ‘Telescope captures view of gravitational waves‘). The announcement caused a sensation because it seemed to confirm the theory of cosmic inflation, which holds that the cosmos mushroomed in size during the first fraction of a second after the Big Bang.
However two independent analyses now suggest that those twisting patterns in the CMB polarization could just as easily be accounted for by dust in the Milky Way Galaxy1, 2.
The BICEP2 team has reportedly been sticking to their guns, and many theoretical physicists initially downplayed the issues, so I’ve been reluctant to put too much credence to these stories. But the doubts seem to be gaining more traction, and this caught my attention:
“I had thought that the result was very secure,” Alan Guth, the cosmologist who first proposed the concept of cosmic inflation in 1980, and who is at the Massachusetts Institute of Technology in Cambridge, told Nature after learning about Flauger’s talk. “Now the situation has changed.”
It’s still possible that when the BICEP2 team actually publishes their results, all of the issues will be addressed and results still found to be compelling. But when the father of cosmic inflation loses confidence in the results, I’d say they’re on the ropes. Inflation definitely may still be reality, but we might not have proof for it yet.