The thing about science is that it is a dynamic, yet strict, process. When a groundbreaking discovery is made, other scientists weigh in and try to replicate the results, thereby growing the body of evidence around the initial discovery. But problems will often crop up when a discovery is announced prematurely having not gone through the full peer review process.
This problem appears to have reared its tricky head for the BICEP2 team who, in March, announced the groundbreaking discovery that their South Pole-based observatory had detected a specific kind of polarized light embedded in the cosmic microwave background (CMB) radiation that originates from our universe’s genesis: the Big Bang. Now their work has been published in the journal Physical Review Letters, but it now comes with a pretty hefty caveat: more work needs to be done to account for obscuring galactic dust.
A Premature Announcement?
The implications for the March announcement were three-fold: The discovery of a certain type of polarization — known as B-mode polarization — in the CMB suggests the presence of gravitational waves. Gravitational waves have yet to be observed, despite the fact they are predicted to pervade the entire universe by Einstein’s Theory of General Relativity. So, a positive detection of gravitational waves would be historic enough.
But there’s more.
The detection of gravitational waves in the CMB would hint at a Big Bang origin. Any ‘signal’ embedded in the slight ripples of varying temperature of the CMB (features known as ‘anisotropies’), suggests that its origin would be just after the Big Bang occurred. One model of the Big Bang, and the moments just after, suggests the universe went through a rapid period of expansion known as ‘inflation.’ The detection of gravitational waves in this way provided the “smoking gun” for the theory of inflation. Yet another historic discovery.
Finally, for the gravitational waves to be embedded in the CMB at all, if we wind back time to the inflationary period when the universe was a fraction of the size it is now, the gravitational waves would have had to have been very tiny when they were created. In fact, they would have been created on the tiniest scales possible hinting at a quantum origin.
One of the biggest conundrums in physics is how gravity fits with the Standard Model. The Standard Model, which is a recipe book of sorts for the quantum world, ignores gravity, hinting that the Standard Model is not a complete theory or that there is some more exotic physics beyond the Standard Model. If this B-mode polarization is real, it would suggest that quantum gravity — transmitted in the form of hypothetical gravitons — left its imprint on the CMB when the universe was very young.
In a nutshell, the detection of gravitational waves in the cosmic microwave background would be a World Cup, World Series and Superbowl win all rolled into one.
“Detecting this signal is one of the most important goals in cosmology today,” John Kovac, leader of the BICEP2 collaboration at the Harvard-Smithsonian Center for Astrophysics, said after the March announcement.
But there was a problem. The BICEP2 physicists announced their discovery before they had published their research to a peer reviewed journal.
Has the Horse Bolted?
Almost immediately after the March announcement, scientists not affiliated with the BICEP2 team started to air their concerns about the research. Although the BICEP2 team had pored over their data for years, accounting for any errors, rumors started to swirl suggesting that even members of the BICEP2 team were concerned about their analysis. These rumors were quickly squashed by the BICEP2 team, but the air of doubt was building.
The doubt centers on galactic dust that exists in the Milky Way. When observing the furthest-most reaches of the Cosmos, the BICEP2 telescope has to look through this intervening dust. For any precise measurements of the CMB, the polarizing effects of galactic dust needs to be corrected for. Unfortunately, only a vague idea of the dust’s polarizing effect was known by the BICEP2 team during their analysis.
The European space-based telescope Planck is also analyzing the CMB, looking for gravitational wave signals and is able to map the polarization effect of galactic dust. The Planck team have yet to publish their results, but are expected to do so soon. Without the Planck data, scientists are concerned that the BICEP2 analysis is incomplete and the B-mode polarization signal may just be a polarization effect by this intervening dust.
Now that the BICEP2 discovery has been published, the team has added a cautionary note in their summary. They now say that their models “are not sufficiently constrained by external public data to exclude the possibility of dust emission bright enough to explain the entire excess signal.” Considering they had already announced the discovery of gravitational waves, is this the equivalent of shutting the barn door after the horse has bolted?
To publish results such as these in a peer reviewed journal, a panel of scientists in the related field comment on the shortcomings of the work. It seems most likely that this added air of caution was a requirement to get their work published rather than any change of heart by the BICEP2 scientists. But it is unfortunate that the gravitational wave discovery announcement was made so prematurely.
So nothing has changed, it’s just that now we are seeing the scientific process play out in a very public arena. The BICEP2 results stand until the Planck team make their results public later this year — only then will we know whether or not primordial gravitational waves have been detected.