Yesterday, researchers from the Background Imaging of Cosmic Extragalactic Polarization (BICEP2) telescope at the South Pole revealed that they have detected the first evidence for the primordial B-mode polarization of the cosmic microwave background (CMB). The astronomers claimed that the primordial B-mode polarization signal – which is related to primordial gravitational waves that flowed through the early universe – is the first direct evidence for cosmic inflation and has been measured to a statistical certainty of 3σ. Now, cosmologist and Perimeter Institute director Neil Turok, who worked on an inflationary model of his own with Stephen Hawking in the 1990s, urges caution and says that extensive experimental confirmation is necessary before BICEP2’s results can be considered as evidence for inflation.
I believe that if both Planck and the new results agree, then together they would give substantial evidence against inflation!
“If…and it’s a big if…this is true, it would be spectacular evidence for what happened at the Big Bang,” Turok told physicsworld.com. While he agreed that at first glance, the BICEP2 observations are in keeping with inflation “as suggested over 30 years ago, wherein space–time would resonate with the aftershocks of inflation and would ring like a bell”, a closer look at the discrepancy between the new results and previous data from the Planck and WMAP telescopes is what worries Turok. Indeed, the tensor-to-scalar ratio of 0.20 that BICEP2 measured is considered to be significantly larger than that expected from previous analyses of data. But the BICEP2 researchers said in their press conference yesterday that they believe certain tweaks could be made to an extension of the ΛCDM cosmological model that could make the two results agree.
“But these tweaks would be tremendously ugly….and in fact, I believe that if both Planck and the new results agree, then together they would give substantial evidence against inflation!” exclaims Turok, further explaining that “[we] must be careful before we treat them as true”.
Astrophysicist Peter Coles, who is based at the University of Sussex in the UK, is also cautious about the BICEP2 data interpretations. He told physicsworld.com, “It seems to me though that there’s a significant possibility of some of the polarization signal in E and B [modes] not being cosmological. This is a very interesting result, but I’d prefer to reserve judgement until it is confirmed by other experiments. If it is genuine, then the spectrum is a bit strange and may indicate something added to the normal inflationary recipe.” He also cautions that if true, the results would constitute an important consistency check on a certain class of inflationary theories, but would not be direct evidence. “In order to be direct, we would have to be able to state categorically that it couldn’t be generated in any other way. I don’t think we can say that.” Indeed, Coles has set up a general “straw poll” on his blog, asking people to vote on the results. At the time of writing, his results show that most people believe that it is too soon to decide.
If it is genuine, then the spectrum is a bit strange and may indicate something added to the normal inflationary recipe
Turok says that the BICEP2 experiment “is absolutely heroic”, but also that possible contamination could have occurred during data taking. Although the telescope is based in the South Pole – one of the clearest places on Earth for observational astronomy – there could be noise thanks to the Earth’s atmosphere. He also says that the signal could be contaminated thanks to signals from dust in the galaxy (known as synchrotron radiation), as well as due to lensing, because the observations are made through galactic clusters. The BICEP2 team says in its paper that it has ruled out contamination from synchrotron radiation and dust at a statistical significance of about 2.3σ. But Turok is unimpressed with those values, and points out that the final 5σ discovery that the team claimed needs to be better explained.
While he extends his “kudos” to the team for going after a really fascinating question, Turok currently remains a sceptic. “I will quote Carl Sagan and say ‘extraordinary claims require extraordinary evidence’, and they don’t have extraordinary evidence just yet.” He reiterates that the BICEP2 and Planck/WMAP discrepancies need to be resolved, and that result will be essential because “something has got to give”. He urges other experiments to confirm the results and suggests that the combined observational results should then fit a simple yet detailed cosmological model before the BICEP2 observations can be thought of as proof of primordial gravitational waves or indeed inflation.
“I am a humble theorist,” says Turok with a laugh, “now the real task is for the experimental community to scan and replicate results…it might take months or years and there is still everything to play for but we should have an answer relatively soon.”
The BICEP2 results are available here.
- BICEP2 finds first direct evidence of cosmic inflation
- Planck reveals ‘almost perfect’ universe
- WMAP data put cosmic inflation to the test
- B-mode polarization spotted in cosmic microwave background
- Sounding out the Big Bang
- Inflation, strings and the anthropic principle
- What is cosmic inflation?
- Planck perspectives