Galactic dust sounds death knell for BICEP2 gravitational wave claim
Feb 3, 2015 3 comments
Astronomers working on the Background Imaging of Cosmic Extragalactic Polarization (BICEP2) telescope at the South Pole have withdrawn their claim to have found the first evidence for the primordial "B-mode" polarization of the cosmic microwave background (CMB). The claim was first made in March 2014 and this update comes after analysis of data from the Keck Array telescope at the South Pole and the most up-to-date maps showing polarized dust emission in our galaxy from the European Space Agency's Planck collaboration. It now seems clear that the signal initially claimed by BICEP2 as an imprint of the rapid "inflation" of the early universe is in fact a foreground effect caused by dust within the Milky Way.
Cosmologists believe that when the universe was very young – a mere 10–35 s after the Big Bang – it underwent a period of extremely rapid expansion known as "inflation" when its volume increased by a factor of up to 1080 in a tiny fraction of a second. About 380,000 years after the Big Bang, the CMB – the thermal remnant of the Big Bang – came into being. BICEP2, Planck and the Keck Array all study the CMB and BICEP2's main aim was to hunt down the primordial B-mode polarization. This "curl" of polarized CMB light is considered to be the smoking gun for inflation.
In March last year BICEP2 scientists claimed success, saying that they had measured primordial B-modes with a statistical certainty of 7σ – well above the 5σ "gold standard" for a discovery in physics. However, doubts soon crept in, especially about how the team had handled the effect of galactic dust on the result. Also, the BICEP2 measurements used in the 2014 analysis were made at just one frequency of 150 kHz – for a signal to be truly cosmological in nature, it must be crosschecked at multiple frequencies.
When the most recent dust maps from Planck were released in September last year, it became apparent that the polarized emission from dust is much more significant over the entire sky than BICEP2 had allowed for. While the dust signal is comparable to the signal detected by BICEP2 even in the cleanest regions, this did not completely rule out BICEP2's original claim. To put the issue to rest, the three groups of scientists analysed their combined data – adding into the mix the latest data from the Keck Array, which also measures CMB polarization.
This analysis was based on CMB polarization observations on a 400 square-degree patch of the sky. The Planck data cover frequencies of 30-353 GHz, while the BICEP2 and Keck Array data were taken at a frequency of 150 GHz. "This joint work has shown that the detection of primordial B-modes is no longer robust once the emission from galactic dust is removed," says Jean-Loup Puget, principal investigator of the HFI instrument on Planck at the Institut d'Astrophysique Spatiale in Orsay, France. "So, unfortunately, we have not been able to confirm that the signal is an imprint of cosmic inflation." While they did find a signal from B-modes that arose due to gravitational lensing from galaxies, which had been spotted before in the CMB, it is not the primordial signal the groups were looking for.
These data imply that the simplest inflation models are now ruled out with 95% confidence
Perimeter Institute of Theoretical Physics
Since its public announcement in March 2014, the BICEP2 team has been criticized by some physicists for prematurely claiming to have found the first "smoking gun" evidence for inflation. Neil Turok, director of the Perimeter Institute of Theoretical Physics in Canada, who had been an early critic of the BICEP2 results, now points out that the latest joint analysis has shied away from making a clear comparison of the data against the most basic models of inflation. "These data imply that the simplest inflation models are now ruled out with 95% confidence," he says, explaining that, while this is not yet conclusive, it may just be just a matter of time thanks to a host of experiments that are currently gathering new and better data on the B-modes. Indeed, Turok believes that within a year we may have the data that begins to winnow away many inflationary theories. One such basic theory – dubbed the Φ2 theory – predicts a 15% contribution in the CMB fluctuations to come from primordial gravitational waves, but the data from the joint analysis show that the maximum contribution is 12%. Turok told physicsworld.com he would not be surprised if this contribution were shown to be a mere 5% in the next year.
Peter Coles, an astrophysicist at the University of Sussex in the UK, told physicsworld.com that, with data at only one frequency, there was no way BICEP2 could have ruled out dust emission. "I think they were right to publish their result, but should have been far more moderate in their claims and more open about the uncertainty, which we now know was huge," he says. "I don't think BICEP2 comes out of this very well, but neither do the many theorists who accepted it unquestioningly as a primordial signal and generated a huge PR bandwagon."
Coles adds that, despite what has happened, he still believes strongly in open science. "The BICEP2 debacle has really just demonstrated how science actually works, warts and all, rather than how it tends to be presented in the media. But I do feel that it has exposed a worrying disregard for the scientific method in some very senior scientists who really should know better. It can be dangerous to want your theory to be true so much that it clouds your judgement. In the end it's the evidence that counts."
Caught in a loop?
Following BICEP2's announcement last March, Subir Sarkar – a particle theorist at the University of Oxford and the Niels Bohr Institute in Copenhagen – claimed to have found evidence that emissions from local "radio loop" structures of dust in our galaxy could generate a previously unknown polarized signal. This new foreground – which should be seen in the radio and microwave frequencies and is present at high galactic latitudes – could easily mimic a B-mode polarization signal, according to Sarkar.
Planck's data from last year convinced Sarkar and colleagues that the loop structures crossing the BICEP2 observation region may be the main cause of the polarization signal. Sarkar told physicsworld.com that he is surprised that the latest paper does not offer a physical explanation for why there should be so much dust at such high galactic latitudes. "Unless we understand this it will be hard to model the foreground emission to the level of accuracy required to make progress in the continuing search for gravitational waves from inflation," he says.
Currently, a variety of satellite and ground-based experiments – such as LiteBIRD, COrE, Atacama Cosmology Telescope and the recently launched SPIDER telescope – are busy taking measurements of the CMB polarization to settle the debate on gravitational waves, and hence inflation. Indeed, the BICEP experiment itself is now taking data at two frequencies and will soon up that number to three.
Theories run amok
"For the past 35 years, theoretical physics has been an extravaganza of model-building," says Turok, adding that theories have "sort of run amok." He alludes to the fact that data from experiments as different in scale as the Large Hadron Collider and Planck have shown that the universe "is much simpler than we expected". The data in the coming years will show whether or not relics of gravitational waves indeed abound in the universe – or if inflationary theories should be consigned to the dusty corners of history.
The paper "A joint analysis of BICEP2/Keck Array and Planck data" by BICEP2/Keck and the Planck collaboration has been submitted to the journal Physical Review Letters. A pre-print is available here.
About the author
Tushna Commissariat is a reporter for physicsworld.com