I blogged back in March about the announcement by a team of cosmologists that they had discovered evidence for gravitational waves in the Cosmic Microwave Background (CMB). The BICEP2 experiment, which is based at the South Pole, claimed to have detected the so-called “B-mode polarisation” in the CMB, and from the strength of the signal they argued that it was the best evidence yet of both gravitational waves in the very early Universe, and of the theory of Cosmic Inflation.
However, since that announcement there has been considerable controversy in the cosmology community as to whether their result is correct or not. I have re-blogged several other people’s blogs on this controversy, for example Peter Coles’ blog here and Matt Strassler here and here. As Peter and Matt’s blogs indicated, this controversy has been swirling around in the astronomical community for the last several months; but last Thursday (the 19th of June) it made it into the New York Times.
The main concern amongst the skeptics is that the BICEP2 team did not correctly subtract the effects of dust in our own Galaxy from their signal. Our Milky Way has a lot of dust in it, it is this dust which causes the dark clouds in the band of the Milky Way which are familiar to anyone who has looked at the Milky Way in any detail, even with the naked eye. Most of the dust is in the plane of the disk, but some is above and below the plane in what we refer to as “high Galactic latitudes”. The BICEP2 team chose their patch of sky to be well below the plane of the Milky Way to try to minimise the effects of dust.
However, it may be that the amount of dust and its degree of polarisation where BICEP2 made their observations is greater than the BICEP2 team thought. If this is the case, then much of the polarised signal that BICEP2 measured may not be due to primordial gravitational waves, but instead may be dominated by this foreground contamination. As the New York Times story states, the BICEP2 team acknowledge that the foreground contamination may be greater than they assumed, but they are sticking to their claim that it is still small compared to the signal they detected.
We should know the answer to this burning issue within the next 6 to 12 months. The Planck satellite has done a detailed all-sky map of the strength of the polarised emission from dust in our Milky Way, far more detailed than any data currently available, and when these maps are released it should allow astronomers to correctly determine how much of BICEP2’s signal is due to foreground contamination. Planck will also do this at several different frequencies, and as Galactic dust is much warmer than the CMB the ratio of its signal at different frequencies will be different to that of the CMB, allowing for much better separation of the two effects.
In the meantime, we have a great insight into how science really works. Any result in science is closely scrutinised by the community, and is not accepted as being real until (a) it has been confirmed by other experiments and (b) that the community is satisfied that the interpretation of the measurement is correct, and that all other possibilities have been considered. As Carl Sagan once said
extraordinary claims require extraordinary evidence
So far, I think it is fair to say, most people in the astronomical and cosmological communities are treating the BICEP2 result with a good deal of caution, and that caution can only be allayed by further analysis and measurements.
If you wish to read more about the BICEP2 results and the surrounding controversy, an excellent place to start is Peter Coles’ blog here.
Nothing to do with Big Bang, of course! Overbye should know better.
See also
http://www.newscientist.com/article/dn25762-big-bang-breakthrough-team-backpedals-on-major-result.html#.U6M-8vldXIY
I will remain surprised if the results turn out to be mostly Galactic dust, if only from the perspective that the authors have much more data available from their ongoing experiments than they included in the now-published paper. This would give them an internal check on the spectral index at a more significant level than the “~1.7 sigma”, but perhaps they weren’t so calculating. If it is all Galactic, then I don’t see any harm having been done in the real world, with the only worries being from those who would take seriously anything found on creationist’s blogs. The combination of the Keck Array data and the Planck final products will provide much more power than Planck would alone, and so the BICEP-based experiments will be valuable in getting a better answer. I still see much of the criticism of the announcement of the BICEP2 result in 2013 as being affected by “sour grapes”.
In “ongoing experiments”, are you referring to “BICEP3”? Clearly data at other frequencies are needed to help separate the signal from Galactic dust from the one from the CMB, which the Keck array, other ground-based experiments and the Planck data will all help to provide.
“I still see much of the criticism of the announcement of the BICEP2 result in 2013 as being affected by “sour grapes”.”
Is “2013” a typo for “2014”, or are you referring to something before the March 2014 announcement? It is almost inevitable, for such an important discovery, that much of the criticism of the analysis and interpretation will come from rival groups who hope they haven’t been scooped. But, at the end of the day, science is the winner, as such scrutiny illustrates nicely an important difference between science and people who just state things without any factual or experimental basis (creationists for example)
[…] in the cosmic microwave background. I have previously blogged about this story, for example here, here and here. But, just to quickly recap, in March the BICEP2 team announced that they had detected the […]
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