In the Dark

One of the advantages of informal workshops like this one I’m attending in Copenhagen right now is that there’s a lot of time for discussions and picking up various bits of gossip. Some of the intelligence gathered in this way is unreliable but often it represents knowledge that’s widely known in the cosmological community but which I’ve missed because I don’t spend as much time on the conference circuit these days.

Anyway, those of you with more than a passing interest in cosmology will remember the results from the BICEP2 experiment announced with a great fanfare of publicity in March this year. A significant number of eminent cosmologists immediately seized on the detection of B-mode correlations in the polarized cosmic microwave background as definitive proof of the existence of primordial gravitational waves. Some went even further, in fact, and claimed that the BICEP2 results prove all kinds of other things too.

As time passed, however, and folks had time to digest some of the details presented by the BICEP2 team, there has been a growing unease about the possibility that the measurements may have been misinterpreted. The problem – the Achilles Heel of BICEP, so to speak – is that it operates at a single frequency, 150 GHz. That means that it is not possible for this experiment on its own to determine the spectrum of the detected signal. This is important because it is not only the cosmic microwave background that is capable of producing polarized radiation at a frequency of 150 GHz, foreground dust inside our own Galaxy being the prime suspect as an alternative source. It should be possible to distinguish between dust and CMB using measurements at different frequencies because the microwave background has a black-body spectrum  whereas dust does not. However, BICEP2 maps only a small part of the sky and at the time of the announcement there were no other measurements covering the same region, so a convincing test has not so far been possible.

The measured spectrum of the cosmic microwave background. It’s indistinguishable from the theoretical black-body curve shown as a solid line

The initial BICEP2 announcement included a discussion of foregrounds that concluded that these were expected to be much lower than their detected signal in the area mapped, but serious doubts have emerged about the accuracy of this claim. Have a look at my BICEP2 folder to see more discussion.

More recently, in July, it was announced that the BICEP2 team would collaborate with the large consortium working on the analysis of data from the Planck experiment to try to resolve these difficulties. Planck not only covers the whole sky but also has detectors making measurements over a wide range of frequencies (all the way up to 857 GHz). This should provide a definitive measurement of the contribution of Galactic dust to the BICEP2 field and at last give us a strong experimental basis on which to decided whether the BICEP2 signal is primordial or not. The result of my informal poll on BICEP2 was a clear majority (~62%) in favour of the statement that it was “too early to say” what the BICEP2 signal actually represents.

Anyway, I have it on very good authority that Planck’s analysis of the Galactic foregrounds in the BICEP2 region will be published (on the arXiv) on or around September 1st 2014. That’s just about 10 days from now. Maybe then this tantalizing wait will be over. I’ll try my best to post about the results when it comes out. In the meantime, I thought I’d do something completely unscientific and try to gauge what how current opinion stands on this issue by means of a poll of the total unrepresentative readership of this blog. Suppose you had to bet on whether the BICEP2 result is due to (a) primordial gravitational waves or (b) Galactic foregrounds, which would you go for?

Of course, those working on this project probably know the answer already so they’ll have to decide for themselves whether they wish to vote!

This entry was posted on August 21, 2014 at 9:45 am and is filed under The Universe and Stuff with tags Bicep2, Cosmology, dust, Planck. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.