New BICEP rumours: nothing to see here

This week there has been a minor kerfuffle about some rumours, originally posted on Adam Falkowski's Resonaances blog, regarding the claimed gravitational wave detection by BICEP. The rumours asserted that Planck had proven BICEP had made a mistake, BICEP had admitted the mistake, and that this might mean that all the excitement about the detection of gravitational waves was misplaced and all that BICEP had seen was some foreground dust emission contaminating their maps. (Since then there has been a strong public denial of this by the BICEP team.)

Now, with the greatest respect to Resonaances, which is an excellent particle physics blog, this is really a non-issue, and certainly not worth offending lots of people for (see for instance Martin Bucher's comment here). I really do not see what substantial information these rumours have provided us with that was not already known in March, and therefore why we should alter assessments of the data  made at that time.

Let me explain a bit more. One of the important limitations of the BICEP2 experiment is that it essentially measured the sky at only one frequency (150 GHz) — the data from BICEP1, which was at 100 GHz, was not good enough to see a signal, and the data from the Keck Array at 100 GHz has not yet been analysed. When you only have one frequency it is much harder to rule out the possibility that the "signal" seen is not due to primordial gravitational waves at all but due to intervening dust or other contamination from our own Galaxy.

The way that BICEP addressed this difficulty was to use a set of different models for the dust distribution in that part of the sky, and to show that all of them predict that the possible level of dust contamination is an order of magnitude too small to account for the signal that they see. Now, some of these models may not be correct. In fact none of them are likely to be exactly right, because they may be based on old and likely less accurate measurements of the dust distribution or rely on a bit of extrapolation, wishful thinking, whatever. But the point is that they all roughly agree about the order of magnitude of dust contamination. This does not mean that we know there is or isn't any foreground contamination; this is merely a plausibility argument from BICEP (that is supported by and supports some other plausibility arguments in the paper).

Now the "new" rumour is based on the fact that it turns out that one of the dust models was based on BICEP's interpretation of preliminary Planck data, and that this data was not officially sanctioned but digitally extracted from a pdf of a slide shown at a talk somewhere. This is not exactly news, since the slide in question is in fact referenced in the BICEP paper. What's new is that now somebody unnamed is suggesting that the slide was in fact misinterpreted, and therefore this one dust model is more wrong than we thought, though we already accepted it was probably somewhat wrong. This is not the same as proving that the BICEP signal has been definitively shown to be caused by dust contamination! In fact I don't see how it changes the current picture we have at all. Ultimately the only way we can be sure about whether the observed signal is truly primordial or due to dust is to have measurements that combine several different frequencies. For that we have to wait a bit for other experiments — and that's the same as we were saying in March.

It's worth noting that when BICEP quote their result in terms of the tensor-to-scalar ratio r, the headline number $r=0.2$ assumes that there is literally zero foreground contamination. This was always an unrealistic assumption, but that hasn't stopped some 300 theorists from writing papers on the arXiv that take the number as face value and use it to rule out or support their favourite theories. The foreground uncertainty means that while we can be reasonably confident that the gravitational wave signal does exist (see here), model comparisons that strongly depend on the precise value of r are probably going to need some revision in the future.

So what new information have we gained since March? Well, Planck released some more data, this time a map of the polarized dust emission close to the Galactic plane.

The polarization fraction at 353 GHz observed by Planck. From arXiv:1405.0871.

Since these maps do not include the part of the sky that BICEP looked at (which is mostly in the grey region at the bottom), they don't tell us a huge amount about whether that part of the sky is or is not contaminated by polarized dust emission! Some people have speculated that this is something to do with the rivalry between Planck and BICEP, which is a bit over-the-top. Instead the reason is more scientific: the mask excludes areas where the error in determining the polarisation fraction is high, or the overall dust signal itself is too small. So the fact that the BICEP patch is in the masked region indicates that the dust emission does not dominate the total emission there, at least at 353 GHz (dust emission increases with frequency). This means there is not a whole lot of dust showing up in the BICEP region — if anything, this is good news! But even this interpretation should be treated with caution: dust doesn't contribute too much to the total intensity in that region, but it may well still contribute a large fraction of whatever B-mode polarization is seen. Based on my understanding and things I have learned from conversations with colleagues, I don't think Planck is going to be sensitive enough to make definitive statements about the dust in that specific region of the sky.

Another interesting paper that has come out since March has been this one, which claims evidence for some contamination in the CMB arising from the "radio loops" of our Galaxy. It also has the great benefit of being an actual scientific paper rather than a rumour on somebody's blog. (Full disclaimer: one of the authors of this paper was my PhD advisor, and another is a friend who was a fellow student when I was at Oxford.) 

The radio loops are believed to be due to ejected material from past supernovae explosions; the idea is that if this dust contains ferrimagnetic molecules or iron, it would contribute polarized emission that might be mistaken for true CMB when it is in fact more local. What this paper argues is that does appear to be some evidence that one of the CMB maps produced by the WMAP satellite (which operated before Planck) does show some correlation between map temperature and the position of one of these radio loops ("Loop I"). In particular, synchrotron emission from Loop I appears to be correlated with the temperature in the WMAP Internal Linear Combination (or ILC) map. I'm not going to comment on the strength of the statistical evidence for this claim; doubtless someone more expert than I will thoroughly check the paper before it is published. For the time being let us treat it as proven.

The relevance of this to BICEP is somewhat intricate, and proceeds like this: given our physical understanding of how the radio loops formed, it seems likely that they produce both synchrotron and dust emission which follow the same pattern on the sky. Therefore perhaps the correlation of the synchrotron emission from Loop I with the ILC map is because both are correlated with dust emission from the loop. If the correlation is because of dust emission, this might be polarized because of the postulated ferrimagnetic molecules etc., leading to a correlation between the WMAP polarization and Loop I. And if Loop I is contaminating the WMAP ILC map, it is perhaps plausible that a different radio loop, called the "New Loop", is also contaminating other CMB maps, in particular those of BICEP. Whereas Loop I doesn't get very close to the BICEP region, the New Loop goes right through the centre of it (see the figure below), so it is possible that there is some polarized contamination appearing in the BICEP data because of the New Loop. At any rate, the foreground dust models that BICEP used didn't account for any radio loops, so likely underestimate the true contamination.

Position of some Galactic radio loops and the BICEP window. "Loop I" is large one in the upper centre, that only skims the BICEP window; the "New Loop" is the one in the lower centre that passes through the centre of it. Figure from Philipp Mertsch.

So far so good, but this is quite a long chain of reasoning and it doesn't prove that it is actually dust contamination that accounts for any part of the BICEP observation. Instead it makes a plausible argument that it might be important; further investigation is required.

At the end of the day then, we are left in pretty much the same position we were in back in March. The BICEP result is exciting, but because it is only at one frequency, it cannot rule out foreground contamination. Other observations at other frequencies are required to confirm whether the signal is indeed cosmological. One scenario is that Planck, operating on the whole sky at many frequencies but with a lower sensitivity than BICEP, confirms a gravitational wave signal, in which case pop the champagne corks and prepare for Stockholm. The other scenario is that Planck can't confirm a detection, but also can't definitively say that BICEP's detection was due to foregrounds (this is still reasonably likely!), in which case we wait for other very sensitive ground-based telescopes pointed at that same region of sky but operating at different frequencies to confirm whether or not dust foregrounds are actually important in that region, and if so, how much they change the inferred value of r.

Until then I would say ignore the rumours.