Monday, March 24, 2014

BICEP2: reasons to be sceptical, part 2

This is the second part of three posts in which I wanted to lay out the various possible causes of concern regarding the BICEP2 result, and provide my own opinion on how seriously we should take these worries. I arranged these reasons to be sceptical into three categories, based on the questions
  • how certain can we be that BICEP2 observed a real B-mode signal?
  • how certain can we be that this B-mode signal is cosmological in origin, i.e. that it is due to gravitational waves rather than something less exciting?
  • how certain can we be that these gravitational waves were caused by inflation?
The first post dealt with the first of the three questions, this one addresses the second, and a post yet to be written will deal with the third.

How certain can we be that the observed B-mode signal is cosmological? 

Let's take it as given that none of the concerns in the previous post turn out to be important, i.e. that the observed B-mode signal is not an artefact of some hidden systematics in the analysis, leakage or whatever. From my position of knowing a little about data in general, but nothing much about CMB polarization analysis, I guessed that the chances of any such systematic being important were about 1 in 100.

The next question is then whether the signal could be caused by something other than the primordial gravitational waves that we are all so interested in. The most important possible contaminant here is other nearby sources of polarized radiation, particularly dust in our own Galaxy. We don't actually know how much polarized dust or synchrotron emission there might be in the sky maps here, so a lot of what BICEP have done is educated guesswork.

To start with, the region of the sky that BICEP looks at was chosen on the basis of a study by Finkbeiner et al. from 1999, which extrapolated from measurements of dust emission at certain other frequencies to estimate that, at the frequency of relevance to CMB missions such as BICEP, that particular part of the sky would be exceptionally "clean", i.e. with exceptionally low foreground dust emission. Whether this is actually true or not is not yet known for certain, but there exist a number of models of the dust distribution, and most of these models predict that the level of contamination to the B-mode detection from polarized dust emission would be an order of magnitude smaller than the observed signal. Similar model-dependent extrapolation to the observation frequency based on WMAP results suggests that synchrotron contamination is also an order of magnitude too small.

Predictions for foreground contamination for different dust models (the coloured lines at the bottom) versus the actual B-mode signal observed by BICEP2 (black points).

Now one real test of these assumptions will come from Planck, because Planck will soon have the best map of dust in our Galaxy and therefore the best limits on the possible contamination. This is one of the reasons to look forward to Planck's own polarization results, due in about October or November. In the absence of this information, the other thing that we would like to see from BICEP in order to be sure their signal is cosmological is evidence that the signal exists at multiple frequencies (and has the expected frequency dependence).

BICEP do not detect the signal at multiple frequencies. The current experiment, BICEP2, operates at 150 GHz only, and that is where the signal is seen. A previous experiment, BICEP1, did run at 100 GHz as well, but BICEP1 did not have the same sensitivity and could only place an upper limit on the B-mode signal. Data from the Keck Array will eventually also include observations at 100 GHz, but this is not yet available. Until we have confirmation of the signal at different frequencies, most cosmologists will treat the result very carefully.

In the absence of this, we must look at the cross-correlation between B2 and B1. Remember that although B1$\times$B1 did not have the sensitivity to make a detection of non-zero power, B2$\times$B1 can still tell us something useful. If B1 maps were purely noise, or B2 maps were due to dust, we would not expect them to be correlated. If both were due to synchrotron radiation, we would expect them to be strongly correlated. In fact the B2$\times$B1 cross power is non-zero at the $3\sigma$ level or about 99% confidence, which is something Peter Coles' sceptical summary ignores. This is indeed evidence that the signal seen at 150 GHz is cosmological.

Still, some level of cross-correlation could be produced even if both B2 and B1 were only seeing foregrounds. Combining the B2$\times$B1 data with B2$\times$B2 and B1$\times$B1 means that polarized dust or synchrotron emission of unexpected strength are rejected as explanations – though at a not-particularly-exciting significance of about $2.2-2.3\sigma$.


It's fair to say, on the basis of models of the distribution of polarized dust and synchrotron emission, that the BICEP2 signal probably isn't due to either of these contaminants. However, we don't yet have confirmation of the detection at multiple frequencies, which is required to judge for sure. At the moment, the frequency-based evidence against foreground contamination is not very strong, but we'd still need some quite unexpected stuff to be going on with the foregrounds to explain the amplitude of the observed signal.

Overall, I'd guess the odds are about 1:100 against foregrounds being the whole story. (This should still be compared with the quoted headline result of 1:300,000,000,000 against $r=0$ assuming no foregrounds at all!)

The chances are much higher – I'd be tempted to say perhaps even as much as better than even money – that foregrounds contribute a part of the observed signal, and that therefore the actual value of the tensor-to-scalar ratio will come down from $r=0.2$, perhaps to as low as $r=0.1$, when Planck checks this result using their better dust mapping.


  1. Tegmark now estimates the probability that some form of inflation happened as 99%. Max Tegmark, Comment #52, 23 March 2014

    1. Really?! I thought he was already completely sure that inflation did happen, I wonder why BICEP should have made him revise his number downwards? ;-)

    2. Einstein never said you needed inflation to confirm the existence of gravitational waves or the associated radiation. If gravitational radiation exist then gravitational waves exist. And gravitational radiation impact on the orbit of PSR 1913+16 was already confirmed. So gravitational waves are present now and highly detectable in other binary star systems.

  2. A good read, Sesh.

    I've read just about everything going, and for myself, I'm now feeling doubtful. Not about the CMB polarization or the gravitational waves, but about the inflation. I've always accepted it, but now I'm saying WHY? Sean Carroll said "Faced with theories that fit all the data but seem unnatural, one can certainly shrug and say, 'Maybe that’s just the way it is'”. I'm now thinking yes, it’s the way it is, because of the way black holes have to be. Try to imagine a “frozen star” early universe. Everything starts out uniform and homogeneous and flat. Inflation is... superfluous. You can still have rapid initial expansion, and gravity waves. Indeed something akin to gravitational time dilation would mean the initial expansion would look very fast to any observers. But it isn't inflation as we know it. It doesn't solve the flatness problem or the horizon problem if there wasn't any problem to begin with.

  3. Excellent article.
    "The chances are much higher – I'd be tempted to say perhaps even as much as better than even money – that foregrounds contribute a part of the observed signal, and that therefore the actual value of the tensor-to-scalar ratio will come down from r=0.2, perhaps to as low as r=0.1, when Planck checks this result using their better dust mapping."
    Good conclusion. Yet, there is a strong argument (at Scientia salon, ) which support the BICEP2 data as genuine.

  4. thanks to the info from my father added insight thank you so much, may be useful for us all

  5. Hey! Thanks for these two great answeres, I enjoyed it alot to read. I wonder whether you plan to write about the 3rd question, namely "how certain can we be that these gravitational waves were caused by inflation?". You said you wanted to create three blog-entries. Hope you will :-) Best regards, Mario

    1. Hi Mario, thanks for the comment and I'm glad you liked these two installments. I haven't quite forgotten about the third one - but I have a series of four seminars in the next few days which I have been preparing my talks for, so blog writing has been on the back burner. But there will be a couple of evenings free in the next week so I'll put it out soon!