Friday, May 16, 2014

BICEP and listening to real experts

First up, I'd like to provide a health warning for all people landing here after following links from Sean Carroll or Peter Woit (thanks for the traffic!): I am not a CMB data analysis expert. What I provide on this blog is my own interpretation and understanding of the news and papers I have read, largely because writing such things out helps me understand them better myself. If it also helps people reading this blog, that's great, and you're welcome. But there are no guarantees that any of what I have written about BICEP is correct! If you truly want the best expert opinions on CMB analysis issues, you should listen to the best CMB experts — in this case, probably people who were in the WMAP collaboration, but are not in either Planck or BICEP. Also, if you want to ask somebody to write a scholarly review article on BICEP (yes, I get strange emails!), please don't ask me.

Having said that, I'm not sure whether any WMAP scientists write blogs, so I can at least try to provide some sources for the non-expert reader to refer to. One thing that you definitely should look at is Raphael Flauger's talk (slides and video) at Princeton yesterday. I think it is this work which was the source of the "is BICEP wrong" rumours first publicly posted at Resonaances, and indeed I see that Resonaances today has a follow-up referring to these very slides.

There are several interesting things to take away from this talk. The first is to do with the question of whether BICEP misinterpreted the preliminary Planck data that they admit having taken from a digitized version of a slide shown at a meeting. Here Flauger essentially simulates the process by digitizing the slide in question (and a few others) himself and analyzing them both with and without the correct CIB subtraction. His conclusion is that with the correct treatment, the dust models appear to predict higher dust contamination than BICEP accounted for; the inference being, I guess, that they didn't subtract the CIB correctly.

How important is this dust contribution? Here there is a fair amount of uncertainty: even if the digitization procedure were foolproof, one of the dust models underestimates the contamination and another one overestimates it. Putting the two together, "foregrounds may be OK if the lower end of the estimates is correct, but are potentially dangerous" (page 40). Flauger tries another method of estimation based on the HI column density, using yet more unofficial Planck "data" taken from digitized slides. This seems to give much the same bottom line.

A key point here is that everybody who isn't privy to the actual Planck data is really just groping in the dark, digitizing other people's slides. Flauger acknowledges by trying to estimate the effect of the process of converting real data into a gif image, converting that into a pdf as part of a talk, somebody nicking the pdf and converting it back to gif and then back to useable data. As you can imagine, the amount of noise introduced in this version of Chinese Whispers is considerable! So I think the following comment from Lyman Page towards the end of the video (as helpfully transcribed by Eiichiro Komatsu for the Facebook audience!) is perhaps the most relevant:
"This is, this is a really, peculiar situation. In that, the best evidence for this not being a foreground, and the best evidence for foregrounds being a possible contaminant, both come from digitizing maps from power point presentations that were not intended to be used this way by teams just sharing the data. So this is not - we all know, this is not sound methodology. You can't bank on this, you shouldn't. And I may be whining, but if I were an editor I wouldn't allow anything based on this in a journal. Just this particular thing, you know. You just can't, you can't do science by digitizing other people's images."
Until Planck answers (or fails to definitively answer) the question of foregrounds in the BICEP window, or some other experiment confirms the signal, we should bear that in mind.

There are some other issues that remain confusing at the moment: the cross-correlation of dust models with BICEP signal doesn't seem to support the idea that all the signal is spurious (though there are possibly some other complicating factors here), and the frequency evidence — such as it is — from the cross power with BICEP1 also doesn't seem to favour a dust contaminant. But all in all, the BICEP result is currently under a lot of pressure. Having seen this latest evidence, I now think the Resonaances verdict ("until [BICEP convincingly demonstrate that foregrounds are under control], I think their result does not stand") is — at least — a justifiable position.

Footnote: I should also perhaps explain that throughout my physics education I have been taught, and had come to believe, that the types of models of inflation BICEP provided evidence for (those with inflaton field values larger than the Planck scale) were fundamentally unnatural and incomplete, and that those, small-field, models that BICEP apparently ruled out were much more likely to be true. So perhaps my conscious attempts to compensate for this acknowledged theoretical prejudice could have biased me too far in the opposite direction in some previous posts!


  1. Good stuff Sesh. I think there's more to all this than meets the eye myself.

    For example, follow up on that footnote. Some types of inflation are unnatural/incomplete, and BICEP has ruled out the others? And inflation is said to be responsible for the uniformity of the CMB, and the non-uniformity? Look closely at the original frozen-star black-hole interpretation and think in terms of a "frozen" early universe that somehow undergoes a phase change. The initial expansion is very rapid, and it can look instant with gravitational-like time dilation. But inflation per se is... superfluous.

    1. No, let me clarify ...

      There are currently no models of inflation, as far as I know, that are complete. Each model has some problems, some with the naturalness of getting a flat enough potential, some with initial conditions, many with reheating the universe after inflation. Not all of these problems are necessarily insoluble, it's possible that some models can be made complete in the future. It's just that many people used to be of the opinion that the problems that small-field models faced were more amenable to solution than those of large-field models. But even on this point opinions differ, sometimes quite strongly. The BICEP result, if it holds up, would settle the argument using data rather than mere opinion.

    2. I just don't see how it can hold up, Sesh. I just don't see how a signal originating from 10¯³² seconds after the Big Bang can survive the super-hot "maelstrom" that lasted the 380,000 years until the CMB photon decoupling, which took place 13.8 billion years ago. It's like saying we can see the ripples in the cosmic jelly that prove inflation. Only the jelly has been through a blender. Twice. I don't believe it. And I don't believe in inflation any more. I believe in expansion. l believe space has to expand, and that it can't do anything else. But I now think inflation and the inflaton is a solution to a problem that doesn't exist.

  2. Ned wright of wmap commented on this in his news of the universe blog. No.

    Looks like we need to wait 3 years for the triple frequency tricept3 results.