The story so far:

  • BICEP2 announces a detection of B modes in the cosmic microwave background (CMB) polarization on large angular scales. If this result is correct, it’s very strong evidence that inflation happened in the very early Universe and is a really big deal. But that “if” part is important: we shouldn’t place too much confidence in this result until it’s independently confirmed.
  • In the blog Résonaances, Adam Falkowski publishes a rumor that an error had been found in the BICEP2 analysis.
  • Various science news outlets pick up the story (particularly this one and this one). They ask the BICEP2 people what they think, and the BICEP2 people vehemently stand by their results.

So what are we supposed to think?

The key claim in the Résonaances post is that the BICEP2 team made an error in modeling Galactic dust. This is potentially important, as an important part of the analysis is testing to make sure that the signal seen in the data is due to the CMB and not to boring, nearby sources such as dust.


To estimate polarized emission from the galactic dust, BICEP digitized an unpublished 353 GHz map shown by the Planck collaboration at a conference.  However, it seems they misinterpreted the Planck results: that map shows the polarization fraction for all foregrounds, not for the galactic dust only (see the “not CIB subtracted” caveat in the slide). Once you correct for that and rescale the Planck results appropriately, some experts claim that the polarized galactic dust emission can account for most of the BICEP signal.

This looks to me like it might be at least partially true.

There is not a definitive map of polarized Galactic dust emission, so the BICEP team had to cobble together models of dust from different sources. They did so in several different ways: section 9.1 of their paper lists six different dust models. One of these models is based on data from the Planck satellite. It appears that they created the model using a digitized image of a slide from a talk by the Planck people, because the relevant data hadn’t been released in any other form. (Footnote 33 of the paper is the evidence for this last statement, in case you want to check it out.) The evidence does seem to me to support Falkowski’s statement: the image in question explicitly says “not CIB subtracted,” meaning that the data that went into that image includes other stuff besides what the BICEP team wanted. This does seem like a flaw in the construction of this particular model.

But it seems to me that Falkowski greatly overstates the significance of this flaw. For one thing, this is just one of six dust models used in the analysis. It was regarded as in some sense the “best” of them, but the more important point is that the other models yielded similar results. The BICEP team’s claim, as I understand it, is that the entire analysis, taking into account all the models, makes it implausible that dust is the source of the signal. Even if you throw out this model, I don’t think that that claim is significantly weakened.

As I’ve said before, I don’t think that the BICEP team has made a thoroughly convincing case that what they’ve seen can’t be foreground contamination. I think we need more data to answer that question. But even if Falkowski has correctly identified an error in the analysis, I don’t think that it changes the level of doubt all that much.

In the past, I’ve found Résonaances to be a good source of information, but I can’t say I’m thrilled with the way Falkowski handled this.

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Ted Bunn

I am an associate professor of physics at the University of Richmond. In addition to teaching a variety of undergraduate physics courses, I work on a variety of research projects in cosmology, the study of the origin, structure, and evolution of the Universe. University of Richmond undergraduates are involved in all aspects of this research. If you want to know more about my research, ask me!

2 thoughts on “Rumors”

  1. “I don’t think that the BICEP team has made a thoroughly convincing case”

    True. However, I think it is also pretty clear that their result is not based mainly on a stupid mistake.

  2. “I don’t think that the BICEP team has made a thoroughly convincing case”

    True. However, I think it is also pretty clear that their result is not based mainly on a stupid mistake.

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