Dust or not?

Following the recent rumor, some more useful information has been coming out about questions that some people are raising about whether the BICEP experiment really has seen signs of gravitational waves from inflation in the polarization of the cosmic microwave background radiation. The Washington Post has by far the best news article I’ve seen on the subject: it actually quotes people on the record, rather than repeating vague anonymous speculation.

The original rumor seems to be generally true, in the sense that it accurately described some criticisms that cosmologists were making about the BICEP analysis. The rumor does seem to have exaggerated and/or oversimplified things, and of course whether those criticisms are valid or not remains to be seen.

The best place I know of to get the technical details is this talk by Raphael Flauger. (Unfortunately, the video doesn’t show the slides as he’s talking, so if you want to follow it, download the slides first and try to follow along as he talks.) He argues that the dust models used by the BICEP team are inaccurate for a few reasons, mostly having to do with problems associated with the reason in the original rumor: the BICEP team appears to have used an image in a slide from a talk for part of their model, and they seem (he claims) to have misinterpreted what was in that slide. In addition (he claims), there are other errors associated with digitizing the image rather than using the real data (which BICEP doesn’t have access to). Flauger further claims that when you use a different (better?) dust model, the possible contribution of dust to what BICEP saw gets significantly larger, possibly large enough to explain their signal.

If BICEP has offered a detailed, technical rebuttal to this criticism, I haven’t seen it yet.

My personal assessment, based on obviously incomplete information: Flauger’s arguments seem to me to need serious consideration. BICEP needs to supply a detailed response. As of now, I don’t know whether he’s right or not, but my view has changed somewhat since the original rumor. The available information now does seem to me sufficient to substantially lower my own estimate of the probability that BICEP has seen primordial gravitational waves. I was fairly skeptical all along, but now I’m more skeptical. If you must know, I’d put the probability significantly below 50%.


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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!

3 thoughts on “Dust or not?”

  1. Is there a legitimate reason BICEP should not have access to the data used to generate the figure they scanned? Or is this just a ridiculous failure to meet reasonable expectations of data sharing?

  2. It is not uncommon for there to be a period of time after data are gathered in which those who did the work get a chance to do something with their own data. If all data were made public immediately, the observers and instrument builders would get a citation and the glory would go to someone who sniped the data as soon as they became available. This also has the danger that he perhaps doesn’t completely understand the data. So, I think it is par for the course that Planck hasn’t released all of their data yet.

    It is also common to show preliminary results in talks. One can’t complain to Planck if someone (mis)uses such preliminary data.

    What has changed is that it has become easier to get some representation of the preliminary data (i.e. digitizing a slide).

  3. “The Washington Post has by far the best news article I’ve seen on the subject”

    Perhaps. There are a couple of blunders, though. First, a casual reader could confuse a South Pole Telescope (that of BICEP2 which, unusual in these modern times, is a refractor—and a small one) with the South Pole Telescope (which is a big reflector within sight of the BICEP2 site).

    Second, this (ostensibly direct) quote is a bit dodgy: “What inflation does is pull apart the fabric of space-time much faster than the speed of light”. While it is true that one of the interesting things about inflation is that it can remove objects from each others backward light cones (which presumably is what she meant), inflation is best defined as “rapid exponential expansion in the early universe”. Not all points separate faster than the speed of light, and it is possible for two points to separate faster than the speed of light in the non-inflationary phase as well.

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