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Some thoughts on BICEP2 and dust

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Colorful visualization of Planck data in the region of BICEP2; click on the image for an even higher-resolution version. [Credit: ESA/Planck Collaboration/M.-A. Miville-Deschênes, CNRS – Institut d’Astrophysique Spatiale, Université Paris-XI, Orsay, France]

Colorful visualization of Planck data in the region of BICEP2; click on the image for an even higher-resolution version. [Credit: ESA/Planck Collaboration/M.-A. Miville-Deschênes, CNRS – Institut d’Astrophysique Spatiale, Université Paris-XI, Orsay, France]

Late last week, researchers with the Planck cosmic microwave background observatory released the newest analysis of their data. This combined the Planck data with the BICEP2 polarization data that caused such a big stir last year. I don’t have time today to write a long piece about it, but here are some quick thoughts, to join all the other people pontificating on it.

  • As I said last year, the BICEP2 estimate of the size of the primordial gravitational wave signal was always untrustworthy. (Yeah yeah, I know, I’m being a jerk. However, I was wrong about where the problems for the BICEP2 data were going to come from.) Much as I wanted to believe it, it was twice the size of the maximum limit derived from WMAP and other cosmological data. The new Planck+BICEP2 estimates are perfectly in line with those earlier values. In other words, we’re more or less back to where we were last year at this time.
  • While the BICEP2 signal is compatible with being entirely due to cosmic dust, that’s not the same thing as saying it is all dust. It’s still just as wrong to say there are no primordial gravitational waves hiding in the data as it was to say BICEP2 proved inflation was real last year. Currently, Planck can provide an upper limit to how much gravitational wave signal, but that’s it. There has always been a chance that any primordial gravitational wave signal is too small to show up in cosmic microwave background polarization, but we have to try to look for it anyway.
  • Planck is not primarily a polarization experiment. Other dedicated observations will be able to do a better job of constraining things. As Marc Kamionkowski pointed out in his presentation to the National Association of Science Writers last fall, there’s a sweet spot in frequency where the signal from galactic dust is low enough to let the potential contribution from gravitational wave polarization shine through. Those other projects should be able to sort out what is what a lot better. (And yes, I have a feature article about those other projects in the works. It will be a little while before it appears, though.)
  • So, hold off on making big judgments for a little while yet. This is not an easy measurement to make: between the tiny size of the signal, the difficulty in extracting it from other sources, and of course the presence of galactic dust, researchers are doing something very hard.

I apologize for being telegraphic and not explaining all my reasoning. SCIENCE COMMUNICATOR FAIL.


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