B-modes? r=0.2 B-modes.

That is the question.

So it’s an exciting day, today! The cosmology community is a-buzz with rumours that BICEP2/KECK will announce the first detection of primordial B-mode polarisation of the CMB in a press conference this afternoon (technical presentation starting 3.45pm CET). Shaun Hotchkiss has a great summary of some of the coverage in the blogosphere, and the Guardian even jumped the gun a bit and posted an article on Friday.

The rumours are by now pretty convincing – there was some minor speculation that they might be announcing the discovery of an exo-moon instead (boooring), but it’s now been confirmed that the press conference is definitely about the CMB. So, some (brief) talking points, then:

  • The rumour is r=0.2 (where r is the scalar-tensor ratio). This is significantly higher than most people expect, and it seems to be in conflict with the Planck inflation mega-plot from early 2013. Not necessarily, though, if you allow the scalar spectral index to run. So which inflationary models would this kill? Will the poor Encyclopaedia Inflationaris guys have to redo everything?
  • Are primordial gravitational waves really a “smoking gun” for inflation? Here’s a relatively vitriolic review by Grishchuk for the entertainment of the contrarian in your life.
  • Foregrounds! Systematics! Here in Oslo, Hans Kristian Eriksen has a 1000 NOK bet running that the accepted value of r will be much lower (inconsistent with r=0.2 at 3 sigma) in 5 years’ time. This polarisation business is tricky, you know. There are also rumblings that the Southern Hole, which is the region of low galactic dust emission that BICEP2 is apparently looking through, might have higher-than-expected polarised foregrounds after all. Does this matter?
  • If it turns out that we need n_run to be non-zero for things to make sense, then we’ve added another parameter to the oh-so-simple, six-parameters-is-all-you-need standard model. I can almost hear the floodgates opening.
  • Of course, everyone should play BICEP2 Bingo during the press conference (by Joe Zuntz).

Here’s hoping for a fireworks-filled afternoon!

About Phil Bull

I'm a theoretical cosmologist, currently working as a NASA NPP fellow at JPL/Caltech in Pasadena, CA. My research focuses on the effects of inhomogeneities on the evolution of the Universe and how we measure it. I'm also keen on stochastic processes, scientific computing, the philosophy of science, and open source stuff. View all posts by Phil Bull

2 responses to “B-modes? r=0.2 B-modes.

  • Phillip Helbig

    “If it turns out that we need n_run to be non-zero for things to make sense, then we’ve added another parameter to the oh-so-simple, six-parameters-is-all-you-need standard model. I can almost hear the floodgates opening.”

    Although I do detect the tongue in your cheek, even so, this is a bit cynical. Yes, 6 numbers can describe a lot and, yes, 6 numbers continued to be enough as more observations came in. But the six numbers had no input from polarization data, and thinking that a running index is strange is like thinking it is strange that Omega is not 1 and lambda is not 0.

    • Phil Bull

      Yes, although I guess it depends on your theoretical prior too. A lot of people seem to think that a running spectral index is ugly/complex, and would prefer it not to be there.

      And remember that EE polarisation data does matter! The value of Tau (optical depth to last scattering) is mostly driven by TE/EE, I believe. This has a knock-on effect on n_s too.

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