Tuesday, September 23, 2014

Sound and lots of Fury: Confirmation of Gravity Wave B Polarization?

It's been a busy few months with a lot about of Sound and Fury in the particle physics and astrophysics communities,  Sterile neutrinos: Dead?  Gravitational Waves: Dead?  TeV Dark Matter: Alive??? (Well...)

My goal in this post is to hopefully calm people down. and ask science journalists to be patient for more data before drawing any strong conclusions.
As such, given the recent article from the Planck discussing dust as a possible source of the B-polarizaed modes in the BICEP2 data, I wanted to remind people that there have been analyses done by other researchers (such as Prof Richard Gott of Princeton University and Dr. Colley formerly of Princeton University) who have mostly confirmed the results from the BICEP2 team. They estimate that the value of the tensor-to-scalar ratio, r, is 0.11+/-0.04 (and hence a detection of gravity waves with only 2sigma certainty. This is less than the 7sigma certainty that BICEP2 original suggested.)

To do so, they looked at the Gaussian/Non-Gaussian nature of the data, and they found that the BICEP2 data is extremely Gaussian (as it should be if it came from gravity waves during inflation...because before inflation many researchers expect that the universe was a giant Gaussian fluctuation of extremely hot stuff.) However, Gott&Colley showed that the dust data from Planck at 353 GHz was very non-Gaussian. Hence, most of the signal that the BICEP2 team measured can't be attributed to dust, and there is 2sigma certainty that the value of r is >0.

Note that Planck last year showed that the E and TT mode data in the CMB was almost entirely Gaussian, and they showed that the initial slope of the primordial density fluctuations vs. wavenumber can be explain by generic theories of inflation. (Note that there are a lot of different inflation theories...in order to rule out the different theories we need to know quantities such as the slope of the density fluctuations and the tensor-to-scalar ratio of the B-mode polarized waves in the CMB.)

So, Prof Gott and Dr. Colley showed that it's unlikely that dust is the cause of the B-mode signal in the BICEP2 data. But in addition to that, I want to point out dust can't explain the drop in signal that BICEP2 (and KECK preliminary) measured at a multipole value of l=50. If what BICEP2 had seen was due to dust, then the signal at l=50 would have been greater than the signal at l=100. As such, both the work by Planck and the work by Gott/Colley help to somewhat confirm the BICEP2 data, however in each case, it's important to note that the certainty has dropped from 7sigma to ~2sigma.

Since there is only likely 2 sigma confirmation of gravitational waves, we are left waiting for more data in order to cross over the 5sigma threshold. Waiting is no fun, but ever worse than waiting is spending a lot of time talking about nothing but Sound & Fury. This may take a few months or a few years to sort out...who knows?  Let's keep an open mind both ways.


Update: May 2015
Planck released it 2015 data set, and there are now some tight constrains on the tensor-to-scalar ratio, r. Note also that in most Theories of Inflation, you can only generate large values of r if you also generate large values of the "running of the scalar pertubations" with k.  And Planck data sets some really tight constraints on d (n_s) / d ln(k). So, it's very likely that the BICEP2 results are not from primordial gravitational waves. However, I think that "it's just dust" is still not a sufficient answer at this point in time. We still need to quantify:  how much is due to dust?

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