Combined constraints from BICEP2, Keck, Planck and WMAP on primordial gravitational waves

This week, the joint analysis of BICEP2 (+ BICEP2's successor Keck) and Planck has finally arrived. The result is more or less what was expected, which is that what BICEP2 saw last year in the B-mode polarisation signal of the CMB was not actually primordial gravitational waves (as had originally been hoped and claimed), but was unfortunately actually due to dust in the Milky Way. Such is life. Though we did of course have the best part of a year to come to grips with this reality.

Combined constraint on \(r\) from polarisation and temperature measurements (in blue). Freshly digitised in the spirit of modern cosmology. Gives \(r\lesssim 0.09\) at \(95\%\) confidence.

As a result of subtracting the dust component in BICEP2/Keck's signal (obtained by comparing the measurements from BICEP2/Keck and Planck), the final constraint on the "tensor to scalar ratio" (or \(r\)) from the BICEP2/Keck measurement is that \(r<0.12\) at \(95\%\) confidence. This \(r\) parameter essentially measures the amplitude of a primordial gravitational wave signal, so the net result is that the subtraction of dust takes BICEP2's high significance measurement of non-zero \(r\) and converts it into simply an upper bound.

I've seen some comments on blogs, in media, on Twitter, etc that there is still evidence of some sort of excess signal in BICEP2/Keck over and above the dust, but I can't see any evidence of that in any of their published results. The final likelihood for \(r\) (shown above in black) shows a plot consistent with \(r=0\) at less than \(1-\sigma\) (i.e. \(r=0\) is less than one standard deviation away from the maximum likelihood value). In fact, it would seem that the measurement of the dust that has been obtained by comparing BICEP2/Keck's measurements with Planck's measurements has been so good that the B-mode constraint on \(r\) from BICEP2/Keck is now competitive with (or even slightly better than) the constraint arising from temperature measurements of the CMB. This was always going to happen at some point in the future and it seems that this future has now arrived.

Comparing Planck's noise and dust to BICEP2

In case anyone reading this doesn't recall, back in March an experiment known as BICEP2 made a detection of something known as B-mode polarisation in the cosmic microwave background (CMB). This was big news, mostly because this B-mode polarisation signal would be a characteristic signal of primordial gravitational waves. The detection of the effects of primordial gravitational waves would itself be a wonderful discovery, but this potential discovery went even further in the wonderfulness because the likely origin of primordial gravitational waves would be a process known as inflation which is postulated to have occurred in the very, very early universe.

The B-mode polarisation in the CMB as seen by BICEP2. Seen here for the first time in blog format without the arrows. Is it dust, or is it ripples in space-time? Don't let Occam's razor decide!

I said at the time, and would stand by this now, that if BICEP2 has detected the effects of primordial gravitational waves, then this would be the greatest discovery of the 21st century.

However, about a month after BICEP2's big announcement a large crack developed in the hope that they had detected the effects of primordial gravitational waves and obtained strong evidence for inflation. The problem is that light scattering of dust in the Milky Way Galaxy can also produce this B-mode polarisation signal. Of course BICEP2 knew this and had estimated the amplitude of such a signal and found it to be much too small to explain their signal. The crack was that it seemed they had potentially under-estimated this signal. Or, more precisely, it was unclear how big the signal actually is. It might be as big as the BICEP2 signal, or it might be smaller.

Either way, the situation a few months ago was that the argument BICEP2 made for why this dust signal should be small was no longer convincing and more evidence was needed to determine whether the signal was due to dust, or primordial stuff.