Weekend plot: BICEP limits on tensor modes

 The insurgency gathers pace. This weekend we contemplate a plot from the recent paper of Michael Mortonson and Uroš Seljak:

It shows the parameter space of inflationary models in the plane of the spectral index ns vs. the tensor-to-scalar ratio r. The yellow region is derived from Planck CMB temperature and WMAP polarization data, while the purple regions combine those with the BICEP2 data. Including BICEP gives a stronger constraint on the tensor modes, rather than a detection of r≠0.

The limits on r from Planck temperature data are dominated by large angular scales (low l) data which themselves display an anomaly, so  they should be taken with a grain of salt. The interesting claim here is that BICEP alone does not hint at r≠0, after using the most up-to-date information on galactic foregrounds and marginalizing over current uncertainties. In this respect, the paper by Michael and Uroš reaches similar conclusions as the analysis of Raphael Flauger and collaborators. The BICEP collaboration originally found that the galactic dust foreground can account for at most 25% of their signal. However, judging from scavenged Planck polarization data, it appears that BICEP underestimated the dust polarization fraction by roughly a factor 2. As this enters in square in the B-mode correlation spectrum, dust can easily account for all the signal observed in BICEP2. The new paper adds a few interesting details to the story. One is that not only the normalization but also the shape of the BICEP spectrum can be reasonably explained by dust if it scales as l^-2.3, as suggested by Planck data. Another is the importance of gravitational lensing effects (neglected by BICEP) in extracting the signal of tensor modes.  Although lensing dominates at high l, it also helps to fit the low l BICEP2 data with r=0. Finally, the paper suggests that it is not at all certain that the forthcoming Planck data will clean up the situation. If the uncertainty on the dust foreground in the BICEP patch is of order 20%, which look like a reasonable figure, the improvement over the current sensitivity to tensor modes may be marginal. So, BICEP may remain a Schrödinger cat for a little while longer.

Follow up on BICEP

The BICEP2 collaboration claims the discovery of the primordial B-mode in the CMB at a very high confidence level.  Résonaances recently reported on the chinese whispers that cast doubts about the statistical significance of that result.  They were based in part on the work of Raphael Flauger and Colin Hill, rumors of which were spreading through email and coffee time discussions. Today Raphael gave a public seminar describing this analysis, see the slides and the video.

The familiar number r=0.2 for the CMB tensor-to-scalar ratio is based on the assumption of zero foreground contribution in the region of the sky observed by BICEP. To argue that foregrounds should not be a big effect, the BICEP paper studied several models to estimate the galactic dust emission. Of those, only the data driven models DDM1 and DDM2 were based actual polarization data inadvertently shared by Planck. However, even these models suggest that foregrounds are not completely negligible. For example, subtracting the foregrounds estimated via DDM2 brings the central value of r down to 0.16 or 0.12 depending how the model is used (cross-correlation vs. auto-correlation). If, instead,  the cross-correlated  BICEP2 and Keck Array data are used as an input, the tensor-to-scalar ratio can easily be below 0.1, in agreement with the existing bounds from Planck and WMAP.

Raphael's message is that, according to his analysis, the foreground emissions are larger than estimated by BICEP, and that systematic uncertainties on that estimate (due to incomplete information, modeling uncertainties, and scraping numbers from pdf slides) are also large. If that is true, the statistical significance of the primordial B-mode  detection is much weaker than what is being claimed by BICEP.

In his talk, Raphael described an independent and what is the most complete to date attempt to extract the foregrounds from existing data. Apart from using the same Planck's polarization fraction map as BICEP, he also included the Q and U all-sky map (the letters refer to how polarization is parameterized), and models of polarized dust emission based on  HI maps (21cm hydrogen line emission is supposed to track the galactic dust).  One reason for the discrepancy with the BICEP estimates could be that the effect of the Cosmic Infrared Background - mostly unpolarized emission from faraway galaxies - is non-negligible. The green band in the plot shows the polarized dust emission obtained from the  CIB corrected DDM2 model, and compares it to the original BICEP estimate (blue dashed line).

The analysis then goes on to extract the foregrounds starting from several different premises. All available datasets (polarization reconstructed via HI maps, the information scraped from existing Planck's polarization maps) seem to say a similar story: galactic foregrounds can be large in the region of interest and uncertainties are large.  The money plot is this one:

Recall that the primordial B-mode signal should show up at moderate angular scales with l∼100 (the high-l end is dominated by non-primordial B-modes from gravitational lensing). Given the current uncertainties, the foreground emission may easily account for the entire BICEP2 signal in that region. Again, this does not prove that tensor mode cannot be there. The story may still reach a happy ending, much like the one of  the discovery of accelerated expansion (where serious doubts about systematic uncertainties also were raised after the initial announcement). But the ball is on the BICEP side to convincingly demonstrate that foregrounds are under control.

Until that happens, I think their result does not stand.

Is BICEP wrong?

The BICEP claim of detecting the primordial B-mode in the polarization of the Cosmic Microwave Background was a huge news. If confirmed, it would be an evidence of gravity waves produced during cosmic inflation, and open a window on physics at an incredibly high energy scale of order 10^16 GeV. Possible implications were described in detail in some 300 papers triggered by the BICEP announcement.  But, among this understandable excitement, we have been aware that the signal has to be confirmed by other experiments before the discovery is established. Back then, Résonaances precisely estimated the chances of the signal being true at `fifty-fifty'.  It appears it's the latter fifty that's gaining an upper hand...

Barring a loose cable, the biggest worry about the BICEP signal is that the collaboration may have underestimated the galactic foreground emission. BICEP2 performed the observations at only one frequency of 150 GHz which is very well suited to study the CMB, but less so for polarized dust or synchrotron emission. As for the latter, more can be learned by going to higher frequencies, while combining maps at different frequencies allows one to separate the galactic and the CMB component. Although the patch of the sky studied by BICEP is well away from the galactic plane, the recently published 353 GHz polarized map from Planck demonstrates that there may be significant emission from these parts of the sky (in that paper the BICEP patch is conveniently masked, so one cannot draw any quantitative conclusions). Once the dust from the BICEP announcement had settled, all eyes were thus on precision measurements of the galactic  foreground. The rumors that have been arriving from the Planck camp were not encouraging, as they were not able to confirm the primordial B-mode signal. It seems that experts now put a finger on what exactly went wrong in BICEP.

To estimate polarized emission from the galactic dust, BICEP digitized an unpublished 353 GHz map shown by the Planck collaboration at a conference.  However, it seems they misinterpreted the Planck results: that map shows the polarization fraction for all foregrounds, not for the galactic dust only (see the "not CIB subtracted" caveat in the slide). Once you correct for that and rescale the Planck results appropriately, some experts claim that the polarized galactic dust emission can account for most of the BICEP signal. The rumor is that the BICEP team has now admitted to the mistake [Update: this last statement is disputed and outwardly denied].

Note that we should not conclude that there is no observable tensor modes in the CMB. Indeed, the tensor to scalar ratio of order 0.1 is probably consistent with the existing experimental data, and may be responsible for a part of the B-mode signal detected by BICEP. New data from Planck, POLARBEAR, ACTpole, and Keck Array should clarify the situation within a year from now.  However, at this point, there seems to be no statistically significant evidence for the primordial B-modes of inflationary origin in the CMB.