mardi 13 mai 2014

Pas de rétractation (mais plus d'incertitude) sur une preuve expérimentale récente de l'inflation cosmologique

D'une rumeur qui enfle à une autre qui (la dé)gonfle (ou bienvenue dans l'ère de la Science Ouverte)
Il y a quelques semaines, le transcyberphysicien relayait ici le battage médiatique orchestré autour de l'annonce d'une possible découverte astrophysique importante. Peter Coles, un cosmologiste anglais, mais aussi un blogueur reconnu, a très bien parlé de cet événement: 
When the BICEP2 team announced that a “major astrophysics discovery” would be announced this Monday I have to admit that I was quite a bit uncomfortable about the way things were being done. I’ve never been keen on “Science by Press Release” and when it became clear that the press conference would be announcing results that hadn’t yet been peer-reviewed my concerns deepened.
However, the BICEP2 team immediately made available not only the “discovery” paper but also the data products, so people with sufficient expertise (and time) could try to unpick the content. This is fully in the spirit of open science and I applaud them for it. Indeed one could argue that putting everything out in the open the way they have is ensuring that that their work is being peer-reviewed in the open by the entire cosmological community not secretly and by one or two anonymous individuals. The more I think about it the more convinced I am becoming that this is a better way of doing peer review than the traditional method, although before I decide that for sure I’d like to know whether the BICEP2 actually does stand up!
One of the particularly interesting developments in this case is the role social media are playing in the BICEP2 story. A Facebook Group was set up in advance of Monday’s announcement and live discussion started immediately the press conference started. The group now has well over 700 members, including many eminent cosmologists. And me. There’s a very healthy scientific discussion going on there which may well prove to be a model of how such things happen in the future. Is this a sign of a major change in the way science is done, the use of digital technology allowing science to break free from the shackles placed on it by traditional publication processes? Maybe.
 Telescoper (alias Peter Coles), Blog In the dark, Bicep2, Social Media and Open Science 14/05/2014

Dans un autre de ses billets il discutait aussi de façon précise et claire les incertitudes qui pèsent sur la véracité de la découverte en question, ou plus précisément sur l'origine cosmologique du signal mis en évidence par l'expérience Bicep2. Mais les discussions techniques autour des incertitudes expérimentales se déroulaient "à bas bruit" semble-t-il dans la blogosphère jusqu'à ce que la situation change il y a trois jours, suite à la publication d'un billet d'Adam Falkowski, physicien des particules (et blogueur "fou" alias Jester ;-) qui prétendait relayer une rumeur selon laquelle les chercheurs de la collaboration Bicep2 auraient reconnus avoir commis une erreur dans l'évaluation d'un signal parasite. 

La parole à un scrutateur sceptique (lanceur d'alerte ou diffuseur de rumeur?)
Voilà ce que dit entre autre chose le billet en question: 
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.
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.
... 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.
 Jester (alias Adam Falkowski), Blog Résonaances Is BICEP wrong? 12/05/14

La réponse des scientifiques scrutés (chercheurs de vérité ou promoteurs de découverte prématurée?)
Le billet un peu "poil à gratter" de Jester a tout de suite suscité beaucoup de réactions et l'intérêt des média scientifiques populaires anglo-saxons qui ont fait réagir les chercheurs dont le travail était remis en question:
Clement Pryke, a cosmologist at the University of Minnesota, Twin Cities, and a co-principal investigator for the BICEP team, acknowledges that the foreground map is an important and thorny issue. Part of the problem is that the Planck team has not made the raw foreground data available, he says. Instead, BICEP researchers had to do the best they could with a PDF file of that map that the Planck team presented at a conference.
... The BICEP team will not be revising or retracting its work, which it posted to the arXiv preprint server, Pryke says: "We stand by our paper."

On 12 May, a rumour emerged on the physics blog Résonaances that the BICEP2 team has already admitted defeat. The blogger, particle physicist Adam Falkowski at CERN, says he has heard through the scientific grapevine that the BICEP2 collaboration misinterpreted a preliminary Planck map in its analysis.
... "We tried to do a careful job in the paper of addressing what public information there was, and also being upfront about the uncertainties. We are quite comfortable with the approach we have taken." [says principal investigator John Kovac at Harvard University]
Lisa Grossman, Rumours swirl over credibility of big bang ripple find, 13/05/2014

Au moment où ces lignes sont écrites (15 mai 11h50 heure de Paris) on apprend que le prestigieux Princeton Center for Theoretical Science organise ce jour même (de façon impromptue semble-t-il) un événement spécial pour discuter justement des dernières avancées dans la compréhension des incertitudes expérimentales incriminées :

Special Event: May 15, 2014
Towards an Understanding of Foregrounds in the BICEP2 Region
Speakers: Raphael Flauger (IAS and NYU) with discussion by Lyman Page (Princeton)
Thursday, May 15 at 9:30 am
PCTS Seminar Room, Room 407
(video recording and slides will be made available after the talk)


Bicep2 versus Planck?
En attendant de nouvelles informations scientifiques de première main sur la question on peut essayer de mettre en lumière les enjeux du débat en croisant les sources d'informations. Commençons par comparer ces deux avis de blogueurs plus ou moins habiles dans l'art de la provocation: 
... the BICEP2 experiment announced a significant detection of the primordial B-model in the CMB power spectrum... 
  • If this holds up, it's huge, comparable in magnitude to the discovery of the Higgs boson. Probably even more exciting because of the surprise element...
  • If you hear a sledgehammer in the corridor of your lab, that may be your local Planck member banging his head on the wall. Yeah, apart from many noble aspects, science also has this lowly competition side. A billion dollar experiment that misses a Nobel-prize-worth low-hanging fruit... I wouldn't wish  to be in their skin if BICEP is right. 
Jester,  Blog RésonaancesCurly impressions, 17/03/2014

The total price of the Planck satellite was €700 million, almost a billion of dollars. On the other hand, BICEP2's expenses are comparable to $10 million, about one hundred times smaller. That's why Planck is the Goliath and BICEP2 was the David ...
If your budget is 100 times smaller than the budget of someone else, it may be and feel more likely that you won't win but it simply does not imply that you can't discover something important before the Goliath does. Even though "chance" could be enough to explain all these unexpected events, the victories of the under dogs, there are also detailed reasons why BICEP2 has apparently done the discovery before Planck.
BICEP2 had a vision. They were focusing on the B-modes, assuming from the beginning that there could be something new over there. Their devices were sufficiently optimized to do the job and they have apparently succeeded in the job. In comparison, Planck has gotten into a pessimistic mode in which people assume that "they can't discover something really new, anyway" which is why they don't even try so hard.
Lubos Motl, Blog The Reference FrameBICEP2 vs Planck: nothing wrong with screen scraping, 14/05/2014

Mais laissons-là la polémique à tendance sociologique pour revenir à des intérêts plus centrés sur la connaissance scientifique.

Les résultats de Bicep2 dépendent(-ils) de ceux de Planck !(?)
Cherchons dans les réactions des lecteurs de blogs des questions, des remarques et des informations intéressantes:

Nick said... 13 May 2014 07:29 (blog Résonaances)
Is it correct then that BICEP2 was systematically unable to precede PLANCK? Surely in their methodology before getting funded someone would have asked about how they would measure the foreground, the answer would have been "We will have to wait for PLANCK!" So in this sense this very modest telescope was never designed to compete for a Nobel Prize but was always complementary to PLANCK? 

Jester said... 13 May 2014 08:29  (blog Résonaances)
Nick, my understanding is that the amount of polarized foreground in the BICEP patch is a bit of a surprise. That region is rather clean in temperature maps, apparently it is less clean in polarization. But, right, it was always clear that for a fully reliable estimation of the foregrounds we need measurements at several frequencies, and Planck is by far best suited to do that.

pion says: May 14, 2014 at 2:40 am (blog Not Even Wrong)

It seems to me that Planck is not our best hope to settle this issue mainly due to the fact that it is a satellite, and information from certain ground-based telescopes might be more credible.
Since the CMB polarization level is obtained from differencing two intensity measurements toward the same direction on the sky, any optical imperfection of the detectors can potentially leak the dominant intensity to the faint B-mode polarization. A few of these spurious signals can be potentially mitigated by the telescope’s scanning strategy; basically, each pixel in the field is observed multiple times with (ideally) different orientations of the polarimeter. Ideally, this would suppress a large fraction of the systematic but not entirely.
Most ground-based telescopes benefit from the earth rotation, others use half waveplate, and in general the scanning strategy could be optimized for minimizing the intensity-to-B-mode leakage. Satellites in orbit, however, are limited and their typical scanning strategy is sub-optimal if not poor. We know, as a fact, that Planck never published their B-maps, and I guess that this is partially due to the issue of B-mode systematics which is always a challenge, for satellites in particular...
My best bet is that a joint effort of ground-based instruments (which are located off the pole) might ultimately provide a conclusive answer to this thorny issue. The problem with this alternative, though, is that ground-based experiments are limited to a relatively narrow frequency window. Hopefully, two or three frequency bands will suffice for the construction of a reliable polarized dust model, but this is not a priori guaranteed.

Le mot de la fin à un cosmologiste encore dubitatif 
I repeat what I’ve said before in response to the BICEP2 analysis, namely that the discussion of foregrounds in their paper is disappointing. I’d also say that I think the foreground emission at these frequencies is so complicated that none of the simple approaches that were available to the BICEP2 team are reliable enough to be convincing. ... I think BICEP2 has definitely detected something at 150 GHz but we simply have no firm evidence at the moment that it is primordial. That will change shortly, with the possibility of other experiments (specifically Planck, but also possibly SPTPol) supplying the missing evidence.

I’m not particularly keen on the rumour-mongering that has gone on, but then I’m not very keen either on the way the BICEP2 result has been presented in some quarters as being beyond reasonable doubt when it clearly doesn’t have that status. Yet.

Rational scepticism is a very good thing. It’s one of the things that makes science what it is. But it all too easily turns into mudslinging. 

Telescoper (alias Peter Coles), Blog In the dark, That BICEP Rumour…, 14/05/2014

Conclusion et morale pour cette histoire
L'annonce très médiatique du premier signe expérimental de l'existence d'ondes gravitationnelles primordiales et de la preuve de la véracité de l'inflation cosmologique était forcément un peu gonflée ...
Le transcyberphysicien, 15/05/2014

Voilà pour la conclusion en forme de boutade; quant-à la morale, il en est une qui s'impose encore et toujours, chaque fois du moins qu'une découverte scientifique extraordinaire est annoncée publiquement:  
Extraordinary claims require extraordinary evidence
Des affirmations extraordinaires exigent des preuves extraordinaires 
Carl Sagan Cosmos,12 - Encyclopaedia Galactica, 14/12/1980

//La rédaction de ce billet a été achevée le jeudi 15/05/2015

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