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Mandeep Gill | |

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What BICEP2 Got Right

The region of the sky in which BICEP2 polarization, shown as dotted lines over Planck data. (Credit: ESA/Planck Collab. M.-A. Miville-Deschênes, CNRS, Univ. Paris-XI)

The region of the sky in which BICEP2 polarization was seen; the region is shown as dotted lines over Planck data. (Credit: ESA/Planck Collab. M.-A. Miville-Deschênes, CNRS, Univ. Paris-XI)

There has been a lot made of the fact that Planck and BICEP2 jointly released a paper last week saying that BICEP2’s original claim of a discovery of proof that the very early universe expanded mindbogglingly quickly – something that’s called inflation – was “wrong.”

[Note here I’m not going to go over any of the physics or terminology about the experiment in this post, there are other great sources for that. I compiled a list of them here back in March 2014.  This particular post is largely discussing the sociology of how one goes about releasing results like this, and then the discourse in the community that goes on afterward.]

A lot of the way the press has breathlessly reported this result has left me feeling a bit discombobulated because the version of the paper that BICEP2 actually submitted to a journal last year had already walked back strongly a lot of the claims of their initial announcement just two short months previous.  The team had clearly started seeing hints in others’ Milky Way Galaxy (MWG) radio loop data, and hearing from others who were expert with the possibilities that unusual dust properties might explain most of the “B-mode polarization” signal they had seen.  So they did the prudent thing and backed away from their initial claims in March 2014.  For the record, I want to quote verbatim the last few sentences of that paper (this is from the last version, the one from June 2014):

“We have pushed into a new regime of sensitivity, and the high-confidence detection of B-mode polarization at degree angular scales brings us to an exciting juncture. If the origin is in tensors, as favored by the evidence presented above, it heralds a new era of B-mode cosmology. However, if these B modes represent evidence of a high-dust foreground, it reveals the scale of the challenges that lie ahead.”

In other words, although they very much hoped and thought they were justified in their analysis from all everyone knew about MWG dust properties previously, they also were quite well aware already that the signal they had detected might be sourced mostly from anomalous dust, more than 7 months before today.

Before going further, I want to note for full disclosure that I say all this as someone who has a number of friends on the B2 (BICEP2) team, all the way from grad students and postdocs to more senior folk.  They are all upstanding and hardworking scientists.  And they have definitely endured some harsh criticism from various not-so-softly-spoken colleagues, publicly.  It’s not my place to say how the entire back and forth should’ve been conducted, except I would argue for civility as much as possible always in all spheres, even for very charged issues. (President Obama always pushes for as well, too, btw.)

Along these lines, I want to make very clear that I do not at all like the phrasing “B2 was totally wrong!!!” — because they did see B-modes in a region of the CMB power spectrum that people never had, and this was and is a major advance, and constitutes the largest part of their published paper.  Where they stretched was in not believing that those B-modes could be fully explained by anomalous dust, which is what the more recent results are pretty clearly demonstrating (but that doesn’t, and can’t, rule out primordial B-modes from inflation at a smaller level still, underneath).

So yes, it can’t have felt very good to be any member of the team after there were big celebrations and they seemed so sure. But the B2 observation was totally consistent with all physics we understand currently, and there was no fundamental reason they could not have been right.  Just turns out that interstellar dust apparently can exist with very different properties than we’re accustomed to.

At the same time — I will say: if you’re going to go out there and make claims like that, you do have to be prepared for the fallout.

Should they have not gone out and made a big splash?  Maybe.  But they were very sure of their signal, they spent over a year crosschecking it in every possible way they could think of — it’s just that they just reached too far for the interpretation, assuming too much about the dust being ‘normal.’  And once the evidence started piling up against that interpretation, they very rapidly started backtracking, and coordinating with others to crosscheck their interpretation of the data, vs. sticking to their guns.  That is indeed the way real science always works, and moves forward.

So yes — they overreached.  But they were not wrong in their observations, and that is the most critical part of observational and experimental science, indeed, I would submit.

That is how we most collaboratively and collegially get to the truth of the Universe.

 

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