BICEP2: A Month Later

A month ago the BICEP2 team announced that our universe is apparently awash with gravitational waves, pointing to the existence of an inflationary phase moments after the Big Bang. This was front page news all over the world, and cosmologists and astrophysicists have been working overtime to make sense of the news. Here is some of that sense…   


Let The Ambulance Races Begin 

For theoretical physicists, ambulance chasing involves getting papers out quickly after a major data release. Some ambulance chasers make significant contributions, some are just trying to draw attention to their earlier work, while others are banging out insubstantial papers in the hope that they will be cited by their slower colleagues. But whatever their motives, cosmologists have certainly been busy: the BICEP2 discovery paper has been cited 188 times on the Arxiv, all in “preprints” written within a month of the original announcement. I am pretty sure this is a world record, and you can always check the current tally.

In fairness, though, cosmologists were so giddy about BICEP2 it wouldn’t have surprised me if someone had stolen an ambulance and driven it in circles, flashing the lights and letting rip with the siren. 


Distributed Peer Review and Open Science

Once upon a time, the right way to announce a big result was to 1) write the paper, 2) send it to a journal, wait for it to be 3) peer reviewed and 4) accepted for publication, after which you could 5) hold a press conference. However, like most recent announcements in fundamental physics and cosmology, BICEP2 went straight from paper to media event, skipping steps 2, 3 and 4.

Old-timers will shake their heads, but this approach fits the principles of open science, which advocates making the processes and products of science transparent and widely available. Given that 1000 scientists are now scrutinising the BICEP2 results, rather than just two or three readers appointed by a journal, this amounts to an intensive, distributed and open peer review process, which is no bad thing. (And the papers will end up in a journal sooner or later.)


Trouble in Paradise? 

The real gold-standard for science is not peer review but reproducibility. BICEP2 claims to have detected a specific twist in the polarization of the microwave background — the so-called “B-mode”. This detection will not be a sure thing until it is confirmed by an independent team with an independent instrument performing an independent analysis. On top of that, inflation is not the only possible origin of such a B-mode, and further data will help confirm the theoretical interpretation of the BICEP2 observations. 

The good news is that no-one has found any show-stoppers. The biggest worry to surface so far is probably that the patch of sky BICEP2 observed may be contaminated by emission from radio “loops” associated with our own galaxy. It is not clear to me that this signal would necessarily reproduce the BICEP2 result, but unsubtracted foregrounds are likely to make any underlying gravitational wave signal look bigger than it really is, and that will need careful checking. And in the worst-case scenario, the BICEP2 results would be purely due to foregrounds, or some other analytical glitch.

We may not have to wait long. The BICEP2 team will be looking closely at these concerns, and more data will be gathered during the coming polar night. In addition, the Planck satellite has gathered the world’s most comprehensive observations of the microwave background and their science team is extending their initial analysis to look at polarization, with results promised before the end of 2014. 


Free Trips to Stockholm

If the BICEP2 result is verified, it is certain to attract the attention of the Nobel committee. In fact, it may be worth two Nobel prizes – one for the idea of inflation, and one for the detection of B-modes, which is a technological tour de force in its own right. (Two prizes have already gone to the microwave background — one for its discovery, and one for the first mapping of the temperature of the microwave background.) 

Speculating about “the prize” is a popular game among scientists, and I have already heard people ruminate about the likely judgment of history if it turns out that the BICEP2 analysis is  basically correct but slightly dust-contaminated. In this scenario, the BICEP2 announcement would have been made with far more confidence than the data ultimately justified, which would provide conversational fodder for decades. 

The intellectual history of inflation has many parallels with that of the the Higgs boson; they are both elegant hypotheses that existed for decades before being experimentally confirmed (assuming, again, that BICEP2 really has seen evidence of inflation). And like the Higgs, the theoretical parentage of inflation is murky. Alan Guth is undoubtedly the Peter Higgs of inflation (even if it is not called “the Guth phase”), but a number people made key contributions to the development of the theory. Unfortunately, only three of them can share the Prize, and there will be discreet (and probably blatant) lobbying for the other two places on the stage if the BICEP2 data holds up. 


What I Have Been Doing?

Beyond giving a slew of interviews the day the story broke, my group at the University of Auckland (in collaboration with Kevork Abazajian at UC Irvine) has looked carefully at the apparent tension between BICEP2 and existing cosmological data. BICEP2 does not just claim to have seen gravitational waves, but to have seen gravitational waves with an amplitude which was apparently ruled out by previous analyses.

We crunched a lot of numbers very quickly, thanks to the high performance computing facilities at NeSI (New Zealand’s e-research organization), and showed that this tension between BICEP2 and previous analyses is statistically significant. Consequently, taking all currently available astrophysical datasets at face value, BICEP2 appears to tell us three startling things about the early universe:

  1. Inflation really did happen right after the big bang.
  2. Inflation happened when the energy density of the universe was very high, as the strength of the gravitational wave background depends directly on the energy density of the universe during inflation. This means that the mechanism of inflation can give us a portal into the realm of ultra-high energy physics, where we expect candidate “grand unified theories” (including string theory) to be important. 
  3. The inflationary phase must be relatively complex, for the gravitational wave background to have escaped indirect analyses made prior to BICEP2. And this means that cosmologists will be able to make far more stringent tests of competing inflationary models than we might have expected.

Alternatively (and much more conservatively!) our results could suggest that the BICEP2 team has over-estimated the strength of the gravitational wave background and that future analyses will remove this discrepancy. 


One More Thing

To me, one of the most astonishing things about the BICEP2 telescope is just how small it is. The secret to BICEP2 is not its size, but the exquisitely sensitive superconducting transition edge sensors used to detect the microwave signal. Admittedly, BICEP2 sits at the South Pole, the whole instrument is chilled to within a hair’s-breadth of absolute zero (a major technological and logistical challenge) and it is surrounded by a complex array of shields, but the actual telescope is 23cm across. This is only a few times larger than the optical instrument Galileo used to explore the heavens over 400 years ago, and BICEP2 may one day rival Galileo in the profundity of its implications for our place in the universe.

BICEP2, to scale - 

BICEP2, to scale – 

3 thoughts on “BICEP2: A Month Later

  1. Your "current tally" link points to your publications, not the BICEP2 paper!

    1. Thanks for catching that — had way too many windows open on INSPIRE. Fixed now.

  2. Excellent to understanding BICEP2 contribution in our approach to the universe knowledge. Witty comparison with the Galileo’s telescope.

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