Friday, March 14, 2014

"A major discovery", BICEP2 and B-modes

[Added note (on Monday): Well, wow, the rumours were, if anything, understated. I'm happy to go on record that, unless a mistake has been made, this is the greatest scientific discovery of the 21st century, and may remain so even once the century is over. I (and others) will write many more detailed summaries of what was observed over time, but BICEP2 have announced a discovery of primordial B-modes, which is extremely strong evidence of cosmological inflation (if it turns out to be scale invariant, inflation is as true as most accepted science). Matt Strassler has a good hastily written summary here. As does Liam McAllister at Lubos Motl's blog, here. Of course, this is just one experiment and maybe they've made a mistake, but the results look very robust at the moment.

Congratulations on being alive today readers! We just learned about how particles work at energies \(10^{13}\) times greater than even the LHC can probe, and about what was happening at a time much, much less than a nanosecond after the beginning of the Big Bang.]


[Added note (on Sunday): It seems highly probable that these rumours are essentially true. Although the precise details of the results aren't yet public, the BICEP2 PI, John Kovac, has sent a widely distributed email with the following information: Data and scientific papers with results from the BICEP2 experiment will go public and be viewable here at 2:45pm GMT on Monday. At the same time a technical webcast will begin at this address.

It's going to be an exciting day!]


The cosmology rumour mill exploded today. Harvard Astrophysics have issued a press release stating that, on Monday, they will announce a "major discovery".

This is the only hard-evidence of anything interesting on the way and it could be an announcement of anything that fits under the label of "astrophysics". This is important to keep in mind. However, for one reason or another (that is hard to nail down), cosmologists are suggesting that it is going to be about cosmology. The speculation is that it will be about the BICEP2 experiment, which has been measuring the polarisation in the CMB. The speculation is that BICEP2 have seen primordial "B-mode" polarisation.

If this speculation is true, this would be a result immense in its significance.

Primordial B-modes would be a smoking gun signal of primordial gravitational waves. This, alone, makes such a discovery important. Gravitational waves have not yet been observed, but are a prediction from general relativity. Therefore, such a discovery would be on the same level of significance as the discovery of the Higgs particle. We were almost certain it would be there, but it is good to finally see it.

However, the potential significance of such a result goes further because these primordial gravitational waves would need a source. The theory of cosmological inflation would/could be such a source. Inflation is a compelling theory, not without some problems, for how the universe evolved in its very earliest stages. If it occurred when the universe had a large enough temperature, it would generate primordial gravitational waves large enough to tickle the CMB enough to make these B-modes visible in the polarisation. As of yet, inflation has passed quite a few observational tests, but nothing has been seen that could be described as smoking gun evidence. A spectrum of primordial gravitational waves would very nearly be such a smoking gun. If the spectrum was scale invariant (i.e. if the gravitational waves have the same amplitude on all distance scales) that would be a smoking gun for inflation and accolades, Nobel Prizes, etc, etc, would flow accordingly.

All of this is just speculation, but some of it does seem to be coming from reputable sources. And some of my colleagues have been talking about tip-offs from people who wish to remain anonymous, so I figured I'd collect all the speculation I know of here in a post (let me know if I've missed anything):

The PI of BICEP2, John Kovac, gave a talk at the annual COSMO conference last year that had some pretty ambitious claims for how sensitive BICEP2 and similar experiments were going to be, so... well... we'll know on Monday. It should also be noted that, although the existence of these gravitational waves is a prediction of inflation, their amplitude is a free parameter and an amplitude this big is potentially a little surprising (for me, lower temperature inflation models just seem more compelling, others might disagree).

Twitter: @just_shaun

[Edit: The video of John Kovac's talk can be found here]


  1. Thanks for sharing the excitement! ;-)

    Prof Liam McAllister of Cornell who is a leading inflationary theorist, at least among the young big shots, wrote a very clear and far-reaching (in my opinion) blog post about the implications of the discovery:

    1. Cool, thanks Lubos. I've added a sentence about this in my final edit. Sharing the excitement is almost an understatement. I'm still somewhat in shock that this has actually happened. We all woke up on Friday with no idea this was coming.

    2. Exactly, Shaun, about Friday.

      I can't resist repeating my ex-adviser Tom Banks' comments about the emotional experience from the day when Polchinski discovered D-branes (I hope it's what it was about). The world has changed.

      Now, since Friday, the world has changed again. On Friday morning, I would respond to Sam, @qftme who heard about the rumor before most of us and who asked "what is the buzz with the B-modes" by some review of some older observations of "gravitational lensing" B-modes etc. which implicitly said "nothing interesting is happening, just review all the old stuff". It's no longer true. Something is happening.

      There's this new more-than-Higgs degree of surprise. First, the B-modes seem stronger than it was believed by almost anyone, including those who would expect them, and stronger than limits (the tension with Planck drops from 3 sigma to 1 sigma if BICEP2 allows the running spectral index, a detail that will probably be increasingly discussed by the experts).

      And the observation was also unexpected because almost everyone would bet that Planck would be the first experiment to detect such B-modes if they exist. Planck may have been the single most likely experiment - even this may actually be untrue and people familiar with the details of BICEP2 etc. could have known better - but even if it were the single most likely experiment, it's far from making it very likely that it must actually be the experiment that makes the discovery. Others have a chance, too - that's why they were paid for, I hope. ;-)

  2. "... unless a mistake has been made, this is the greatest scientific discovery of the 21st century ..." I think the space roar is more important. What might be the explanation for the space roar?

    1. Thanks for the comment David. To be honest, I''d never even heard of the space roar. The results from Google suggest it might turn out to be pretty interesting, who knows. The reason I would still argue that, at the moment, a primordial B-mode detection would be the greatest 21st Century discovery is that we already have a possible interpretation for the cause of such a signal and it is those implications that make it such a great discovery.

      Sure, by the end of the century, the space roar might turn out to be suggesting something even bigger, but at the moment we don't know, so it could be much more mundane. It's definitely worth keeping an eye on though, so thanks for bringing it to my attention.


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