Curly impressions

Today the BICEP2 experiment announced a significant detection of the primordial B-model in the CMB power spectrum (the excess at lower multipoles in the plot). From that one can infer that the tensor-to-scalar ratio of primordial fluctuations is somewhere between 0.1 and 0.2. Cosmologists are strongly represented in the blogosphere, so for the description of technical aspects of the BICEP results and their impact on the models of inflation better see elsewhere, for example here or here or here.  In this post, let me throw in a few random impressions from a particle physicist.  

• 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 this holds up" is the central question now. This sort of  experiments is subject to pesky instrumental effects and systematic effects due to foreground emission.  It's not impossible that BICEP screwed up;  in fact, experts point out some worrying aspects of the data, for example the excess in the BB spectrum at high multipoles. So I would say at this point it's fifty-fifty.  Fortunately, there are many experiments  out there with similar sensitivity (Planck, ACTPole, SPT, POLARBEAR) that should be able to  confirm or refute the claim in the near future. In particular, the release of Planck polarization data this year should straighten many things out. For the sake of this post I'll drop the conditional, but there really should be "If this holds up" in front of every sentence below. 
• The big thing here is not gravitational waves (we observed them before), and not an evidence for inflation (we've already had a few from the CMB alone:  the temperature isotropy across the sky, the scalar fluctuations, the spectral index). The point is that the amplitude of the primordial tensor modes is directly related to the energy density during inflation. This turns out to be (2*10^16 GeV)^4 -- a whopping energy scale unavailable to particle accelerators in this century. Consequently, the Hubble scale during inflation has also been nailed down, and it's 10^14 GeV. While cosmologist  study the universe when it was being born, particle physicists are getting a glimpse of  physics at a very high-energy scale. 
• The proximity of the inflation scale to the unification scale in minimal supersymmetry is certainly intriguing. It may be a numerical accident.... but I'm sure that there will soon be models on arXiv  "predicting" this coincidence. 
• The plot shows the basic parameters of inflation as of today. 
  
   If you think that the Planck region is not what you remember, that's right, it's not the one usually shown. The red region in this plot is the Planck constraint when the spectral index ns is allowed to run, that is to say, to depend on the distance scale. It is a challenge for  inflationary models to get the required amount of running. In the more likely scenario with small running the BICEP result is in about 3 sigma tension with the Planck constraints.   
• There's more challenge for inflation model building. In the single-field inflation one can relate how much the inflaton field was displaced during inflation to the value of r and the number of e-folds (the so-called Lyth bound). For r~0.2 one finds that the displacement is larger than the Planck scale. For particle physicists, who generally like effective field theories,  the focus now will be on identifying all exceptions from this rule. But from another (e.g. string-theoretical) point of view this is an opportunity: may be inflation can be our probe of transplanckian physics? In the coming week there will surely  be an arXiv flood on both  of these fronts.  
• Speaking about model building, Higgs inflation is ruled out, at least in the current version. A robust prediction of Higgs inflation is no tensor modes at an observable level. In other words, we have a new evidence for new physics beyond the Standard Model. 
• It is worth remembering that the gravitational waves during inflation is the most plausible but not the unique explanation of BICEP results. For example, an early  phase transition or decay of massive particles during inflation may also  generate tensor perturbations. That's another model building direction worth following in the coming weeks.   
• 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.  
• One more thing we learned from the BICEP announcement: mankind can study the universe moments after the bing bang, but setting up an internet connection is a totally different story ;)  

The BICEP2 paper is here, for more material see this page.