The rumor mill has once again turned its wheels a few cogs to throw out some new grist for physicists and cosmologists. This is following the announcement last Wednesday of an announcement this Monday in which the Harvard-Smithsonian Center for Astrophysics will reveal to the world a “Major Discovery”. The best available rumours now say that astrophysicsts working with the BICEP observatory in Antartica will reveal the discovery of primordial gravitational waves in the cosmic microwave background. If true this would be a very big deal indeed because it could be a direct experimental hook into the physics of inflation and even quantum gravity. These are of course the least well understood and most exciting unchartered waters of fundamental physics. Any observation that could provide phenomenology for these areas would be the greatest empirical discovery for the foundations of our universe for decades.
Before going any further it is worth recalling that we have tuned into some webcast announcements recently only to be disappointed that the expected discovery comes in the form of a negative result setting new limits on what we wanted (e.g. LUX, AMS-2 etc) This could turn out to be another case of the same but if so the “Major Discovery” tag will be pointed out as a major piece of over-hype. It is also possible that the announcement has nothing to do with what the rumors say. but that looks increasingly unlikely at this point. So let’s try to understand a little better what it might be about.
The Cosmic Microwave Background has been mapped out in exquisite detail by a series of space and Earth-based observatories including the European Planck mission which provided the best resolution all-sky survey of the CMB. So far Planck has only shown us the fluctuations of the scalar modes but it also looked at the polarisation of the background. Although it stopped working back in 2012 we are still waiting for those maps. Meanwhile some smaller scale results for the polarisation have already come in from land based observatories.
Microwave polarisation can be broken down into two modes using a Helmholtz decomposition which splits a vector field into a sum of two parts: The E-mode whose vector curl is zero and the B-mode whose divergence is zero. The E-mode in the CMB was first observed in 2002 by the DASI interferometer, but it is not particularly interesting. E-mode polarisation is generated by scattering from atoms before the radiation decoupled from matter but long after the period of inflation. Last summer the South Pole Telescope (SPT) found B-modes in the CMB for the first time, but these were known to be due to gravitational lensing of the radiation around massive galactic clusters. These can twist the E-mode polarisation to form B-modes so they are only slightly more interesting than the E-modes themselves. Really these lensing B-modes are not much better than a background that needs to be subtracted to see the more interesting B-modes that may be the signature of primordial gravitational waves.
The B-modes will have an anisotropy spectrum just as the scalar modes do and Planck may eventually provide us with a plot of this spectrum but as an initial result we are interested in the peak ratio of the tensor modes to the scalar modes which is given by a parameter known simply as r. The latest rumor say that a value for r has been measured by the BICEP2 observatory in Antarctica which is a smaller rival to the SPT, both housed at the Dark Sector Lab (pictured) Some more precise and less reliable versions of the rumor say that the answer is r=0.2. This is somewhat bigger than expected and could be as good as a 3 or 4-sigma signal because the sensitivity of BICEP2 was estimated at r=0.06. If this is true it has immediate implications for inflationary models and quantum gravity. It would rule out quite a lot of theories while giving hope to others. For example you may hear a lot about axion monodromy inflation if this rumor is confirmed, but there will be many other ideas that could explain the result and it will be impossible to separate them at least until a detailed spectrum is available rather than a single data point. Another implication of such a high value for r might be that primordial gravitational waves could have a bigger impact on galaxy formation than previously envisioned. This could help explain why galaxies formed so quickly and why there is more large scale structure than expected in galaxy distribution (see my previous spectulations on this point)
The most important thing about a high signal of primordial gravitational waves for now would be that it would show that there is something there that can be measured so more efforts and funding are likely to be turned in that direction. But first the new result (if it is what the rumors say) will be scrutinised, not least by rival astronomers from the SPT and Polarbear observatories who only managed to detect lensing B-modes. Why would BICEP2 succeed where they failed? Can they be sure that they correctly subtracted the background? These questions are premature and even immature before we hear the announcement, but it is good to go along prepared for the kind of questions that may need to be asked.