BICEP2, Gravity Waves and Galactic Dust (personal reflections)
(figure 3 from the PRL)
On March 17 2014 the BICEP2 results hit the world by storm. The BICEP2 collaboration claimed to see gravitational radiation of cosmological origin, consistent with predictions from cosmic inflation theory. There was widespread jubilation in the cosmology community and the public at large. The implications of such a discovery are indeed profound, and I posted these exuberant reflections.
After that, concerns were raised that the signal they reported may have been at least partially due to dust in our own galaxy, not the cosmological origin that got us so excited.
On 9/21/14 a paper appeared, written by a team of scientists with official access to data from the Planck CMB Satellite (vs the digitized conference slides I mentioned in my earlier post).This paper presents (among other things) by far the most authoritative studies of polarized emission from galactic dust in the region of the sky studied by BICEP2.Their results strongly support the idea that the BICEP2 signals are predominantly (perhaps entirely) of galactic origin.Furthermore, on 1/30/2015 results were announced of project with Planck and BICEP2 scientists working together is using more sophisticated methods to tease out whether some of the BICEP2 signal is possibly cosmological.Those results also gave no evidence that the signal is cosmological.
Generally, it should be emphasized that while the observation of gravity waves at the level reported by BICEP2 would give remarkable support for the theory of cosmic inflation, the lack of a signal at this level does not undermine the idea of inflation since many implementations of this idea predict lower levels of gravity waves.However, the absence of a gravity wave signal at the BICEP2 level definitely rules out some specific models.
Many of us have been on an emotional roller coaster as this story has unfolded. This video, from the early days when we thought the result was cosmological has touched millions, and has given people a wonderful window on the emotional energy that goes into doing science.Considering this video with the knowledge that this exciting result has essentially disappeared may help you more fully understand what a ride we have been on.
This is certainly a story about the passion that goes into doing science, but it is also a success story for science.While I do feel the BICEP2 team should have been more careful about how they presented their results, their over-optimistic beliefs about the limited impact of galactic dust on their data reflected a (mistaken) perspective that was widely held in our community regarding the overall strength of the dust emissions.The relentless march of progress is clearing up these misconceptions, and progress was in fact stimulated by the excitement surrounding the original announcement.Also, the 9/21/14 paper has identified other regions of the sky where the galactic dust signal is lower, so new studies focused on those regions may yet yield a cosmological signal. Perhaps more importantly, the 9/14 paper shows that the spectral properties of the dust are sufficiently well understood so one should be able to “clean out” the dust and measure a subdominant gravity wave signal if it is there. Should a gravity wave signal be confirmed by new experiments we will indeed have reason for the grand celebration which we unfortunately had prematurely!
Note about the LIGO gravity wave discovery: On February 11, 2016 the LIGO collaboration announced the direct detection of gravity waves produced by a collision of two black holes. This announcement also generated much deserved excitement. There are several important differences between the LIGO result and the BICEP2 result. For one, the origin of the LIGO gravity waves is a well understood (if highly cataclysmic) process describable by classical physics. A cosmological signal such as was reported by BICEP2 would most naturally be interpreted in terms of quantum events much earlier in the history of the universe, probably connected with a period of cosmic inflation. Our understanding of this earlier period is overall less well understood than the black hole collisions. Another major difference is that the LIGO team were much more conservative about checking everything carefully before making their announcement. I feel it is extremely unlikely that the LIGO team will have to revise their interpretation of their results the way the BCEP2 team did.