The Quintessence of Dust?

Yale University




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Today, another chapter has been added to the increasingly convoluted BICEP2 saga [see hereherehere and here for my accounts of previous developments]. The story began on March 21st with a media conference heard around the world that heralded a “5 sigma” detection of gravitational waves in the polarisation patterns in the microwave sky. This was presented as prima facie evidence our universe began with an inflationary phase that created these patterns a few trillionths of a trillionth of a trillionth of a second after the Big Bang itself.

Unfortunately, once the initial excitement died away, a number of voices asked whether BICEP2’s signal had a more humble origin — dust in our own galaxy. Dust can mimic a gravitational wave signal if it interacts with the galaxy’s magnetic field. From a cosmic perspective, anything inside our galaxy is a “foreground” – dirt on the window through which we peer at the microwave background, the fossil light from the big bang coming to us from the furthest reaches of space. And however amazing ‘space dust’ sounds, it is a lot less exciting to a cosmologist than hard evidence of gravitational waves.

BICEP2 is a specialised device — it looks at one patch of the sky in a single frequency, forming an exquisitely detailed but monochromatic map of that subset of the heavens. By contrast, the European Space Agency’s Planck spacecraft observed the full sky in multiple frequencies, but with less clarity than BICEP2. Tantalisingly, much of the Planck data is still “in the can” as the Planck Science Team works to extract useful and reliable information from the torrent of observations captured by the spacecraft. Planck’s frequency coverage means it can predict the amount of dust BICEP2 should expect to see, even though it cannot match the pinpoint clarity of the BICEP2 measurement itself.  

Which brings us to yesterday: Planck scientists posted a preprint estimating the amount of dust in the BICEP2 field of view. The results are discouraging for anyone hoping the original BICEP2 announcement would survive.

The news has been covered in many places — Sean Carroll has a great blog, there is this story at Nature, this at the Simons Foundation, and an enormous amount of chatter throughout the community. So far as I can see, the current state of play looks like this:

  • This is the first time since the original BICEP2 announcement that genuinely new data has been added to the analysis, so it is a big step toward a full understanding.
  •  If the new Planck dust analysis had been available to the BICEP2 team in March, they would presumably not have confidently claimed a detection of the “B-mode”, the hallmark of an inflationary gravitational wave signal.
  • Even if dust does not contribute all of the B-mode seen by BICEP2, any inflationary gravitational wave signal is likely to be significantly smaller than the number reported in the original BICEP2 analysis. This is not too surprising, as that value was hard to reconcile with other, indirect constraints on inflation.
  • The actual BICEP2 observations still represent a stunning technical achievement and marked a huge leap forward in our ability to measure the microwave background; the debate here is around the interpretation, not the observations themselves. 
  • The original BICEP2 estimate for the dust signal matched the broad expectations of the community, but the dust (and particularly its contribution to the polarisation) had not been well observed, and the Planck results now suggest that those expectations were overly optimistic. 
  • It ain’t over till it’s over. The new Planck analysis is itself an extrapolation from high frequencies (where the dust is more visible) to the single frequency observed by BICEP2 (where the gravitational wave signal would be most obvious), so there is plenty of room for further surprises. What is needed now is a direct comparison between BICEP2 and the full Planck dataset — that is in the works, and could bring another twist to the tale.
  • Finally, there is a real risk that cosmology bloggers and science magazine sub-editors will run out of ideas for slightly melancholy dust-related headlines… 

As the euphoria around the original BICEP2 announcement faded a few months ago, I said that the mood amongst cosmologists was (I imagined!) not unlike that of someone slowly waking up in an unfamiliar Las Vegas hotel room with a throbbing headache, hazy memories of the night before, and a fresh tattoo reading “r=0.2”. And just as Johnny Depp found it necessary to make minor amendments to his “Winona Forever” ink, cosmologists will be thinking that “r=0.?” might better express our feelings for the time being.