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.


Planck Live Blog

RELOAD FOR UPDATES [Lo-tech live blogging!]

THE DAY AFTER:  t work, digesting the Planck data.    The commentary below ran a bit technical; once I have my own thoughts together I will write them down…

12:49 AM NZST oo distracted to update the live blog after downloading the key papers.  They are a thing of beauty and a joy to behold. Huge kudos to the Planck team. Quick reactions, and then time to turn in (nothing like a global twitterfest to confirm that the world is round, I guess.)  

  • Firstly, the Hubble constant result from Planck really is quite low when compared to direct measurements. The parameters paper digs into the error budget here, and you can expect to see a good deal of effort going into improving the lowest rungs in the cosmic “distance ladder” in the next little while.
  • Neutrino physics results are in broad agreement with expectations from particle physics — three kinds of neutrinos, with a small overall mass.
  • The analysis of inflation is lovely (although I am bound to say this, as parts of it are influenced by a paper I wrote with Hiranya Peiris, a member of the Planck collaboration) — simple single-field, slow-roll models are still alive and kicking.
  • Speaking of which, non-Gaussianity is not detected, and constraints are significantly tightened.
  • People who like a simple universe will be happy.
  • If you were hoping that Planck would expand the fundamental set of parameters n the concordance model, looks like you were out of luck.
  • One elephant in the room is that there is no use of polarization data — this is being worked on, but it will make a big difference when it becomes available.
  • Finally, the emphasis on “anomalies” in the media conference and press release seems like headline-bait to me. By and large these were already known from the WMAP data, and are at large angular scales — where Planck and WMAP should overlap with one another.  Any dataset has anomalies and by stressing these, the ESA media-monkeys detract from the huge advance this dataset represents. Planck has provided us with a picture of the early universe with unprecedented clarity and precision, and every cosmologist in the world will have to do their job differently on the strength of it.  Working out what it tells us may take years, but it is going to be fun. 


10:56 PM NZST  tarting to think there is no big discovery to be announced.  But beautiful measurement of the early universe.  

10:51 PM NZST  old us that there are 3.2 ± .2 species of neutrinos — confirms predictions of particle physics.  Nothing about their mass so far. 

10:48 PM NZST  akeaway so far, inflation seems to be in good shape as a theory of the early universe.  Anomalies discussed were mostly seen in WMAP data, but now confirmed at higher precsion by Planck.

10:45 PM NZST Stunning accuracy in results for “standard’ cosmological parameters. Hubble’s constant lower than expected, universe a little older.   ots of time being spent on “anomalies”; wonder what will be said next.  

10:27 PM NZST  Foreground subtraction. #planck has 9 frequency bands — foregrounds have differnt freq. dependence from CMB, subtracted away. 


10:12 PM NZST  George Efstathiou about to take the stage. 

10:16 PM NZST Giving a quick overview of CMB physics…  Electrons and protons join to form atoms 380000 years after the big bang — CMB decouples from the matter, and largely unchanged since then.  ives us a baby photo of the universe.

10:07 PM NZST  Super choppy video. Promise of an “almost perfect” universe. Planck detectors cooled by liquid helium until they are a fraction of a degree above absolute zero — much colder than the CMB [microwave background] itself.

9:41 PM NZST  New Zealand is currently 12 hours ahead of Paris.  Press conference starts in 15 mins, but the actual papers go live at 12pm apparently.  Worry the ESA server will look this ery shortly afterwards, but suspect they can handle it. 

:40 PM NZST ooking at astrophysicists on facebook, the East Coast of the US appears to be waking up (or never went to sleep).  

3:50 PM NZST  Planning to live blog the lanck live blog data release tonight.  In the meantime, read Renee Hlozek and Shaun Hotchkiss‘s blogposts which give good discussions of what is at stake, or watch Ed Copeland giving a quick survey of cosmology. For my part, I am enormously curious to understand how Planck tightens constraints on the inflationary phase in the early universe, whether it confirms the existence of possible “glitches” in the early universe (and it will almost certainly provide new candidates, even if it rules out the old ones), the results for non-Guassianity, and the implications of Planck for neutrino physics. All this and more will be explained in a few hours. The webcast starts at 10am in Paris — 10pm in New Zealand, with the technical papers scheduled to become available at noon. 

And the European Space Agency says that #askplanck is the official hashtag for questions.