Done and Dusted?

The on-again off-again discovery of the “smoking gun of inflation” is now firmly set to “off”. The latest development is an analysis combining data from the Planck spacecraft, the BICEP2 telescope at the South Pole (and the source of the original excitement) and the Keck Array, a second South Pole experiment. The announcement was supposed to come next week, but the news leaked yesterday when the Planck mission’s French website put up the press release and background information early. The page is now locked, but rather pointlessly, given how fast the news spread. And while English is the (ahem) linga franca of science, monolingual Anglophones could get the gist from Google Translate.

The Planck announcement, today.

The Planck announcement, today.

Back in March, the BICEP2 team claimed to have found a characteristic twist in the polarisation pattern of the microwave background — fossil light from the big bang itself — suggesting the universe was awash with gravitational waves. The most likely origin for these gravitational waves was inflation; a phase of accelerated expansion immediately after the Big Bang. The idea of inflaton has been around for 35 years, and the gravitational wave signal claimed by BICEP2 would convince most cosmologists inflation had really happened in our universe. The news turned the cosmology community on its head but slowly unravelled over the next few months. 

Photons in the microwave background hail from the depths of intergalactic space, as they have been in flight since around 380,000 years after the Big Bang when the universe first becomes transparent. However, to get to us they pass through our Milky Way galaxy which contains a good deal of dust and gas. Some of the dust is electrically charged and interacts with the magnetic field of the galaxy, producing a similar pattern to the primordial gravitational wave signal. At the time the size of the dust signal was unknown, and the BICEP2 team accounted for it using the best estimates for its size. But those estimates were too conservative, and today’s news is that the vast majority of their “signal” is from the dust.

The new information comes from combining observations at several different microwave frequencies. The map of the microwave sky made by BICEP2 is exquisitely clear, but uses a single frequency –150 gigahertz, about 1500 times higher than an FM radio signal — so it is effectively black and white. The galactic dust has a different “colour” from the microwave background. The new analysis compares the BICEP2 signal to a map of sky made by the Planck satellite at 353 GHz and at this frequency dust is much brighter than the microwave background. If the BICEP2 looked different from the 353GHz Planck map,we would know that BICEP2 was not seeing a lot of dust – but the correlation between the maps is high, telling us that the BICEP2 signal is mainly dust and the champagne should have stayed in the fridge

The new analysis does not rule out inflationary gravitational waves. The signal claimed by the BICEP2 team was always surprisingly large. Cosmologists use the variable “r” for the strength of a gravitational wave background. Before BICEP2 it seemed likely that r was smaller than 0.1, but the “headline” number from BICEP2 was r = 0.2, which dropped to r=0.16 when they subtracted their best-guess for the dust distribution. This table comes via Google Translate and the now hidden press release, and the upshot is that we are effectively back to where we were a year ago:

From the pLANCK WeB pAGE

From the pLANCK WeB pAGE

That said, BICEP2 still represents a major milestone in our ability to probe the early universe; the technology it uses gives an exquisitely clear measurements, and we can expect huge progress on the observational side in the coming years. 

When the news started to unravel, it struck me that the cosmology community was in the same position as someone 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”. So even if r is not exactly zero, it looks like we will need a visit to this guy:

 Cosmologists, reacting to the latest BICEP2 news... 


Cosmologists, reacting to the latest BICEP2 news… 

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.


The Smoking Gnu

Today the cosmology community is full of chatter about an impending announcement – I heard the story from people on three continents. Moreover, the Center for Astrophysics at Harvard has scheduled a media conference on Monday with the promise of a major discovery, so there is some substance to the stories.

As always though, rumours run beyond the available facts, starting with the claim that the discovery was made by BICEP2, a telescope at the South Pole. BICEP2 is one of a number of instruments around the world that are dedicated to observations of the microwave background, the afterglow of the Big Bang. The word is that the BICEP team will announce evidence for a primordial B-mode – a delicate twist in the polarisation-pattern of the microwave sky,

Unlike the dark matter and dark energy that dominate the cosmos today, a B-mode would be a tiny pinch of spice that adds an oh-so-subtle flavour to the recipe of the universe. What makes a B-mode interesting (at least to a theorist) is how it gets there. Most realistic cosmological models rely on inflationa hypothetical period of accelerated growth, to solve “initial conditions problems” that plague simple models of the Big Bang. During inflation, quantum ripples in spacetime are stretched until they span the visible universe. It is these ripples, or gravitational waves, that would induce a B-mode in the microwave sky. Conversely, the B-mode predicted by competing solutions to the initial conditions problems is unobservably small, making the B-mode a “smoking gun” for inflation. So if the rumours hold up, BICEP could have detected a “signal”  that confirms a key tenet of modern cosmology and which was written into the sky a trillion, trillion trillionth of a second after the Big Bang. And that would be a very big deal indeed. 

When it comes to the details, the stories diverge. Some claim a (relatively) large B-mode that could be hard to square with other datasets, or would imply that the early universe is weirder than we imagine. Other rumours tell of a signal that is consistent with everything else we know, but might permit only a more tentative detection. (And not all possible B-modes match the predictions of simple inflationary models: a big B-mode that was not an inflationary B-mode might be the most astonishing outcome of all.)

This is a blog about a rumour and what it would mean if the rumours are correct (and the results then stand the test of time). It may be that rumours are wrong and the announcement is about something entirely different but at Noon, Eastern Daylight Time on March 17th we will know. 


Rare footage of the cosmology rumour mill in action 

PS For a Smoking Gnu look at the Discworld — which is certainly a fascinating cosmology, even if it is not the universe we actually inhabit. 

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.