The Piper At the Gates of Dawn

In March 2014 the BICEP2 experiment reported the detection of tell-tale fingerprints of gravitational waves left over the Big Bang, a result hailed as one of the biggest discoveries of the century. The world-wide cosmology community was stunned; nearly 200 papers were written within a month of the announcement as we worked to make sense of the news. However, the results quickly unravelled and by September the excitement had largely evaporated.

Books could and probably will be written about this saga (science writers, call your agents): there is ambition, drama, excitement, Nobel fever, science-by-media, a telescope at the South Pole, and astrophysicists so hungry for data that they analysed images lifted from in Powerpoint slides when the originals were unreleased.

What was been overlooked in the fuss is that, despite the demise of the headline, BICEP2 marked a huge step forward for observational cosmology. BICEP2 does not see gravitational waves themselves; it observes the Cosmic Microwave Background. These microwaves are fossil light from the Big Bang and looking at them opens a window through which we can see the primordial universe. Gravitational waves leave a tiny twist in the polarisation of these microwaves and BICEP2 was the first instrument sensitive enough to be able to see it. Unfortunately, a similar twist is also supplied by dust in our galaxy. The BICEP2 team believed they had accounted for this “foreground” but they underestimated its strength, but there is no problem with the data itself. 

Last weekend saw another announcement from the BICEP2 scientists, in collaboration with the team from the Keck Array, a separate experiment, There was no media circus, and this time the news is that they see no gravitational waves at all, putting the tightest-ever limits on the size of any “background” gravitational waves in the universe. The original BICEP2 data looked at a single wavelength in the microwave spectrum, but the KECK data adds another wavelength, turning a monochromatic image into the equivalent of a colour photograph. And with this information it is easier to isolate the contribution from the dust and identify any signal from the Big Bang itself. This news didn’t make the front page of the New York Times, but its implications are massive. 

Cosmologists were excited about BICEP2 because these gravitational waves are a “smoking gun” for inflation, a period of ultra-fast expansion thought to happen immediately after the Big Bang. 

A baby, showing obvious signs of fine-tuning. 

A baby, showing obvious signs of fine-tuning. 

Like a small human, a baby universe need not be clean and tidy, but the baby photo of the universe obtained from the microwave background shows a young universe that is smooth and regular. Inflation washes away any bumps and lumps left over from the Big Bang, a cosmic Supernanny who made sure that the infant universe was photoshoot-ready when the microwave background was laid down. 

Cosmologists don’t know for sure if inflation happened, but we have hundreds of ideas about how it might have happened; one way to test them is via their different predictions for present-day gravitational wave background. If the original BICEP2 announcement had held up, most cosmologists would have seen it as compelling evidence that inflation is part of the history of our universe – if you see a background of gravitational waves, inflation is the simplest way for the universe to have made them.

On the other hand, the new limits on the gravitational wave background are putting pressure on some of our favourite models of inflation. The latest results from BICEP2 and Keck – and the progress we can now expect in the next few years – put us on the threshold of testing some of our deepest ideas about the early universe. It’s going to be an interesting ride. 

The Dark Sector Laboratory, South Pole. The BICEP2 telescope is housed in the structure closest to the camera. 

The Dark Sector Laboratory, South Pole. The BICEP2 telescope is housed in the structure closest to the camera. 


CODA: The question of whether the absence of gravitational waves is evidence for the absence of inflation is an ongoing argument in cosmology, and deserves a blog post on its own.  

And the van Gogh image looks a little bit like the patterns you see when the polarisation of the microwave background is mapped out on the sky. Just a little. 

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?

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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.

 

BICEP2: Two Months Later (and the Morning After)

It’s two months since the BICEP2 team announced it had seen the fingerprints of gravitational waves in the microwave background, thus apparently opening a portal into the universe ten trillion, trillion, trillionths of a second after the Big Bang. In the last week, however, the mood among cosmologists has taken on a morning-after tone, with a wave of doubt rolling through the community. It’s possible that the cosmology community is slowly waking up to find itself 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”.

 

What’s the issue?

The second thoughts are about how the BICEP2 analysis accounts for “foregrounds”, which is to say, things between us and what we’re looking at. In this case, the question is: how might dust in our own galaxy interfere with the detection of tell-tale signs of gravitational waves in the microwave sky?

The BICEP2 team concluded that foregrounds contribute around 20% of their signal, which leaves plenty of room to make a confident claim to have detected gravitational waves. 

By far the best way to isolate foreground dust is to use the microwave equivalent of a colour photo – but unfortunately the BICEP2 image is monochromatic. Consequently, the BICEP2 team could not extract foregrounds using just their own data. Instead, they presented a slate of indirect methods, in order to arrive at a reasonable estimate. One of these approaches uses the Planck satellite’s measurements of the microwave sky, and it’s this one that has come in for serious scrutiny. 

Firstly, it’s become clear that the BICEP2 team snagged the data they needed –  which has not been formally released by the Planck collaboration –  from a “teaser” image in a presentation posted online. This is certainly unorthodox, but does not immediately undermine the BICEP2 result. The issue burst into the public domain when Dan Falkowski (who often discusses — and disseminates — particle physics rumours on his blog Resonaances) not only drew attention to the “data-scraping”, but claimed that the BICEP2 team had misinterpreted the images and would be revising their paper.

The BICEP2 people hotly denied they had made a mistake, and didn’t even concede there was a mistake to argue about.

Separately, in a talk at Princeton (see video and slides), Raphael Flauger presented a virtuoso re-analysis of the foregrounds, including an estimate of the extra uncertainty injected by the fact that information was grabbed from a PDF file rather than from raw data. His measured conclusion was that the BICEP2 result is possibly overly optimistic. 

The blogosphere and science media has kicked into overdrive as the debate rages. Sesh Nadathur, Peter Woit and Sean Carroll all provide good summaries, while the Washington Post provides a great analysis of the current state of play.

 

So Where Are We Now?  

This is not (yet) a show-stopper, but the debate shines a light on a weak spot in BICEP2’s claim to have seen the fingerprints of gravitational waves.

In the long run, we need more data. Planck data was only part of the BICEP2 team’s estimation of the foregrounds, and new data (being gathered as you read this) should provide a much better answer over the next 12 months. 

 

Is This How Science is Done Now?

Apparently, yes it is. This is science in the age of the internet, and the world gets to watch in real time. We are caught between two powerful forces — on the one hand, as Lyman Page (a Princeton astrophysicist and microwave background expert) says at the end of Flauger’s talk:

So this is not – we all know, this is not sound methodology. You can’t bank on this, you shouldn’t. […] You just can’t, you can’t do science by digitizing other people’s images.

But on the other hand, does anyone really expect us just to sit and wait?

As far as the screen-scraping is concerned, there are precedents — a few years ago, the Pamela satellite was rumoured to have seen an excess of high energy positrons in cosmic rays that might have been due to dark matter in our galaxy. The Pamela team showed a slide at a conference and a couple of enterprising individuals snapped photos and extracted the datapoints — hundreds of papers quickly followed. Given the cameras that live inside our phones, the ubiquity of video at big conferences, these days “teasers” effectively amount to an unofficial data release. So people who drop hints about unpublished results in conference talks while coyly flashing a visual aid should not be surprised at the consequences. [As it turns out, the Pamela excess is real, but can explained by less glamorous mechanisms than dark matter]. 

One might also ask why Planck doesn’t just release the sky map used by BICEP2 and be done with it. That’s because what they showed was a work-in-progress: maps like this are not “raw” data, but the end-product of a long and painstaking analysis, and we can’t demand that anyone turn over a half-finished product. 

As many people have pointed out, the BICEP2 results have not gone through peer review. On the other hand, many other people (including me) also pointed out that over the last two months the BICEP2 papers have been dissected by hundreds of scientists, so they are getting more stringent, open-air scrutiny than any journal could provide (given that important journal papers might still only go past three referees). Moreover, many recent announcements (Planck, WMAP, the Higgs) were made before undergoing peer review, so this is the new normal. (It follows the near ubiquitous practice among the astro-and particle physics communities of posting full “preprints” on arxiv.org before you send your article to an actual journal.)

 

The Worst-Case Scenario

Whatever happens, the BICEP2 observations are by far the most precise measurements of the microwave background ever made. Even if the claimed detection of gravitational waves evaporates, the technological strides that underpin BICEP2 (and similar experiments now gathering data) will bring dramatic progress in cosmology. Even in the worst-case scenario, we are not looking at a re-run of the faster-than-light neutrinos flap which was traced to a loose cable and a dodgy clock and became entirely uninteresting once those problems were solved.  

 

My Own Guess

Personally, I would not be surprised if BICEP2 had overestimated the strength of the gravitational wave signal, even if I am not expecting it to vanish completely when the dust has settled (if you will pardon the pun). Cosmologists use the parameter “r” to describe the strength of the gravitational wave background. Before BICEP2, indirect measurements suggested that r was no more than 0.1, but BICEP2 prefers a higher number. A higher value of r would be fantastic for me and my fellow theorists, but it almost seems too good to be true. 

On the other hand, if BICEP2 is correct, it successfully probes the universe at energies a trillion times higher than we can reach at the LHC. Any intuition we might claim to have about physics at these scales is tenuous at best, so we will simply have to wait and see what develops. 

So even if the honeymoon is over, cosmology and gravitational waves are not yet headed for a Vegas-style quickie divorce. On the other hand, perhaps they need a restorative breakfast at the hotel buffet and a heart-to-heart about where they go from here.  (And they may yet need some touch-ups on that tattoo).


Live from New York City

If you are in Auckland on May 31 (or in New York City on May 30, when you can see it in the flesh) the University of Auckland is partnering with New York’s World Science Festival to present a simulcast of a Festival programme on the BICEP2 results, Ripples from the Big Bang. Moderated by Brian Greene it brings together John Kovac, one of the leaders of BICEP2, Alan Guth and Andrei Linde, who played key roles in the development of inflation, microwave background experimentalist Amber Miller, and Princeton theorist Paul Steinhardt. The New York event will be streamed live followed by a local Q&A, with me providing the Answers. Free entry, but ticket required for entry

BICEP2: A Month Later

A month ago the BICEP2 team announced that our universe is apparently awash with gravitational waves, pointing to the existence of an inflationary phase moments after the Big Bang. This was front page news all over the world, and cosmologists and astrophysicists have been working overtime to make sense of the news. Here is some of that sense…   

 

Let The Ambulance Races Begin 

For theoretical physicists, ambulance chasing involves getting papers out quickly after a major data release. Some ambulance chasers make significant contributions, some are just trying to draw attention to their earlier work, while others are banging out insubstantial papers in the hope that they will be cited by their slower colleagues. But whatever their motives, cosmologists have certainly been busy: the BICEP2 discovery paper has been cited 188 times on the Arxiv, all in “preprints” written within a month of the original announcement. I am pretty sure this is a world record, and you can always check the current tally.

In fairness, though, cosmologists were so giddy about BICEP2 it wouldn’t have surprised me if someone had stolen an ambulance and driven it in circles, flashing the lights and letting rip with the siren. 

 

Distributed Peer Review and Open Science

Once upon a time, the right way to announce a big result was to 1) write the paper, 2) send it to a journal, wait for it to be 3) peer reviewed and 4) accepted for publication, after which you could 5) hold a press conference. However, like most recent announcements in fundamental physics and cosmology, BICEP2 went straight from paper to media event, skipping steps 2, 3 and 4.

Old-timers will shake their heads, but this approach fits the principles of open science, which advocates making the processes and products of science transparent and widely available. Given that 1000 scientists are now scrutinising the BICEP2 results, rather than just two or three readers appointed by a journal, this amounts to an intensive, distributed and open peer review process, which is no bad thing. (And the papers will end up in a journal sooner or later.)

 

Trouble in Paradise? 

The real gold-standard for science is not peer review but reproducibility. BICEP2 claims to have detected a specific twist in the polarization of the microwave background — the so-called “B-mode”. This detection will not be a sure thing until it is confirmed by an independent team with an independent instrument performing an independent analysis. On top of that, inflation is not the only possible origin of such a B-mode, and further data will help confirm the theoretical interpretation of the BICEP2 observations. 

The good news is that no-one has found any show-stoppers. The biggest worry to surface so far is probably that the patch of sky BICEP2 observed may be contaminated by emission from radio “loops” associated with our own galaxy. It is not clear to me that this signal would necessarily reproduce the BICEP2 result, but unsubtracted foregrounds are likely to make any underlying gravitational wave signal look bigger than it really is, and that will need careful checking. And in the worst-case scenario, the BICEP2 results would be purely due to foregrounds, or some other analytical glitch.

We may not have to wait long. The BICEP2 team will be looking closely at these concerns, and more data will be gathered during the coming polar night. In addition, the Planck satellite has gathered the world’s most comprehensive observations of the microwave background and their science team is extending their initial analysis to look at polarization, with results promised before the end of 2014. 

 

Free Trips to Stockholm

If the BICEP2 result is verified, it is certain to attract the attention of the Nobel committee. In fact, it may be worth two Nobel prizes – one for the idea of inflation, and one for the detection of B-modes, which is a technological tour de force in its own right. (Two prizes have already gone to the microwave background — one for its discovery, and one for the first mapping of the temperature of the microwave background.) 

Speculating about “the prize” is a popular game among scientists, and I have already heard people ruminate about the likely judgment of history if it turns out that the BICEP2 analysis is  basically correct but slightly dust-contaminated. In this scenario, the BICEP2 announcement would have been made with far more confidence than the data ultimately justified, which would provide conversational fodder for decades. 

The intellectual history of inflation has many parallels with that of the the Higgs boson; they are both elegant hypotheses that existed for decades before being experimentally confirmed (assuming, again, that BICEP2 really has seen evidence of inflation). And like the Higgs, the theoretical parentage of inflation is murky. Alan Guth is undoubtedly the Peter Higgs of inflation (even if it is not called “the Guth phase”), but a number people made key contributions to the development of the theory. Unfortunately, only three of them can share the Prize, and there will be discreet (and probably blatant) lobbying for the other two places on the stage if the BICEP2 data holds up. 

 

What I Have Been Doing?

Beyond giving a slew of interviews the day the story broke, my group at the University of Auckland (in collaboration with Kevork Abazajian at UC Irvine) has looked carefully at the apparent tension between BICEP2 and existing cosmological data. BICEP2 does not just claim to have seen gravitational waves, but to have seen gravitational waves with an amplitude which was apparently ruled out by previous analyses.

We crunched a lot of numbers very quickly, thanks to the high performance computing facilities at NeSI (New Zealand’s e-research organization), and showed that this tension between BICEP2 and previous analyses is statistically significant. Consequently, taking all currently available astrophysical datasets at face value, BICEP2 appears to tell us three startling things about the early universe:

  1. Inflation really did happen right after the big bang.
  2. Inflation happened when the energy density of the universe was very high, as the strength of the gravitational wave background depends directly on the energy density of the universe during inflation. This means that the mechanism of inflation can give us a portal into the realm of ultra-high energy physics, where we expect candidate “grand unified theories” (including string theory) to be important. 
  3. The inflationary phase must be relatively complex, for the gravitational wave background to have escaped indirect analyses made prior to BICEP2. And this means that cosmologists will be able to make far more stringent tests of competing inflationary models than we might have expected.

Alternatively (and much more conservatively!) our results could suggest that the BICEP2 team has over-estimated the strength of the gravitational wave background and that future analyses will remove this discrepancy. 

 

One More Thing

To me, one of the most astonishing things about the BICEP2 telescope is just how small it is. The secret to BICEP2 is not its size, but the exquisitely sensitive superconducting transition edge sensors used to detect the microwave signal. Admittedly, BICEP2 sits at the South Pole, the whole instrument is chilled to within a hair’s-breadth of absolute zero (a major technological and logistical challenge) and it is surrounded by a complex array of shields, but the actual telescope is 23cm across. This is only a few times larger than the optical instrument Galileo used to explore the heavens over 400 years ago, and BICEP2 may one day rival Galileo in the profundity of its implications for our place in the universe.

BICEP2, to scale - www.bicepkeck.org 

BICEP2, to scale – www.bicepkeck.org 

The Weekly World News

Whenever you throw a party, there is always someone who double-dips the guacamole. In this case the jerk was Ephraim Hardcastle, a pseudonymous correspondent in the Daily Mail. This nimrod thought the most important thing to say about one of the biggest science stories in 50 years was that two of the experts asked to appear on the BBC news that night were both women of colour. Hardcastle’s shtick is similar to that of the old Weekly World News columnist Ed Anger — with the difference that Anger was a conscious parody. And while it is hard to take Hardcastle seriously, he caused real pain to real people in order to get off a few shots in a drive-by attack on “diversity”, and followed it with a non-apology worthy of Arthur Fonzarelli. 

For the record, there is no person in the world better qualified to comment on the BICEP results than Hiranya Peiris. As a PhD student, she was the lead-author on the first-ever paper to put serious constraints on inflation with microwave background data and she has worked on two major space-based CMB experiments. [Full disclosure: I have collaborated with Hiranya for 10 years and count her as a close friend.] Ironically, the resulting brouhaha saw both UCL and the Royal Astronomical Society spell out her qualifications: not just Cambridge, Princeton and Chicago but half a dozen fancy fellowships and prizes, any one of which makes for a CV that hums and crackles when it sits in a pile of job applications. So next time anyone needs a leading British astrophysicist for a TV appearance they will know who to call. 

The Mail has a gift for missing the point. While this may not be in the same league as backing the wrong side in the run-up to World War Two, if there is a story here it is that British astronomy is no longer the almost exclusive domain of white men. 

You can read Peiris’ own commentary on the affair is in the THES and you can sign a petition calling for a genuine apology here.