Its Dark Materials

Each galaxy lives within its own three-dimensional halo of dark matter whose gravitational field corrals the stars within it. Without the stars, the halo would still be there, albeit invisible to our eyes; but if the halo vanished, the stars would scatter into the depths of the universe – just as a Christmas tree remains a tree with or without the pretty lights. Whereas without the tree, the lights would merely be a puddle of colour on the snowy ground.

Growing up in the Southern Hemisphere, the traditional trappings of Christmas were always out of step with the onset of summer around us: Santa in his cozy suit, imagery of roaring fireplaces, snowy scenes on Christmas cards, a heavy meal we consumed on a hot day before a swim.

But after I’d experienced my first Northern Hemisphere Christmas in Ithaca, New York — which, with several feet of snow on the ground, was something close to Narnia — the childhood strangeness of that transplanted holiday melted away and the seasonal symbolism at last made sense. A festival of light in the darkness of midwinter; gathering around a warm hearth while it snowed outside.

Fast forward a few years and I was living in New York City, where the Rockefeller Christmas tree (and the ice skaters beneath it) stands as a marker of the turning seasons. Another few years further on, my family and I were living in Connecticut, where the town of New Haven marks the season by selecting an enormous local evergreen to make the ultimate sacrifice in exchange for the chance to stand, dramatically lit, in the New Haven Green through the Yuletide season. We lived close by, and it became an annual ritual to walk to see the tree with the kids, always in winter jackets, often with snow on the ground.

New Haven Green. Image: Richard Easther

New Haven Green. Image: Richard Easther

Once, as we approached the illuminated tree, my wife Jolisa – a literature person, never not searching for metaphors to help make sense of science – asked: “You know this dark matter stuff that you talk about, is it something like a Christmas tree at night — we can see the bright twinkling lights, but we can only make sense of why they’re hanging in the air in that shape if we know about the tree that holds them up?”

And she was exactly right.

Our sun is one of roughly 100 billion stars in the Milky Way Galaxy, and the Milky Way is itself one of roughly a trillion galaxies in the visible universe. For over 100 years, astronomers and physicists have been trying to understand how galaxies, the giant islands of stars that are the large-scale buildings blocks of the universe, hold themselves together. If galaxies are made entirely of stars – in other words, if what we see is all we’ve got – the stars would be moving too fast for their mutual gravitational attraction to hold a galaxy together.


A selection of galaxies; each image contains over a hundred billion stars.


To make sense of this, the vast majority of astrophysicists and astronomers have come to believe that the cosmos is now contains far more than our eyes can see. As we now see the universe, each galaxy lives within its own three-dimensional halo of dark matter, whose gravitational field corrals the stars within it. Without the stars, the halo would still be there, albeit invisible to our eyes; but if the halo vanished, the stars would scatter into the depths of the universe – just as a Christmas tree remains a tree with or without the pretty lights. Whereas without the tree, the lights would merely be a puddle of colour on the snowy ground.

So if you are seeking a secular interpretation of the iconography of Christmas, you could do worse than seeing a well-trimmed Christmas tree, illuminated with lights and bedecked with tinsel, as a metaphor for the cosmos.

Dark matter – by definition – neither emits nor absorbs light, and cannot thus be made of atoms, or indeed any of the fundamental particles known to physicists; it must be something entirely novel. So likewise, let the spectacle of a galaxy serve as a reminder that there is literally more to the physical world than meets the eye, and that there are deep mysteries for us to solve in the years and decades to come.

Happy holidays and compliments of the season.

View fullsize

The Milky Way over Portobello, Dunedin. Image: Ian Griffin.

The Milky Way over Portobello, Dunedin. Image: Ian Griffin.


Footnote: Actually this sort of was my TED talk. And, full disclosure, we can’t (and shouldn’t) be sure about dark matter until we have a better idea of its properties and composition.

Pop Science

This week I acquired a copy of Steven Weinberg’s 1977 book The First Three Minutescourtesy of an emeritus colleague downsizing his library. It was the first detailed popularisation of the Big Bang and is a pop sci classic, written by one of the leading theoretical physicists of the modern era.  

An absolute classic, even if this copy has seen better days. 

An absolute classic, even if this copy has seen better days. 

As you can guess from the title, The First Three Minutes tells the story of the moments following the Big Bang. The early universe sets the stage for the development of the cosmos we see around us now, and the Cosmic Microwave Background is a key link between the distant past and the present day.  Discovered just a dozen years before the book appeared in 1977, the microwave background is a a time capsule buried moments after the Big Bang, and Weinberg explains how it reveals the nature of the infant universe 

And, as it happens, the latest addition to my library fell open to reveal these words:

Page 77

Page 77

This text is almost a time capsule on its own. In 1992 CoBE made headlines by providing a map of the microwave background sensitive enough to reveal minute variations in the temperature of different regions of the sky. In 2006, Mather shared the Nobel prize for his work on CoBE. Meanwhile, Rai Weiss moved on from CoBE to become a founder of LIGO which earned its own place in history by successfully detecting gravitational waves in 2015.

Bon voyage indeed. 


CODA: And it goes without staying that Weiss is an odds-on favourite to get the call from Stockholm a few weeks from now when this year’s prizes are announced.  

CODA 2: And indeed Weiss did win the 2017 prize, along with Barish and Thorne.  

IMAGE: The header image shows the temperature differences across the sky, as measured by the CoBE satellite. The temperature range corresponds to changes of a few parts in 100,000. 

New York State of Mind

It’s not often an advertisement sums up a deep truth about the universe, but here’s one that does.

One of the commonest questions about the Big Bang is “If the universe is expanding, is everything in it getting bigger? The solar system, the sun, the earth, our bodies and the atoms we are made of?”

The answer is no, anything that can hold itself together won’t get any bigger as the Universe grows. For big things (like our Milky Way galaxy, or the Solar System within it) gravity provides the glue that stops them from stretching; for little things like rocks and people electrical forces between atoms hold them together. Only the space between galaxies (and clusters of galaxies) grows as the Universe expands which is just as Einstein’s General Relativity predicts, and our most sensitive measurements confirm. (Phew)

But Manhattan Mini Storage nails it in eight words. Those New Yorkers, always in a hurry…

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. 

Einstein’s Magic Bag

In 1915, Berlin was at the centre of an empire locked into a global war. But at least one resident of that city had his mind elsewhere: Albert Einstein was working to reconcile gravity with the theory of relativity he had invented a decade earlier. Einstein solved the problem by year’s end, and in doing so he changed our understanding of space and time forever. 

For Newton, space and time are the stage upon which the play-of-the-world takes place. The actors – planets, stars, people, atoms, and even light itself – move in space, but space itself is fixed and unchanging. In Newton’s universe, space tells us where things happen and time tells us when things happen but nothing happens to space or time; gravity reaches across space, but does not change space itself. 

In Einstein’s universe, space is shaped by matter and that shape changes when matter changes its position. Einstein brings the wooden stage of Newtonian physics to life; space now responds as the actors move about. For a physicist, Einstein’s understanding is as shocking and wonderful as a play in which the theatre springs to life and takes a speaking role in the drama.

Einstein welded space and time together into spacetime and showed that mass curves spacetime, and that gravity is encoded in this curvature. In Newton’s universe, gravity drags the earth around the sun. In Einstein’s universe the sun bends spacetime and the earth circles the sun by following the simplest path it can find though spacetime. 

This year marks the centenary of Einstein’s breakthrough, his General Theory of Relativity. The mathematical expression of the theory, the Einstein field equations,  would fit on a postcard:

 The Einstein Field Equations: the left hand side depends on the shape of spacetime while the right-hand side tracks the distribution of energy and momentum. Or, as John Wheeler put it, space (the left hand side) tells matter how to move, while…

 

The Einstein Field Equations: the left hand side depends on the shape of spacetime while the right-hand side tracks the distribution of energy and momentum. Or, as John Wheeler put it, space (the left hand side) tells matter how to move, while matter (the right hand side) tells space how to curve.  

But the Einstein equations are a magic bag: far bigger on the inside than they appear on the outside. One hundred years later physicists and mathematicians are still working to unpack this expression and to fit it to the rest of our knowledge about the universe. Einstein’s new theory immediately solved a 100 year old riddle, explaining why Mercury’s orbit around the sun did not quite match Newton’s predictions. Within 10 years, the underpinnings of the Big Bang and the expanding universe tumbled out of the magic bag of General Relativity: the universe is not expanding because distant galaxies move through space; the galaxies move because space itself is expanding. “Expanding space” is an idea you can have only after General Relativity tells you that spacetime is dynamical, rather than fixed and static.

Beyond clearing up the niggling behaviour of Mercury, Einstein made two predictions, each of which is a test that General Relativity must pass. First, gravity bends the passage of light itself; second, clocks run more slowly in strong gravitational fields. The bending of starlight was detected in 1919, via the changing positions of stars near the sun during a total solar eclipse; the slowing of time was inferred from observations in the 1920s and spectacularly confirmed in 1959. (And understanding this phenomenon is key to getting the GPS system to work, believe it or not.)

And there is much, much more. In 1915, also Einstein showed that as large objects accelerate they generate waves in spacetime. One hundred years later these “gravitational waves” have never been directly observed, but Advanced LIGOthe first observatory that should be sensitive enough to find them, is being commissioned just in time for the theory’s 100th birthday. 

 The LIGO gravitational wave detector - the strongest gravitational waves that we hope to observe will the move mirrors at each end of the "arms" by a distance that is a billion times smaller than the diameter of a typical atom.  

 

The LIGO gravitational wave detector – the strongest gravitational waves that we hope to observe will the move mirrors at each end of the “arms” by a distance that is a billion times smaller than the diameter of a typical atom.  

Likewise, a fully unified theory, a self-consistent quantum mechanical description of gravity alongside the other forces in nature is as elusive today as it was for Einstein, who searched in vain for such a model during the last two decades of his life.

So 100 years on, physicists are still unpacking the magic bag Einstein that discovered in 1915… 


Coda: Watch the World Science Festival programme Reality Since Einstein recorded in May this year…

What the Spacecraft Saw In The Night…

Gregory: Is there any other point to which you would wish to draw my attention?
Holmes: To the curious incident of the dog in the night-time.
Gregory: The dog did nothing in the night-time.
Holmes: That was the curious incident.
– From “Silver Blaze”, Arthur Conan Doyle

Over the biggest science stories of the last year is the on-againoff-again discovery of gravitational waves left over from the Big Bang. But the biggest story in cosmology – one that has been building for 15 years – almost always flies under the radar. Since the year 2000, we have increased our stock of knowledge on the microwave background – fossil light from the Big Bang – by a factor of maybe 10,000. Likewise, our data on the distribution of galaxies in space has grown by between 10 and 100. Despite these advances, the “big picture” concordance cosmology describing the evolving universe has hardly changed at all. 

Let me absolutely clear: this is good news for cosmologists. It means we can dig into the detailed history of the universe and test the two huge hypotheses which underpin the concordance cosmology – dark matter and dark energy. These are ad hoc assumptions (and profound challenges for theoretical physicists) but the predictions of the concordance model have survived a vast increase in our ability to test them.

When the first microwave background data from the Planck spacecraft was released in 2013 this pair of piecharts was part of the media package:

European space agency / Planck

European space agency / Planck

The spin from the European Space Agency media team was that the estimated amounts of dark matter and “ordinary matter” in the universe had gone up, while the fraction of dark energy had dropped. To a cosmologist, this is interesting news. But the bigger story is that the two pies are very similar, with no slices added or subtracted, even though Planck had made huge strides in measuring the cosmos.

The next batch of Planck data was released yesterday (very early in the morning here in New Zealand), and this time the big news is that Planck has accurately measured how long it took for the first stars to light up after the Big Bang. The headline story is that the dark ages – the time before stars – lasted roughly 550 million years, 100 million years longer than Planck’s previous estimate. (This is a long time, but a fraction of the 13.8 billion years since the Big Bang itself.)

Planck probes the dark ages because the first stars burn so brightly they ionise most of the gas in the primordial universe leaving a tell-tale imprint on the microwave background. However, this news adds confidence to the concordance cosmology as observations of distant galaxies are a better fit with this result than the previous estimate. If the gap hadn’t narrowed, it might have looked as if some exotic process (decaying particles left over from the big bang?) helped to ionise the primordial universe – and many of these theories are now ruled out. 

Sooner or later, there are bound to be surprises, even if we are back to square one when it comes to primordial gravitational waves. But just as Sherlock Holmes solved a mystery because a dog did not bark in the night, the biggest news today is what Planck does not say. To quote Holmes again, once once you eliminated the impossible, whatever remains, however improbable, must be the truth. So the concordance cosmology, with its dark energy and dark matter, is looking more and more like the truth, however improbable it might seem. 


Postscript 1: Cosmologists could argue for ages over just how to measure the amount of information at our disposal and the numbers here are guesstimates. For the microwave background, the spectacular growth reflects our near-complete ignorance of its properties as little as 25 years ago, whereas 3D galaxy positions have been mapped for much longer. Either way, though, you can make a case that the storehouse of data used to test models of the evolving universe has a grown a million times bigger since the year 2000. 

Postscript 2: The new Planck dataset has any number of interesting hints, and the key paper on inflation is still in preparation. I am still gathering my thoughts on that. 

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?

0
0
1
742
4234
Yale University
35
9
4967
14.0

Normal
0

false
false
false

EN-US
JA
X-NONE

/* Style Definitions */
table.MsoNormalTable
{mso-style-name:”Table Normal”;
mso-tstyle-rowband-size:0;
mso-tstyle-colband-size:0;
mso-style-noshow:yes;
mso-style-priority:99;
mso-style-parent:””;
mso-padding-alt:0in 5.4pt 0in 5.4pt;
mso-para-margin-top:0in;
mso-para-margin-right:0in;
mso-para-margin-bottom:10.0pt;
mso-para-margin-left:0in;
mso-pagination:widow-orphan;
font-size:10.0pt;
font-family:Cambria;
mso-ascii-font-family:Cambria;
mso-ascii-theme-font:minor-latin;
mso-hansi-font-family:Cambria;
mso-hansi-theme-font:minor-latin;
mso-ansi-language:EN-US;
mso-fareast-language:JA;}

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; redux redux

When the news of the BICEP2 result broke, the mood was euphoric. There was open speculation about Nobel Prizes, a certain video went viral and cosmologists spoke of a radical transformation in our understanding of the early universe. And all of this may still come to pass. But the wave of doubt blowing through the cosmological community in the last week is growing into a consensus that the BICEP2 team has overstated the case for a discovery. (For recent coverage see Science MagazineScientific American and the Washington Post.) 

BICEP2 squeezes information from the cosmic microwave background, light that has been flying across the universe since just after the Big Bang, forming the backdrop against against which we view the rest of the cosmos. Isolating and removing other sources of microwaves — so-called “foregrounds” — is a key challenge for all observers of the microwave sky.

Crucially, BICEP2 measures both the intensity and polarisation of the microwave sky, and the polarisation of foregrounds is poorly understood. After two months of scrutiny, there is a realisation that the BICEP2 team may have underestimated the polarisation contribution from the dust in our galaxy and, in addition, was overly optimistic about the quality of their foreground estimate. Consequently, it seems that the BICEP2 team cannot confidently claim to have detected a “primordial B mode”, the signature of gravitational waves and the hallmark of an inflationary era in the early universe.

A further twist is that the BICEP2 foreground analysis relied on unreleased data from the Planck satellite, “scraped” from preliminary maps shown at conferences and preserved in PDFs of the speakers’ slides. My initial reaction to this was fairly tolerant, but it really was not (and never will be) a good idea. Specifically, scraping itself adds uncertainty, and this uncertainty was not quantified or accounted for by the BICEP2 team. Moreover, preliminary data is preliminary: the measured value shown at any point in a map of the microwave sky is effectively the difference between two much larger and almost equal numbers, but if either of these quantities changes slightly (as the calibration of the instrument improves, for instance), their difference can change substantially — another source of uncertainty that the BICEP2 error budget ignored. Finally, the data-scraping detracts from the BICEP2 observations themselves, which represent a dramatic advance in our ability to measure the microwave sky. Astrophysics is a poster-child for the Open Science movement, but this moment is a reminder of the distinction between open science and other people’s unfinished work.

So where does this leave us? These developments do not rule out the possibility of a large gravitational wave background, and the data needed to understand the foregrounds is currently being gathered and analysed. If our universe does turn out to contain a significant gravitational wave background, BICEP2 will undeniably have participated in its discovery. Conversely, if the signal seen by BICEP2 is shown to consist entirely of foreground, that heartwarming viral video may come to look like an episode of Candid Camera in which not even the host was in on the joke. And for now, cosmologists are living in interesting times.