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. 

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

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.   

How To Train Your Universe

Imagine you set off a Big Bang. Imagine you set off many Big Bangs. Newborn universes are all small, grubby and just a little disorganised. But you love them, and want each of them to mature into a well-balanced cosmos with an evenly distributed population of stars and galaxies, and an almost (but not quite!) featureless microwave background. What should you do?

Most cosmologists would suggest a dose of inflation, delivered at an early age. During inflation, your baby universe grows almost exponentially, ensuring that the mature universe is smooth and flat. This dramatic expansion not only smooths away any irregular remnants of the Big Bang, but generates the primordial ripples that eventually grow into galaxies; points that were once almost touching now find themselves on opposite sides of the sky, as part of a microwave background that is almost identical in every direction. 

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During inflation, the universe expands dramatically, diluting any remnants of the Big Bang itself. 

Cosmologists don’t give tips to newbie universe-builders, but we do ask how our universe evolved. It was quickly discovered that a simple Big Bang needed special and apparently arbitrary initial conditions in order to grow into the universe we now inhabit. But In 1980, physicist Alan Guth, then a post-doc at SLAC, realised that a mechanism he dubbed inflation made these “initial conditions problems” manageable, even if it didn’t solve them completely. Inflation rapidly became part of the theorists’ tool kit – and like all successes, it has many parents.

What does inflation look like? In a smooth, expanding universe the distance between any two points gets larger, but inflation ensures that the speed at which they recede from one another is increasing. (NB it is the empty space between galaxies that expands; “bound” objects, such as atoms, solar systems, galaxies, people, kittens, etc, don’t expand).

The key ingredient of inflation turns out to be negative pressure, which might sound weird, but it’s not. You know what positive pressure looks like: stuff that expands, releasing energy that can be turned into motion. Like a shaken up bottle of ginger beer, or this bike pump powered water rocket:

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Just as stuff with positive pressure wants to expand, stuff with negative pressure wants to contract — like a stretched rubber band that goes “ping”. See, you knew what negative pressure was all along.

So negative pressure may not be mysterious, but is it something that makes a universe expand. Surely you’d want positive pressure for that?

Um. Yes. Well, sort of. If the universe is pushed apart by the stuff inside it, the pressure drops as space expands. And if pressure helps universe expand, expansion slows as the universe gets bigger and the pressure gets smaller. But inflation is accelerated expansion, and paradoxically needs negative pressure. Bear with me here. 

The next apparent paradox is the density of the universe during inflation. The density of a box of rocks goes down if the box gets bigger (and nothing enters or leaves the box). Double the size of the box and the density drops by a factor of two. But wait! Stretching something with negative pressure stores energy within it, and thanks to E=em-cee-squared this extra energy has mass. In an inflating universe energy gets diluted as the universe expands, but the negative pressure adds extra mass. For realistic models of inflation the density is almost constant — the stretching of space adds energy almost as fast as the expansion dilutes it: the universe can grow a trillion, trillion, trillion times bigger during inflation but the density only falls by a factor of 1000 or so..  

[You might worry that inflation somehow undercuts the conservation of energy, but when you get down to brass tacks, it’s actually conservation of energy that makes it work this way.]

So what has negative pressure? To a particle physicist at least, a “scalar field” is the easiest answer. All fundamental subatomic particles are associated with a field (and vice versa) — a scalar field corresponds to a particle with no spin. A scalar field can store a fixed energy at each point in space (thanks to its “potential”), and the total energy in the field grows as the universe expands and the volume of space increases. Which means the pressure is negative, and inflation is underway.

Indeed, Guth discovered inflation by thinking about scalar fields, not the other way round. 

So which scalar field made inflation happen? We don’t know. Haven’t a clue. The Higgs-boson (discovered in 2012) is a scalar, and so far, it’s the only one we know about. But not all scalar fields lead to inflation, and the Higgs is the kind that doesn’t. This is where cosmology doubles down — we’re not just looking at the beginning of the universe, we are exploring undiscovered vistas in particle physics. 

In fact, inflation could have happened a trillion, trillion, trillionth of a second after the Big Bang, at energies a trillion times beyond the reach of the Large Hadron Collider (and difficult to replicate experimentally!). At these energies you must take “theories of everything” seriously (string theory, among other contenders) — and if we know inflation happened, we also know that any successful “theory of everything” must let inflation happen. And since inflation underlies the hot and cold spots in the microwave background and leads to the formation of galaxies, any theory of inflation must account for exactly what we see in the sky. Thus theory meets the real world.  

The word is that the BICEP team has discovered B-modes, specific patterns in the polarisation of the microwave background. B-modes are the “smoking gun” of inflation, and their strength is tied to the energy density of the universe during inflation – if the rumours hold up, we could soon know that inflation DID happen a trillion, trillion, trillionth of a second after the Big Bang, and at energies a trillion times beyond the reach of the Large Hadron Collider.

There would still be several steps to nailing it down — not all B-mode signals must come from inflation, and not all inflationary models must produce detectable B-modes. And there’s one more thing: the B-mode is sourced by gravitational waves — ripples in the fabric of space — generated during inflation. Gravitational waves are a key prediction of Einstein’s General Relativity and their existence is still not conclusively confirmed. Even so, gravitational waves are getting second billing to what we will learn about the early universe: we will know that inflation is a compelling answer to cosmology’s initial-conditions problems, and humankind will have looked directly into the cosmic dawn. 

No pressure. No pressure at all. Negative pressure, in fact.


For a more detailed discussion, check out Sean Carroll’s blog which describes the connection between B-modes and inflation, and much else. Tomorrow is going to be an interesting day.