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 

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. 

//player.vimeo.com/video/89257338?wmode=opaque&api=1“>

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. 

 

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. 

//player.vimeo.com/video/27755517?wmode=opaque&api=1“>

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.  

How Two Guys Working for the Telephone Company Discovered the Origin of the Universe

​In 1964 Arno Penzias and Robert Wilson were working for AT&T — Ma Bell, the then-monolithic US telephone company.  Nowadays you can pick up satellite signals with a TV dish the size of a frying pan.  But the first communications satellites — Echo, for the obvious reason — were simply balloons; orbiting radio mirrors and with no on-board amplification. Consequently, they needed super-sensitive detectors on the ground.  Penzias and Wilson were trained in radio astronomy, a field whose stock-in-trade is the detection of faint radio signals from space and their job at Bell Labs was to work with a radio antenna in Holmdel, New Jersey which would be used with this new-fangled satellite.  (Radio astronomy itself was invented in the 1930s by another Bell employee, Karl Jansky — he was working on short-wave radio communications, not satellites, and stumbled over emissions from  the centre of the Milky Way galaxy, one of the brightest sources in the radio sky.)

​The story has been told many times. The detector had an annoying and remarkably intransigent “hiss” and Penzias and Wilson knew ​it was the detector, since the hiss didn’t change as they pointed their antenna at different places in the sky.  A radio hiss can be converted into a temperature: a red-hot coal has a temperature of a few thousand degrees Celsius but this hiss was microwave-hot, putting it just a few degrees above absolute zero.  

​Holmdel Antenna [Bell Labs]

​Holmdel Antenna [Bell Labs]

The never got rid of the hiss. Instead, through the academic grapevine, they found themselves talking to a group of Princeton astrophysicists.  In the 1940s it was realized that if the universe ​began with a big bang (an idea invented in the 1920s) it would have a detectable afterglow.  In the early 1960s, the big bang was gaining traction among astrophysicists but the competition between it and the competing steady state proposal was far from settled.  The Princeton crew were working on a detector to look for this afterglow,  and realized that Penzias and Wilson had already found it.  

Comparing notes, the two groups published back-to-back letters to the Astrophysical Journal. Penzias and Wilson simply reported their results (and their painstaking efforts to eliminate other explanations) while the Princeton scientists provided the interpretation. Penzias and Wilson’s letter was entitled A Measurement Of Excess Antenna Temperature At 4080 Mc/s​ — a masterful understatement. As legend has it, the impact of their work only hit home when it made the front page of the New York Times.  In 1978 Penzias and Wilson won the Nobel Prize — certainly the first and very likely the time last two telephone company employees get to stumble across one of secrets of the universe. 

For cosmology, the microwave background is the gift that keeps on giving — it has done far more than simply tell us that the big bang happened.  Right away, the smoothness of the microwave background confirmed that the early universe was almost the same at every point.   The largest departure from smoothness arises because the earth (and the sun and the Milky Way) are all moving, relative to the “frame” defined by the microwave background. The microwave background is redshifted on one side of the sky and blueshifted on the other – telling us that the Milky Way galaxy is moving at around 600 km/s relative to the average position of all the matter in the universe — a number first measured in the 1970s. 

​Fast forward to the 1992, and the COBE satellite team announced that it had made the first measurements of the differences ​in the temperature of the microwave background at different points in the sky;  10,000 times smaller than the temperature of the background itself.  Today the departures from smoothness is more interesting than the background itself — information contained within a map of the microwave background can tell us how the big bang happened, and what the universe looked like in the first minutes and years after the big bang.    Mather and Smoot, two leaders of the COBE team, won the second microwave background’s second Nobel Prize in 2006.  After COBE, a host of ground-and balloon-based experiments have contributed to the analysis of the microwave background and from 2001 to 2010 a second satellite, WMAP, compiled an exquisitely detailed map of the microwave background.​

Transition from COBE to WMAP sky maps  – credit: NASA / WMAP Science Team

What do we get from this?  In combination with other data the microwave background  tells us the age of the universe and the amount of dark matter it contains. It lets us explore the mechanism that produced the tiny departures from smoothness in the microwave background, the same mechanism which sewed the seeds that led to the formation of galaxies and planets and stars (and, ultimately, people) as the universe evolved.  Not so long ago, estimates for the age of the universe varied by a factor of two — in 2013 the consensus is that our universe is 13.7 billion years old, a number good to within a few percent:  thanks to the microwave background, cosmology has become a precision science. 

And on Thursday morning — March 21 t 10am CET — it all changes again. Planck, the next spacecraft to study the microwave background, will release its first map of the microwave sky, and announce its first batch of cosmological results.   Planck improves on WMAP the way that WMAP improved on COBE: whatever the Planck team tells us tomorrow, our understanding of the universe is going to change. 

plan to live-blog and tweet (@reasther) the Planck results as they are released.