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.