No one particularly needs me to tell them about the BICEP2 results, given that so many others have already done so very nicely. But here is the way I put it in the latest issue of Prospect, where I wanted to try to put the findings within the broader picture of our unfolding cosmological view over the past century. That’s why I mention dark energy and the cosmological constant, even though one can perfectly well explain inflation without that. I’d contend that, if this work bears up, we’ll see the major landmarks as:
1912/1919: general relativity proposed and ‘confirmed’
1927/29: the Big Bang and cosmic expansion predicted and confirmed
1965: the CMB detected (and a minor landmark with the 1992 COBE results)
1998: the accelerating expansion of the universe
2014: inflation and gravitational waves ‘confirmed’ (?)
Who’s going to put money on Guth and Linde for the Nobel? Probably needs an independent confirmation first, though.
I feel like I spend a fair bit of time these days trying to bring a critical eye to the excesses of science boosterism. So how nice it is to be able for once to relish the sheer joy of how fab science can be. That was an exciting week. And if this piece is a little loose around the edges, forgive me – it had to be knocked out essentially overnight.
The discovery reported on 17 March by a US-led team of scientists will join the small collection of epochal moments that, at a stroke, changed our conception of what the universe is like. It offers evidence that, within an absurdly small fraction of a second after the universe was born in the Big Bang, it underwent a fleeting period of very rapid growth called inflation. This left the fabric of spacetime ringing with “gravitational waves”, which are predicted by Albert Einstein’s theory of general relativity but have never been seen before.
Finding evidence for either inflation or gravitational waves would each be a huge deal on its own. Confirming both together will leave cosmology reeling, and – barring some alternative explanation for the data, which looks unlikely – it is inconceivable that they will fail to win a Nobel prize in their own right and probably to motivate another for the theories they support. According to astrophysicist Sean Carroll of the California Institute of Technology in Pasadena, the results supply “experimental evidence of something that was happening right when our universe was being born”. That we can find this nearly fourteen billion years after the event is astonishing.
The discovery was made by a team led by John Kovac of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, using the Background Imaging of Cosmic Extragalactic Polarization (BICEP2) telescope located at the South Pole. It’s the kind of milestone in observational cosmology that comes only once every few decades, and fits perfectly into the narrative created by the previous ones.
We might start in 1919, when the British astronomer Arthur Eddington observed, from the island of Principe, the bending of starlight passing by the sun during a total solar eclipse. This confirmed Einstein’s prediction that gravity distorts spacetime, forcing light to trace an apparently curved path. The discovery made Einstein internationally famous.
Because of this effect of gravity, general relativity predicts that violent astrophysical events involving very massive objects – an exploding star (supernova), say, or two black holes colliding – can excite waves in spacetime that travel like ripples in a pond: gravitational waves. Scientists were confident that these waves exist, but detecting them is immensely difficult because the distortions of spacetime are so small, changing the length of a kilometre by a fraction of the radius of an atom. Several gravitational-wave detectors have been built around the world to spot these distortions from a passing gravity wave via interference effects in laser beams shone along long, straight channels and bouncing off mirrors at the end. They haven’t yet revealed anything, but the hope is that gravitational waves might eventually be used just like radio waves or X-rays to detect and study distant astronomical events.
The BICEP2 findings unite gravitational waves and general relativity with the theory of the Big Bang, for which we need to go back to the second cosmological milestone. In 1929 American astronomer Edwin Hubble reported evidence that the universe is expanding: the further away galaxies are, he said, the faster they are receding from us. Hubble’s expanding universe is just what is expected from an origin in a Big Bang. In fact Einstein had already found that general relativity predicts this expansion, but before Hubble most people believed that the universe exists in a static steady state, and so Einstein added a term to his equations to impose that. Yet in 1927 a relatively obscure Belgian physicist, George Lemaître, dared to take the theory seriously enough to predict a Big Bang. Hubble’s data confirmed it.
Yet it wasn’t until 1965 that one of the key predictions of the Big Bang theory was verified. Such a violent event should have left an ‘afterglow’: radiation scattered all across the sky, by now dimmed to a haze of microwaves with a temperature of just a little less than three degrees above absolute zero. While setting up a large microwave receiver to conduct radio astronomy, Arno Penzias and Robert Wilson found that they were picking up noise that they couldn’t eliminate. Eventually they realised it was the fundamental noise of the universe itself: the cosmic microwave background (CMB) radiation of the Big Bang. That’s milestone number three.
Number four came in 1998. While observing very distant supernovae, two teams of astronomers discovered that these objects weren’t just receding from us: they were speeding up. That was a real shock, because most cosmologists thought that the gravitational pull of all the matter in the universe would be slowing down its expansion. If, on the contrary, it is speeding up, then some force or principle seems to be opposing gravity. We call it dark energy, but no one knows what it is.
Einstein had already unwittingly provided a formal answer with his balancing act for getting rid of cosmic expansion: he added to his equations a fudge factor now called the cosmological constant. This amounts to saying that the vacuum of empty space itself has an energy – and because this energy increases as space expands, it can in fact produce an acceleration.
BICEP2’s results now look like milestone number five, and they stitch all these ideas together. The telescope has made incredibly detailed measurements of the CMB, spotting temperature differences from place to place in the sky of just a ten-millionth of a degree. Hence the exotic location: the telescope sits at the Amundsen-Scott South Pole station, 2,800 up on an ice sheet, where the atmosphere is thin, dry and clear, and free of interference from light and radio signals.
For the fact is that the CMB isn’t simply a uniform glow: some parts of the universe are a tiny bit “hotter” than others. This was confirmed in 1992 by observations with the Cosmic Background Explorer (COBE) satellite, which provided the first map of these “anisotropies” (hot and cool spots) in the CMB – and thereby some of the best evidence for the Big Bang itself. Since then the maps have got considerably more detailed.
Yet the puzzle is not so much why the CMB isn’t entirely smooth but why it isn’t even more uneven. A simple theory of a Big Bang in which the universe expanded from a tiny primeval fireball predicts that it should be much more blotchy, consisting of patches that are receding too fast to affect one another. So space should be far less flat and uniform. In 1980 the American physicist Alan Guth proposed that very early in the Big Bang – about a trillion-trillion-trillionth of a second (10**-36 s) after it began – the universe underwent a burst of extremely rapid expansion, called inflation, which took it from much smaller than an atom to perhaps the size of a tennis ball – an expansion of around 10**60-10**80-fold. This would have smoothed away the unevenness. In effect, inflationary theory supposes that there was a short time when the vacuum energy was big enough to boost the universe’s expansion.
Inflation doesn’t smooth out space completely, though. Quantum mechanics insists on some randomness in the pre-inflation pinprick universe, and these quantum fluctuations would have been frozen into the inflated universe, imprinted for example on the CMB. In turn, those variations seeded the gravitational collapse of gas into stars and galaxies – a staggering idea really, that infinitesimal quantum randomness is now writ large and glowing across the heavens. It’s possible to calculate what pattern these quantum fluctuations out to give rise to, and observations of the CMB seem to match it.
All the same, there was no direct evidence for inflation – until now. The theory also predicts that the microwave background radiation should be polarized – its electromagnetic oscillations have a preferred orientation – with a characteristic pattern of twists, called the B-mode. This swirly polarization is what BICEP2 has detected, and there’s no obvious explanation for it except inflation. Cue a Nobel nomination for Guth, and other architects of inflationary theory, in October.
What’s all this got to do with gravitational waves? Cosmic inflation was rather like a shock wave that set the universe quaking with primordial gravitational waves. They have now, 13.8 billion years later, died away to undetectable levels. But they’ve left a fingerprint behind, in the form of the polarized swirls of the CMB, just as ocean waves leave ripples in sand. It seems the only way these swirls could have got there was via gravitational waves.
OK, but where does inflation itself come from? Physicists’ usual response to a question they can’t answer is to invent a particle that does the required job, and give it a snazzy name: neutrino, WIMP, graviton, whatever. Carroll, who now proudly records Kovac among his former students, admits that this is what they’ve done here. “We don’t know what field it is that drove inflation”, he says, “so we just call it the inflaton.”
In other words, just as the photon (a ‘particle of light’) is the agent of the force of electromagnetism, and the Higgs boson was initially postulated as the force field that gave some particles their mass, so the inflaton is the alleged particle behind the force that unleashed inflation. It’s just a name, but here’s the point: it’s a particle whose behaviour, like that of all fundamental particles, must be governed by quantum theory.
And that’s where we really hit the exciting stuff. Confirming these two astonishing ideas, inflation and gravitational waves, is terrific. But they always looked a pretty safe bet. It’s what lies behind them that could be truly revolutionary. For gravitational waves are a product of general relativity, the current theory of gravity. But here they get kicked into existence by an effect of quantum mechanics, orchestrated by the quantum inflaton. In other words, we’re looking at an effect that bridges the biggest mystery in contemporary physics: how to reconcile the ‘classical physics’ of relativity with quantum physics, and thus create a quantum theory of gravity. Sure, BICEP2’s results don’t yet show us how to do that. But how many simultaneous revolutions could you cope with?