The scale and development of the universe over time. ©BICEP2 2014
The discovery reported on 17th March by US scientists changed at a stroke our conception of the universe. It offered convincing evidence that, within a fraction of a second after the universe was born in the Big Bang, it underwent a period of very rapid growth called “inflation”. This produced a vast number of “gravitational waves,” a phenomenon predicted by Albert Einstein’s theory of general relativity. Evidence of these waves has never been seen before. Finding evidence for either inflation or gravitational waves is a huge deal. But confirming both has left cosmology reeling and—barring some alternative explanation for the data, which looks unlikely—the discovery will merit Nobel Prizes. According to Sean Carroll, an astrophysicist at the California Institute of Technology, the results supply “experimental evidence of something that was happening right when our universe was being born.” That we can find this evidence nearly 14bn years after the event is astonishing. It may also be a step towards solving one of the greatest and most profound problems in science—how to reconcile the two physical models of the universe: quantum physics and relativity. Achieving this would bring science closer to developing a theory capable of describing all physical phenomena. 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 is a milestone in cosmology, and part of a story that has been playing out for almost a century. In 1919, the British astronomer Arthur Eddington observed, from the island of Príncipe, the bending of starlight as it passed by the sun during a total solar eclipse. This confirmed Einstein’s prediction that gravity distorts spacetime, the fabric of the universe. General relativity also predicts that 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. These are gravitational waves. Scientists were confident that these waves exist, but detection 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 to spot distortions from passing gravity waves, but haven’t yet registered any.
The discovery reported on 17th March by US scientists changed at a stroke our conception of the universe. It offered convincing evidence that, within a fraction of a second after the universe was born in the Big Bang, it underwent a period of very rapid growth called “inflation”. This produced a vast number of “gravitational waves,” a phenomenon predicted by Albert Einstein’s theory of general relativity. Evidence of these waves has never been seen before. Finding evidence for either inflation or gravitational waves is a huge deal. But confirming both has left cosmology reeling and—barring some alternative explanation for the data, which looks unlikely—the discovery will merit Nobel Prizes. According to Sean Carroll, an astrophysicist at the California Institute of Technology, the results supply “experimental evidence of something that was happening right when our universe was being born.” That we can find this evidence nearly 14bn years after the event is astonishing. It may also be a step towards solving one of the greatest and most profound problems in science—how to reconcile the two physical models of the universe: quantum physics and relativity. Achieving this would bring science closer to developing a theory capable of describing all physical phenomena. 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 is a milestone in cosmology, and part of a story that has been playing out for almost a century. In 1919, the British astronomer Arthur Eddington observed, from the island of Príncipe, the bending of starlight as it passed by the sun during a total solar eclipse. This confirmed Einstein’s prediction that gravity distorts spacetime, the fabric of the universe. General relativity also predicts that 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. These are gravitational waves. Scientists were confident that these waves exist, but detection 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 to spot distortions from passing gravity waves, but haven’t yet registered any.
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Einstein had found that his theory of general relativity predicted that the universe was expanding, but at the time, most scientists believed that the universe exists in a static steady state. Einstein therefore added a term to his equations to make them conform to this idea of a static universe. Yet in 1927 an obscure Belgian physicist, Georges Lemaître, dared to set out a theory of cosmic expansion, the idea that would later be termed the Big Bang. Two years later Edwin Hubble, an American astronomer, reported his findings that the further away galaxies were, the faster they were receding, confirming that view. Yet it wasn’t until 1965 that one of the key predictions of Big Bang theory was verified. Such a violent event should have left an “afterglow”: radiation scattered across the sky, now at microwave frequencies and with a temperature just 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 or explain. Eventually they realised that it was no error—they were hearing the fundamental noise of the universe itself: the cosmic microwave background (CMB) radiation, the remnant of the Big Bang. Then, in 1998, while observing distant supernovae, astronomers found that these exploded stars weren’t just receding from the earth: they were speeding up. That was a shock, because most cosmologists thought that the gravitational pull of all the matter in the universe would be slowing its expansion. But on the contrary, this growth was speeding up, meaning that some force or principle was opposing gravity. We call it dark energy, but really no one knows what it is. Einstein had already unwittingly provided a formal answer when he added an extra term to his equations to cancel cosmic expansion. This term is now called the cosmological constant and amounts to saying that the vacuum of empty space itself has an energy—vacuum energy. And because this energy increases as space expands, it can produce an acceleration. BICEP2’s results stitch all these ideas together. The telescope has made incredibly detailed measurements of the cosmic microwave background radiation, spotting temperature differences from place to place in the sky of just a 10-millionth of a degree. Hence the exotic location: the telescope sits at the Amundsen-Scott South Pole station, 2,800m up on an ice sheet, where the atmosphere is thin, dry and clear, and free of interference from stray 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 the 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 why the CMB isn’t much more uneven. A simple theory of a Big Bang in which the universe expanded steadily from a tiny primeval fireball predicts that it should be much more blotchy and space itself far less flat and uniform. In 1980 the American physicist Alan Guth proposed that very early in the Big Bang—about a trilliontrillion- trillionth of a second (10¯ seconds) after it began—the universe underwent a burst of “inflation,” which took it from a size smaller than an atom to perhaps the size of a tennis ball: an expansion of around 10-10- fold. This smoothed away the unevenness. In effect, inflationary theory supposes that there was a short time when the universe’s vacuum energy—first formally expressed as Einstein’s cosmological constant—was big enough to boost the universe’s expansion. Yet if inflation smoothed out space, it could not do so completely. 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. It’s a staggering idea that infinitesimal quantum randomness is now writ large and glowing across the heavens. It’s possible to calculate what pattern these quantum fluctuations ought to give rise to, and observations of the CMB match it. All the same, there was no direct evidence for inflation—until now. The microwave background radiation is polarised, meaning that the up-down oscillations of the waves have a particular orientation in space, rather than being random. Inflationary theory predicts that the plane of polarisation should change from one point to another in the sky with a characteristic pattern of twists, called the B-mode. This swirly polarisation is what BICEP2 has detected, and there’s no obvious explanation for it except for 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? Within inflationary theory, they are the explanation for how the swirls got into the CMB. Cosmic inflation was rather like a shock wave that set the universe quaking with primordial gravitational waves. They have now, 13.8bn years later, died away to undetectable levels. But they’ve left this fingerprint behind, just as ocean waves leave ripples in sand. OK, but where does inflation itself come from? Physicists’ usual response to a question they can’t answer is to invent a particle or field 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 agent of 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 is 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 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 in this theory, 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?
