2014 Kavli
Prize in
Astrophysics

Citation from the Committee

The theory of cosmic inflation, proposed and developed by Alan Guth, Andrei Linde, and Alexei Starobinsky, has revolutionized our thinking about the universe. This theory extends our physical description of the cosmos to the earliest times, when the universe was only a tiny fraction of a second old. According to this theory, very soon after our universe came into existence it underwent a short-lived phase of exponential expansion. During this brief period the universe expanded by a huge factor – hence the name inflation. The consequences of this episode were momentous for the evolution of the cosmos.

Without inflation, the Big Bang theory – a great achievement of 20th-century science – is incomplete. According to the Big Bang theory, our universe came into existence approximately 14 billion years ago. Its initial density and temperature were unimaginably high. Since then, the universe has been expanding at a rate that can be calculated using Einstein’s theory of General Relativity. In spite of its astounding success, the Big Bang theory suffers from two major shortcomings: the “horizon” and the “flatness” problems. Cosmic inflation solves them both.

As the universe expanded, it cooled. Today it is bathed in a sea of microwave radiation, the heat left over from the Big Bang. At first sight, the near uniformity of this microwave background across the sky implies a disturbing contradiction: opposite parts of the sky would never have been in causal contact with each other. How could the properties of this radiation be so similar when no physical processes could have acted to homogenize it? This puzzle is known as the horizon problem. A related puzzle is the flatness problem: if, at the Big Bang, the geometry of space had deviated ever so slightly from a flat configuration, the curvature of the universe would have subsequently been rapidly amplified. Yet, by the 1970s, astronomers were inferring that the geometry of our universe is close to flat. The Big Bang theory had no explanation for this observation.

These two fundamental problems were elegantly solved in one fell swoop by Alan Guth in a paper entitled “Inflationary universe: A possible solution to the horizon and flatness problems” published in 1981. Guth hypothesized that the universe was initially trapped in a peculiar state (the “false vacuum”) from which it decayed, in the process expanding exponentially and liberating the energy present in our universe today. The phase of rapid expansion would have exposed different parts of the universe to one another, so that physical processes could homogenize the properties of the primordial radiation. This solved the horizon problem. The same expansion would have ironed out any primordial curvature, thereby also solving the flatness problem. However, Guth’s simple and elegant model was flawed: as he himself recognized, it would lead to gross inhomogeneities in the distribution of matter on large scales.

In 1982, Andrei Linde proposed a working model of inflation in which the universe would gracefully exit from the exponential expansion phase without producing unacceptable inhomogeneities. He went on to build ever more sophisticated models, which dominate current thinking in the field.

In 1980, Alexei Starobinsky independently postulated a similar early phase of exponential expansion, in this case driven by quantum gravity effects. The solution he devised included an important prediction: the early universe would have generated gravitational waves which, he speculated, might one day be detected.

That the universe has a flat geometry has now been confirmed to extraordinary precision. With all its implications for the geometry and structure of our universe, the concept of cosmic inflation transformed the way in which physicists think about the early universe.

Pioneering the theory of cosmic inflation

In the late 1970s, the Big Bang theory was in trouble. Few doubted that the universe was born in an explosive primordial fireball, but theorists had come up with some seemingly insurmountable problems and didn’t know which way to turn.

By Daniel Clery

Evidence for the Big Bang began building in the early decades of the 20th century when astronomers looked at the spectra of distant galaxies and found that their light was shifted toward the red end of the spectrum. This could be because they were flying away from us at high speed, so the light they were emitting was being stretched out to longer wavelengths. In the 1920s, American astronomer Edwin Hubble showed that the further away a galaxy was, the greater was its redshift. The unavoidable conclusion was that the whole of the universe was expanding and, some argued, if you wind the clock backwards, the whole universe must have emerged from a single point some time in the distant past.

That was not the only possible explanation, but the Big Bang did seem to explain many aspects of the observed universe, including the distribution of galaxies and the fact that most of the normal matter in the universe is the light elements hydrogen and helium. The theory’s position became almost unassailable in the 1960s when astronomers discovered the cosmic microwave background (CMB) radiation, a relic from 400,000 years after the Big Bang when protons and electrons combined to form neutral atoms and space became transparent. The CMB fitted the theory so closely that few then doubted this explanation of the universe’s evolution. But that made the problems which emerged in the 1970s all the more troubling.

The first problem was the fact that the universe looks the same whichever way you look, which physicists refer to as the horizon problem. The farther astronomers look out into space, the farther back in time are the objects they are seeing because of the time it takes for those objects’ light to get here. But that sets a limit of how far it is possible to see, because some objects are so far away it would take longer than the age of the universe for their light to reach us. So there is a “particle horizon” beyond which we cannot see, and we cannot have any knowledge of the regions beyond the horizon because for the entire life of the universe they have been out of reach.

As time passes, the horizon recedes because it takes longer for light to get to us. But, theorists ask, if that’s the case, why do those previously unseen regions of space that are emerging over the horizon look exactly like the space we already know? Our local region of space and those new regions can never have been in contact before during the life of the universe, so how do they come to have the same values of, say, temperature and mass density? Measurements of the CMB only emphasized the problem, since it appeared to show almost exactly the same temperature wherever astronomers looked on the sky.

Another problem deals with the curvature of space. It is the matter within the universe that gives it curvature, and that curvature can be either positive or negative. A positive curvature means that there is enough mass in the universe for its gravity to eventually halt the expansion and pull it back into a Big Crunch. Negative curvature means there is not enough mass for that and the universe will keep expanding forever. But there is a critical mass density between the two at which the curvature is zero and the universe is said to be “flat.”

Evidence had been growing since the 1960s that the universe has a flat geometry, and this was confirmed by the first detailed observations of the CMB by NASA’s Cosmic Background Explorer (COBE) satellite in the early 1990s. But a flat universe puzzled cosmologists because the universe’s expansion tends to push the curvature away from flatness. If it started out just very slightly positive or negative, by today it should be highly positive or negative. For the curvature to remain as flat as it is after the 13.8 billion years of the universe’s existence, it must have started out so close to zero to have been the most accurately defined number in all of physics.

A third problem concerned an exotic particle called a magnetic monopole, like an isolated north or south pole of a magnet. Some theories that sought to describe the first moments after the Big Bang predicted that magnetic monopoles would be created in great numbers and the universe today should be swimming with them. The fact that not a single magnetic monopole has ever been detected suggests that either the theories are wrong or something has removed them from view. These three problems were considered so serious that in the 1970s theorists began to ask searching questions about the Big Bang theory.

There had previously been suggestions by some theorists that the universe might have undergone a period of rapid expansion early in its life, but the first to come up with a convincing scenario was Russian cosmologist Alexei Starobinsky of the Landau Institute for Theoretical Physics near Moscow. Starobinsky wasn’t trying to overcome the problems with the Big Bang theory, he was trying to explain the origins of the Big Bang using the theory of quantum gravity, an as-yet unsuccessful attempt to marry quantum mechanics and general relativity. In 1979, his calculations led him to the conclusion that the universe, just instants after its creation, could have gone through a period or runaway growth, or exponential expansion. He also realised that this expansion would have produced gravity waves, ripples in spacetime that could be detectable today. Starobinsky’s theory caused a stir among Soviet cosmologists, but the relative isolation of the Soviet Union at the time and the fact that he published in Russian meant that his work was not known in the West.

At around the same time, theoretical physicist Alan Guth of the Stanford Linear Accelerator Center and later the Massachusetts Institute of Technology was looking into the overproduction of magnetic monopoles and came up with a similar scenario of exponential expansion to Starobinsky. It seemed like a crazy idea at the time, that the universe should suddenly start to expand, doubling its speed and doubling and doubling it again many times until it was growing faster than the speed of light. All this happens in a mere twinkling of an eye, between 10-36 seconds and 10-32 seconds after the Big Bang, during which time the universe grows from many times smaller than a proton to about the size of a grapefruit. Despite the weirdness of the idea, Guth came up with a catchy name: inflation.

For Guth, inflation neatly solved the magnetic monopole problem because however many monopoles there were before the expansion, they would be so spread out afterwards that we might never see one. But helpfully, inflation offered solutions to the other two problems as well. Whatever curvature the universe had before inflation, the rapid expansion pulls it out taught like a rubber sheet and so creates a flat universe. For the horizon problem, inflation gives the universe an opportunity to equalize its temperature and density before the expansion starts because everything is within the particle horizon and so able to communicate. Inflation then grows the universe so fast that it pushes most of it outside the horizon, but the common properties remain, and wherever we look now the universe appears the same.

Inflation had obvious appeal because it so neatly solved the Big Bang’s problems, but it still had shortcomings, which Guth acknowledged. For one thing, Guth’s inflation seemed to leave the universe after expansion as a mess of bubbles and we don’t see any evidence of these today. There was also no mechanism for ending inflation at the right time so that the universe was left as a hot fireball continuing to expand at a more leisurely rate with only gravity to slow it down.

In October 1981, cosmologists gathered for a conference in Moscow and inflation was the hot topic for discussion. Stephen Hawking presented a paper asserting that inflation could not be made to work. But Russian cosmologist Andrei Linde countered his arguments by describing a new version of inflation in which the scalar field—the primordial force that drives inflation—can work without creating bubbles and will drop out at the right time. Hawking was convinced, and Linde has remained a leading light in inflation theory from that day to this.

Support for inflation is not universal but observational evidence for it has grown over the past three decades, including measurements of the CMB by NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and ESA’s Planck satellite which found the universe to be flat to an accuracy of a few percent and to have similar properties across the sky to an accuracy of one in 100,000. More evidence was provided earlier this year by an experiment at the South Pole called BICEP2 which, however, awaits confirmation by independent data. BICEP2 detected swirls in the polarisation of the CMB that are believed to be caused by the gravity waves spawned during inflation, as predicted by Alexei Starobinsky.

The theory of inflation does not describe the origin of the universe nor how the particles and forces that we see today arose but it is now widely believed that inflation will be an essential component of any more complete theory of the origin of the universe. The field of inflation theory now occupies thousands of theorists and many variations of inflation are actively debated. The Norwegian Academy of Science and Letters honours Alexei Starobinsky, Alan Guth, and Andrei Linde for setting that ball in motion.