THE 2014 KAVLI PRIZE IN ASTROPHYSICS is awarded to Alan Guth, Andrei Linde and Alexei Starobinsky “for pioneering the theory of cosmic inflation.”

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.