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The Principle of Mediocrity For the 2011

A&G • October 2011 • Vol. 52 5.1 The universe as we know it originated in a great explosion that we call the Big Bang. For nearly a century cosmologists studied the aftermath of this explosion: how the universe expanded and cooled down, how galaxies were gradually pulled together by gravity, etc. The nature of the Big Bang itself came into focus only relatively recently. It is the subject of the theory of inflation, which was developed in the early 1980s by Alan Guth, Andrei Linde and others, and has led to a radically new global picture of the universe. According to this new picture, remote regions beyond our horizon are strikingly different from what we observe here and may even obey different laws of physics. Here I will discuss the origin of the new worldview, its possible observational tests, and its implications for the beginning and the end of the universe. I will start with a brief review of the theory of inflation. The key role in this theory is played by a peculiar entity called “false vacuum”. Vacuum is just empty space, but according to modern particle physics it is very different from “nothing”. It is a physical object, endowed with energy density and pressure, and can be in a number of different states. Particle physicists refer to these states as different vacua. The properties and the types of elementary particles differ from one vacuum to another. The gravitational force induced by a false vacuum is rather peculiar: it is repulsive. The higher the energy of the vacuum, the stronger is the repulsion. The word “false” refers to the fact that this kind of vacuum is unstable. It decays into a low-energy vacuum like ours, and the excess energy goes to produce a hot fireball of particles and radiation. I should emphasize that false vacua with these strange properties were not invented for the purposes of inflation: their existence follows from particle physics and General Relativity. The theory of inflation assumes that at some early time in its history the universe was in the state of a high-energy false vacuum. Why it should be so is a good question, and I will have something to say about it later in this article. The repulsive gravitational force produced by that vacuum would then cause a super-fast, exponential expansion of the universe. There is a characteristic time – the doubling time – in which the size of the universe would double. This is similar to economic inflation: if the rate of inflation is constant, the prices would double, say, every 10 years. Cosmic inflation is a lot faster than that: depending on the model, the doubling time can be as short as 10 seconds. In about 330 doubling times the size of the universe will grow by a factor of 10. No matter what its initial size is, the universe will very quickly become huge. Because the false vacuum is unstable, it eventually decays, producing a hot fireball, and that’s the end of inflation. The fireball continues to expand by inertia and evolves along the lines of standard Big Bang cosmology. Decay of the false vacuum plays the role of the Big Bang in this scenario. The theory of inflation explained some otherwise mysterious features of the Big Bang, which simply had to be postulated before. It explained the expansion of the universe (it is due to the repulsive gravity of the false vacuum), its high temperature (due to the high energy density of the false vacuum), and its observed homogeneity (false vacuum is very homogeneous: apart from quantum fluctuations, it has a constant energy density). The theory has also made a number of testable predictions. It predicted that on the largest observable scales the universe should be accurately described by flat, Euclidean geometry. It also predicted a nearly scale-invariant spectrum of small Gaussian density perturbations. These predictions have been spectacularly confirmed by observations. By now inflation has become the leading cosmological paradigm.