THE EVOLUTION OF THE UNIVERSE

Abstract

With the recent measurements of temperature and polarization anisotropies in the microwave background by WMAP, we have entered a new era of precision cosmology, with the cosmological parameters of a Standard Cosmological Model determined to 1%. This Standard Model is based on the Big Bang theory and the inflationary paradigm, a period of exponential expansion in the early universe responsible for the large-scale homogeneity and spatial flatness of our observable patch of the Universe. The spectrum of metric perturbations, seen in the microwave background as temperature anisotropies, were produced during inflation from quantum fluctuations that were stretched to cosmological size by the expansion, and later gave rise, via gravitational collapse, to the observed large-scale structure of clusters and superclusters of galaxies. Furthermore, the same theory predicts that all the matter and radiation in the universe today originated at the end of inflation from an explosive production of particles that could also have been the origin of the present baryon asymmetry, before the universe reached thermal equilibrium at a very large temperature. From there on, the universe cooled down as it expanded, in the way described by the standard hot Big Bang model. Our present understanding of the universe is based upon the successful hot Big Bang theory, which explains its evolution from the rst fraction of a second to our present age, around 13 billion years later. This theory rests upon four strong pillars, a theoretical framework based on general relativity, as put forward by Albert Einstein and Alexander A. Friedmann in the 1920s, and three strong observational facts. First, the expansion of the universe, discovered by Edwin P. Hubble in the 1930s, as a recession of galaxies at a speed proportional to their distance from us. Second, the relative abundance of light elements, explained by George Gamow in the 1940s, mainly that of helium, deuterium and lithium, which were cooked from the nuclear reactions that took place at around a second to a few minutes after the Big Bang, when the universe was a hundred times hotter than the core of the sun. Third, the cosmic microwave background (CMB), the afterglow of the Big Bang, discovered in 1965 by Arno A. Penzias and Robert W. Wilson as a very isotropic blackbody radiation at a temperature of about 3 degrees Kelvin, emitted when the universe was cold enough to form neutral atoms, and photons decoupled from matter, 380 000 years after the Big Bang. Today, these observations are conrmed to within a few percent accuracy, and have helped establish the hot Big Bang as the preferred model of the universe. The Big Bang theory could not explain, however, the origin of matter and structure in the universe; that is, the origin of the matter{antimatter asymmetry, without which the universe today would be lled by a uniform radiation continuosly expanding and cooling, with no traces of matter, and thus without the possibility to form gravitationally bound systems like galaxies, stars and planets that could sustain life. Moreover, the standard Big Bang theory assumes, but cannot explain,