The origin of matter and structure in the universe.

Cosmology is nowadays one of the most active areas of research in fundamental science. We are going through a true revolution in the observations that are capable of providing crucial information about the origin and evolution of the universe. In the first years of the next millennium we will have, for the first time in the history of such an ancient science as cosmology, a precise knowledge about a handful of parameters that determine our standard cosmological model. This standard model is based on the inflationary paradigm, a period of exponential expansion in the early universe responsible for the large-scale homogeneity and flatness of our observable patch of the universe. A spectrum of density perturbations, seen in the microwave background as temperature anisotropies, could have been 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. With the observations that will soon become available in the next millennium, we will be able to test the validity of the inflationary paradigm, and determine, with unprecedented accuracy, the parameters of a truly standard model of cosmology.