ABSTRACT
This chapter presents the inflationary model and discusses the problem of the origin of structure and the correspondingly more detailed tests of relativistic models. It discusses a general development of inflation models and looks at the particle physics. The theory of inflation tries to arrange that the energy density in the symmetric ground state drives an exponential expansion of the Universe so as to dilute, essentially to zero, the density of any unwanted relics. Despite the appeal of eliminating the horizon and flatness problems, one of the most attractive features of inflation is now taken to be the possibility of understanding how structure arises in the Universe. As quantum fluctuations cross the horizon they become frozen as classical perturbations and they re-enter after inflation as density perturbations. For a weakly first-order or second-order transition inflation takes place on the same timescale as the phase transition.
