(ed.). Gravitational waves (IOP, 2001)(422s).

Chapter 16Elementary introduction to pre-big bangcosmology and to the relic gravitonbackgroundMaurizio GasperiniDipartimento di Fisica, Università di Bari, Via G. Amendola 173,70126 Bari, Italy and Istituto Nazionale di Fisica Nucleare,Sezione di Bari, Bari, ItalyThis is a contracted version of a series of lectures for graduate and undergraduatestudents given at the ‘VI Seminario Nazionale di Fisica Teorica’ (Parma,September 1997), at the Second International Conference ‘Around VIRGO’ (Pisa,September 1998), and at the Second SIGRAV School on ‘Gravitational Wavesin Astrophysics, Cosmology and String Theory’ (Center ‘A. Volta’, Como, April1999). The aim is to provide an elementary, self-contained introduction tostring cosmology and, in particular, to the background of relic cosmic gravitonspredicted in the context of the so-called ‘pre-big bang’ scenario. No specialpreparation is required besides a basic knowledge of general relativity and ofstandard (inflationary) cosmology. All the essential computations are reportedin full details either in the main text or in the appendices. For a deeper and morecomplete approach to the pre-big bang scenario the interested reader is referred tothe updated collection of papers available at http://www.to.infn.it/∼gasperin/.16.1 IntroductionThe purpose of these lectures is to provide an introduction to the background ofrelic gravitational waves expected in a string cosmology context, and to discussits main properties. To this purpose, it seems to be appropriate to include a shortpresentation of string cosmology, in order to explain the basic ideas underlying280