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

290 Elementary introduction to pre-big bang cosmology• Class I: a ∼ t β , β>1, t →+∞. This class of backgrounds correspondsto what is conventionally called ‘power inflation’, describing a phase ofaccelerated expansion and decreasing curvature scale, ȧ > 0, ä > 0,Ḣ < 0. This class contains, as a limiting case, the standard de Sitterinflation, β →∞, a ∼ e Ht , Ḣ = 0, i.e. accelerated exponential expansionat constant curvature.• Class II: a ∼ (−t) β , β < 1, t → 0 − . This is the class of backgroundscorresponding to the string cosmology scenario. There are two possiblesubclasses:IIa: β < 0, describing superinflation, i.e. accelerated expansion withgrowing curvature scale, ȧ > 0, ä > 0, Ḣ > 0;IIb: 0