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

282 Elementary introduction to pre-big bang cosmologyCurvature scale versus timeBIG BANGsingularitypresent timeSTANDARD COSMOLOGYtimede SitterSTANDARD INFLATIONtimeM SString pert.vacuumPRE-BIG BANGPOST-BIG BANGSTRING COSMOLOGYtimeFigure 16.1. Time evolution of the curvature scale in the standard cosmological scenario,in the conventional inflationary scenario, and in the string cosmology scenario.From a classical point of view, however, the answer to the above question—what happens to the universe before the phase of constant curvature, which cannotlast for ever—is very simple, as we are left with only two possibilities. Either thecurvature starts growing again, at some point in the past (but in this case thesingularity problems remain, it is simply shifted back in time), or the curvaturestarts decreasing.In this second case we are just led to the string cosmology scenario,illustrated in the bottom part of figure 16.1. String theory suggests indeed forthe curvature a specular behaviour (or better a ‘dual’ behaviour, as we shall seein a moment) around the time axis. As we go back in time the curvature grows,reaches a maximum controlled by the string scale, and then decreases towardsa state which is asymptotically flat and with negligible interactions (vanishing