Recent Progress in the Pre-Big Bang Scenario

The pre-big bang scenario describes a dilaton-driven phase emerging from the perturbative string vacuum and preceding the standard hot big bang. Its lowest-order realization is, however, singular and drives the dilaton toward strong coupling. This review summarizes recent progress on two complementary aspects of the scenario. On the phenomenological side, string-phase modifications of scalar and tensor perturbations can generate primordial black holes and a stochastic gravitational-wave background, allowing to probe the structure of the high-curvature epoch. On the theoretical side, the all-order O(d, d) formulation organizes homogeneous α′ corrections and, through a Routhian description, provides simple conditions for regular bouncing solutions. Adding a non-perturbative dilaton potential can then stabilize the string coupling and lead to FLRW or de Sitter attractors, including from anisotropic initial conditions. Together, these results connect the graceful-exit problem with potentially observable relics of the string phase.