Recent progress in the Pre-Big Bang Scenario

I will discuss recent progress in string cosmology. Incorporating all-order α′ corrections leads to a far richer and more self-consistent picture of the pre-Big Bang universe. Within this more complete framework, the early universe can evolve smoothly through a genuinely non-singular bounce, allowing a continuous transition between two dual cosmological phases without encountering the usual curvature singularity. This mechanism provides a natural way to resolve the classical Big Bang singularity while remaining firmly grounded in string-theoretic principles. Moreover, when an appropriate potential is introduced for the dilaton, the same setup can stabilize the string coupling at late times, preventing it from running to strong coupling. This dynamical stabilization enables the emergence of realistic cosmological epochs, such as matter domination or even a period of de Sitter-like accelerated expansion, within a unified string-inspired description. These theoretical developments significantly broaden the phenomenological landscape. During the high-curvature string phase, small variations in the sound speed of cosmological perturbations can dramatically boost the production of primordial black holes, potentially bringing their abundance into the open window for dark matter. Likewise, recent hints from NANOGrav of a stochastic gravitational-wave background can be naturally accommodated by non-minimal versions of the pre-Big Bang scenario, in which stringy corrections enhance gravitational wave production. Taken together, these results suggest that genuinely stringy effects may leave measurable traces in today’s universe, offering a promising bridge between fundamental theory and near-future observations.