Expanding and Contracting Universes in Third Quantized String Cosmology

We discuss the possibility of quantum transitions from the string perturbative vacuum to cosmological configurations characterized by isotropic contraction and decreasing dilaton. When the dilaton potential preserves the sign of the Hubble factor throughout the evolution, such transitions can be represented as an anti-tunnelling of the Wheeler–De Witt wave function in minisuperspace or, in a third-quantization language, as the production of pairs of universes out of the vacuum.