Inflationary and Deflationary Branches in Extended Pre--Big Bang Cosmology

TL;DR

This paper explores pre--big bang cosmology within Brans--Dicke theory, identifying classical limitations due to singularities and quantum possibilities for transition between inflationary and deflationary phases.

Contribution

It introduces models with cosmological constants in Brans--Dicke theory and analyzes classical and quantum transitions between cosmological branches.

Findings

Classical solutions show no transition due to singularities.

Quantum tunneling allows non-zero transition probability.

Inflationary and deflationary branches coexist in extended models.

Abstract

The pre--big bang cosmological scenario is studied within the context of the Brans--Dicke theory of gravity. An epoch of superinflationary expansion may occur in the pre--big bang phase of the Universe's history in a certain region of parameter space. Two models are considered that contain a cosmological constant in the gravitational and matter sectors of the theory, respectively. Classical pre-- and post--big bang solutions are found for both models. The existence of a curvature singularity forbids a classical transition between the two branches. On the other hand, a quantum cosmological approach based on the tunneling boundary condition results in a non--zero transition probability. The transition may be interpreted as a spatial reflection of the wavefunction in minisuperspace.