Classical and semiclassical stability of emergent universes in Jordan–Brans–Dicke theory

The emergent universe scenario is based on the assumption that the universe originates from a past-eternal Einstein static (ES) state, subsequently evolving toward an inflationary phase and a hot Big Bang era. Such models are appealing as they provide nonsingular and geodesically complete cosmological histories. However, it has been argued by Mithani and Vilenkin [1–5] that, even when the ES state is classically stable, certain models can admit semiclassical tunneling channels leading to quantum decay toward configurations of vanishing scale factor. In this work, we investigate the classical and semiclassical stability of the ES regime in the context of Jordan–Brans–Dicke (JBD) theory. We analyze the structure of the Wheeler–DeWitt potential in minisuperspace and study representative semiclassical tunneling channels compatible with the Hamiltonian constraint. We show that, for suitable choices of the JBD potential and model parameters, the ES configuration can be robust against both classical perturbations and the specific semiclassical tunneling channels analyzed in this work. Our results indicate that the quantum instability discussed by Mithani and Vilenkin may be avoided within certain regions of parameter space, while leaving open the possibility of more general tunneling processes beyond the scope of the present analysis.