Nucleation at finite temperature beyond the superminispace model

The transition from the quantum to the classical regime of the nucleation of the closed Robertson–Walker Universe with spatially homogeneous matter fields is investigated with a perturbation expansion around the sphaleron configuration. A criterion is derived for the occurrence of a first-order type transition, and the related phase diagram for scalar and vector fields is obtained. For scalar fields both the first and second order transitions can occur depending on the shape of the potential barrier. For a vector field, here that of an O(3) nonlinear σ-model, the transition is seen to be only of the first order.