We consider the quantum field theoretical formulation of boson field mixing and obtain the exact oscillation formula. This formula does not depend on arbitrary mass parameters. We show that the space for the mixed field states is unitarily inequivalent to the state space where the unmixed field operators are defined. We also study the structure of the currents and charges for the mixed fields.

By resorting to some results in quantum field theories with spontaneous breakdown of symmetry we show that an explanation based on microscopic dynamics can be given of the fact that topological defect formation is observed during the process of non-equilibrium phase transitions characterized by a non-zero order parameter. We show that the NambuGoldstone particle acquires an effective non-zero mass due to boundary (finite volume) effects and this is related with the size of the defect. We also relate such volume effects with temperature effects. PACS: 03.70.+k, 11.30.Qc, 95.50.Eb Much attention is currently devoted to the problem of topological defect formation during the process of symmetry breaking phase transitions [1]. In such a process it may happen that a region, surrounded by ordered domains, remains trapped in the "normal" or symmetric state. This occurrence manifests as a topological defect. Topological defects are observed as macroscopic extended objects with classical behavior, e.g. vortices in superconductors and superfluids, magnetic domain walls in ferromagnets, dislocations, grain boundaries, point defects in crystals. In cosmology, topological defects, such as cosmic strings, may have been playing a rôle in the phase transition processes in the early Universe [2]. The Kibble-Zurek scenario [3,4] provides the phenomenological understanding of the defect formation in phase transitions. There is a surprising analogy between defect formation in solid state physics and in high energy physics and cosmology [5]. For an interesting table of analogies see [6]. As an example, we just mention the analogy between vortex in superfluids and global cosmic strings. Studying the physics of defect formation in condensed matter physics may be then helpful in the understanding of possible scenarios in the early Universe cosmology [1]. The 1