The Geometric Schwinger Effect II: Modular Geometry of the Effective Bias in the Bosonic Vacuum

This work is the second part of the Geometric Schwinger Effect programme, which studies vacuum decay from a geometric and modular perspective. Part II examines the effective bias Feff treated as a central parameter controlling semiclassical decay rates. The paper shows how this bias arises from the modular geometry of the bosonic vacuum and how it can receive contributions from gauge, gravitational and topological sectors. It also develops general bounds on the domain-wall tension in multi-harmonic potentials and presents a practical procedure for estimating thin-wall bounce actions. The results provide a structured and modular way of organising the ingredients that enter vacuum-decay analyses.