Cosmic Lockdown: When Decoherence Saves the Universe from Tunneling

We investigate how quantum decoherence influences the tunneling dynamics of quantum fields in cosmological spacetimes. Specifically, we study a scalar field in an asymmetric double well potential during inflation, coupled to environmental degrees of freedom provided either by heavy spectator fields or by short-wavelength modes as they cross out the Hubble scale. This setup enables a systematic derivation of both Markovian and non-Markovian master equations, along with their stochastic unravelings, which we solve numerically. We find that, while decoherence is essential for suppressing quantum interference between vacua, its impact on the relative vacuum populations is limited. Fields heavier than the Hubble scale relax adiabatically toward the true vacuum with high probability, while lighter fields exhibit non-adiabatic enhancements of false-vacuum occupation. Once the system has decohered, quantum tunneling between vacua becomes strongly suppressed, effectively locking the system into the stochastically selected local minimum. This "cosmic lockdown" mechanism is a manifestation of the quantum Zeno effect: environmental monitoring stabilizes enhanced false-vacuum occupation for light fields by preventing them from tunneling.