Density-Dependent Vacuum Bubble Nucleation in Matter-Coupled Dark-Sector Cosmology: Granularity of the Source, Wall Dynamics, Transmission, and Gravitational Viability

Can a declining dark-matter density switch on the decay of a vacuum, and what do the resulting vacuum walls do? For a scalar whose coupling to dark matter is centred on a false vacuum, dilution turns the held vacuum metastable at a degeneracy density theta = eta/2 in the mean-field description. That description has a narrow domain of validity: for a vacuum at the dark-energy scale the field's Compton length is about 0.07 mm, inside which the dark-matter energy is only about 0.05 m_phi^4. Particle dark matter of any mass is a set of point sources between which the vacuum would decay at its zero-density rate; the mean field is restored only for wave dark matter heavier than the field and still classical (about 3 meV to 0.25 eV). For such dark matter the decay begins in the emptiest voids, protecting the past requires a mean coupling close to the degeneracy density, and galactic haloes keep the held vacuum. The walls are about 0.2 mm thick, run away, become transparent to the dark matter and carry the released energy (Israel junction); transitioned regions recollapse, isolated Hubble-scale bubbles end as black holes, and a continuation through a bounce is not supported. Prepared for Physics of the Dark Universe.