Late-Time Vacuum Decay and Cosmological Reheating: Gravitational Tunneling, Stochastic Sensitivity, and Falsification Boundaries

Manuscript prepared for The European Physical Journal C (Regular Article). Can the decay of a metastable late-time vacuum seed a new cosmological cycle? The paper specifies a scalar potential with an explicit metastable minimum, solves the Coleman-De Luccia bounce numerically in flat space and with Einstein gravity (shooting, an independent boundary-value solution, analytic de Sitter tails; 13 executable consistency checks), and converts the action to a nucleation rate per Hubble four-volume with the measured dark-energy density as the false-vacuum energy. With all scales set by the dark energy the gravitational parameter is about 1e-60, gravitational corrections to the action are about 1e-59 and the Hawking-Moss regime is irrelevant; the decay is a flat-space process controlled by one action B = b(eta)/lambda, and nucleation within a Hubble four-volume requires B below about 287. Across physically consistent priors 15% of the parameter space is excluded by the survival of the present vacuum and 80% never decays. The interior of a nucleated bubble is a curvature-dominated open universe whose energy density is bounded by the dark-energy density: reheating reaches at most 2.2 meV, 4.6e8 times below nucleosynthesis. A cyclic reset that regenerates a hot big bang by decay of the present dark energy is therefore excluded within this framework, as is a trigger by dilution of matter (which would need a scalar-matter coupling 1e28-1e30 times larger than allowed). The falsification boundaries are stated as explicit, reproducible conditions. Contents: manuscript (Word and LaTeX/PDF), supplementary information, cover letter.