Matter-Dilution-Triggered Recollapse and Conditional Nonsingular Cycles in a Metastable Dark-Energy Model

Abstract Can the dilution of matter end the cosmological expansion and start a new hot phase? We study a scalar field coupled to dark matter through \(\:A\left(\varphi\:\right)=1+{\varphi\:}^{2}/2{M}^{2}\). The coupling holds a vacuum with the observed dark-energy density while the dark-matter density exceeds a critical value, and releases it into a negative vacuum once the expansion has diluted the matter. While the field is held, the homogeneous background reproduces the standard cosmological model to numerical accuracy; after the release the expansion turns around tens of billions of years later. Freezing the density seen by the field keeps all 100 coupled models of a parameter grid expanding forever, whereas the uncoupled field rolls before today over most of the grid. With an assumed effective bounce, thermal dissipation and dark matter that obeys its own Boltzmann equation with a field-dependent mass, 100 consecutive cycles are integrated without resetting any variable. Each has a radiation-dominated bounce, freeze-out to the observed abundance, a matter era, a held dark-energy epoch of 5.28 ± 0.01 e-folds, a release and a turnaround; the repeating cycle is an attracting fixed point of the bounce-to-bounce map, and an independent constraint-preserving solver reproduces it. The cycle requires a bounce hot enough to rethermalize the dark matter but below the percolation of black holes, dissipation, inherited rather than regenerated anisotropy and a coherent seed for the release. It does not produce the observed primordial perturbations (its scalar spectral index is near 3), so these are conditional background solutions, not an observationally complete cosmology.