8-Cosmology Under Vacuum Constraint: The Janus Universe

Standard cosmology treats vacuum energy as an effective gravitational source whose value must be fixed phenomenologically, leading to persistent conceptual tensions such as the cosmological constant problem. Recent structural analyses have shown that stable vacuum contributions cannot arise from a single spectral sector, but require a conjugated two-sector vacuum structure in which dominant contributions cancel naturally.Building on the minimal gravitational completion of such a conjugated vacuum, we investigate the cosmological dynamics implied once gravity is required to be compatible with this structure.Adopting a deliberately minimal cosmological setting, we show that global cosmic time acquires a conjugated structure and that key features of large-scale cosmic evolution follow as structural consequences rather than as model-dependent inputs.In particular, accelerated cosmological expansion emerges naturally without the introduction of a fundamental cosmological constant or exotic dark energy components. The resulting dynamics are stable, free from runaway behavior, and compatible with local physics and established symmetries.Cosmological horizons admit a natural structural interpretation within this framework.These results indicate that cosmology is no longer a freely adjustable model-building exercise, but a dynamics constrained by the internal structure of the vacuum. The Janus framework thus provides a minimal and coherent foundation for exploring the quantitative cosmological consequences of a conjugated vacuum structure.