Observational Expansion History H(z) and the Interpretation of Cosmic Acceleration

This paper examines the observational expansion history of the universe using model-independent measurements of the Hubble parameter H(z)H(z)H(z) derived from cosmic chronometers. Unlike distance–based cosmological probes, cosmic chronometers provide direct estimates of the expansion rate by measuring the differential aging of passively evolving galaxies. These observations allow the evolution of the expansion rate to be studied empirically across a broad range of redshifts. Compiled chronometer data show a smooth transition from a decelerating expansion regime at earlier cosmic times to the present epoch of accelerating expansion, with the transition occurring near z≈0.6z \approx 0.6z≈0.6. Within the standard ΛCDM framework this behavior is interpreted as the epoch at which the cosmological constant begins to dominate over matter density. In the Relational Lens framework developed in earlier papers in this series, spacetime is interpreted as a bounded dynamical domain between two complementary limits: a contraction boundary associated with gravitational singularities and a propagation boundary defined by the speed of light ccc. From this perspective, the observed transition in cosmic expansion may represent a change in the dynamical regime of spacetime rather than the influence of an additional dark-energy component. The analysis also highlights the appearance of a characteristic acceleration scale involving the quantity cH0cH_0cH0, which arises in multiple cosmological contexts and lies within the same order of magnitude as the empirical acceleration scale a0a_0a0 observed in galaxy dynamics. The recurrence of similar acceleration scales across independent astrophysical systems raises the possibility that galactic dynamics and cosmic expansion may be influenced by a common structural property of spacetime. This work does not attempt to replace the ΛCDM model but instead explores whether observational expansion data are consistent with a boundary-driven interpretation of cosmic dynamics. The results motivate further investigation of possible connections between galactic acceleration scales and cosmological expansion in the next paper in this series.