A Phenomenological Interpretation of the Hubble Tension Through a Time Dependent Effective Expansion Rate

The Hubble tension refers to the persistent and statistically significant discrepancy between the Hubble constant inferred from early‑universe observations—most notably the cosmic microwave background within ΛCDM model fits—and the higher values obtained from late‑universe distance measurements. This mismatch is widely discussed as a potential indication of unaccounted systematic effects or as evidence for modifications of the early or late cosmological expansion history. This paper develops a strictly phenomenological framework in which the Hubble constant is interpreted as an effective, epoch‑dependent quantity rather than a universal parameter. A minimal parametrization is introduced that describes the expansion history through a reference rate and a time‑dependent additive term. Within this approach, the Hubble tension does not arise from conflicting measurements of the same physical constant, but from different observational probes sampling distinct dynamical regimes of cosmic expansion. The framework remains fully compatible with ΛCDM in the limit of vanishing time dependence, avoids any microphysical assumptions, and focuses on empirical testability. Observable consequences and explicit falsification criteria are formulated, particularly for measurements at intermediate redshifts and for independent geometric probes such as BAO, supernovae, cosmic chronometers, time‑delay lenses, and gravitational‑wave standard sirens.