This document presents a comprehensive analysis of late-stage cosmic acceleration under Expanse Tension Theory (ETT), a novel density-dependent field framework that explains accelerated expansion without invoking “dark energy.” The study rigorously evaluates ETT predictions against the three principal observational probes of cosmology: Type Ia Supernovae (SN Ia) – luminosity distances (brightness vs. redshift) Baryon Acoustic Oscillations (BAO) – angular and radial standard ruler distances Cosmic Chronometers (CC) – direct measurements of the Hubble parameter from galaxy ages For each probe, the methodology follows a consistent structure: observational baselines are defined, expected matter-only models (OCDM) are compared, the standard ΛCDM fit is reviewed, and ETT predictions are derived and overlaid. Results demonstrate that ETT achieves statistical performance comparable to ΛCDM (reduced χ² ≈ 0.5–1.1, Pearson r ≈ 0.94–0.98), yet does so without reliance on a phantom dark energy component. The underpinning field theory — based on a Holland–Higgs density-dependent coupling — is presented in the appendices, along with complete parameter tables, derivations, and calculation methods. All data sources (Pantheon+ supernova compilation, BOSS/eBOSS/6dF/WiggleZ BAO surveys, and the consolidated CC measurements) are documented to enable full reproducibility. This work establishes ETT as a physically motivated alternative to ΛCDM, offering a causal, mechanistic explanation for late-time acceleration grounded in first principles rather than parameter insertion. By unifying brightness, geometry, and time probes under one predictive framework, ETT provides a robust testable challenge to the standard cosmological model. Keywords / TagsCosmology; Expanse Tension Theory; ETT; Generalised Expanse Tension Theory; GETT; Dark Energy; ΛCDM; OCDM; Cosmic Acceleration; Late-Time Acceleration; Type Ia Supernovae; Supernova Cosmology; Baryon Acoustic Oscillations; BAO; Cosmic Chronometers; Hubble Parameter; H(z); Expansion Rate; Quantum Field Theory; Holland Field; Holland–Higgs Coupling; Density-Dependent Coupling; Alternative Cosmology; Observational Cosmology; Galaxy Surveys; Pantheon+; BOSS; eBOSS; WiggleZ; 6dF; Planck 2018; Reproducible Science; Physics of Expansion