MATM v7.0: Metric Adjustment with Tension and Ballast as an Explanatory Extension of ΛCDM

We present a Bayesian MCMC analysis of the Metric Adjustment with Tension and Ballast (MATM) cosmological model, which introduces gravitational ballast and decaying cosmic tension as alternatives to the standard ΛCDM framework. Using 1701 Type Ia supernovae from the Pantheon+SH0ES sample combined with the Planck 2018 CMB prior on Ωm, we perform a comprehensive comparison between MATM and ΛCDM. Both models achieve excellent fits to the data (χ²/N ≈ 0.10), with MATM marginally reducing χ² by 0.5 but incurring penalties from information criteria (ΔBIC ≈ +7). Model comparison yields Δχ² = -0.5 (marginal improvement), ΔAIC = +1.5 (no preference), and ΔBIC = +6.9 (moderate preference for ΛCDM), consistent with Occam's razor. We conclude that MATM is a viable alternative cosmology that offers a unified framework linking Einsteinian geometry (ballast), Hubble expansion (H₀), and Planckian quantum limits (decaying tension). While not intended to replace ΛCDM, MATM broadens its explanatory scope and provides specific falsifiability criteria for future observational tests.