Cosmic acceleration from Vaidya-FRW junction conditions: dark energy as horizon growth dynamics

Using Israel-Darmois junction conditions to match a Vaidya exterior to a Friedmann-Robertson-Walker interior, we derive H = Ṁ/M: the Hubble parameter equals the fractional mass accretion rate of the bounding horizon. For power-law accretion Ṁ = κM^α, the effective equation of state is wₑff = −1/3 − 2α/3. Constant fractional accretion (α = 1) yields w = −1 exactly, reproducing the ΛCDM expansion history from boundary geometry without introducing a dark-energy fluid. Joint analysis of DESI DR2 BAO, Pantheon+ supernovae, and Planck CMB priors returns α = 0. 968 ± 0. 034, confirming this prediction to within 3%. With the ΛCDM baseline locked (α ≡ 1), the data identify a localized accretion enhancement at zc = 0. 65 +0. 12/−0. 06 with Planck-consistent background cosmology (H₀ = 67. 7 ± 0. 7, Ωₘ = 0. 307 ± 0. 006), preferred over ΛCDM at Δχ² = 11. 5 (p = 0. 003, 2. 9σ for 2 extra parameters). The feature appears at consistent redshift and amplitude across three independent supernova compilations and in pure BAO+CMB data without supernovae. At equal parameter count with w₀wₐ (5 vs 5), the Gaussian model fits Pantheon+ better (Δχ² = 11. 2 vs 9. 8 over ΛCDM) with 10: 1 Bayesian preference. The framework predicts |Δw| > 0. 2 divergence from w₀wₐ at z > 1. 5, already emerging in hemisphere splits of current data 38.