Semiclassical Determination of Ω_Λ from Standard Model Horizon Loading

The cosmological constant problem—QFT vacuum energy exceeding observations by \( 10^{120} \)—remains unsolved without fine-tuning or anthropics. We present a semiclassical determination of \( \Omega_\Lambda \) from Standard Model field content on a spherical cosmological horizon. Under a small set of discrete modeling assumptions (M1–M4), we project Standard Model fields onto the monopole (\( \ell = 0 \)) block of a spherical horizon at radius \( R \sim H_0^{-1} \), apply Kubo-Martin-Schwinger (KMS) thermal weighting at the Gibbons-Hawking temperature \( T_{\mathrm{GH}} = \hbar c/(2\pi k_{\mathrm{B}} R) \), and use symmetry-fixed greybody factors. All boundary terms vanish by gauge/color/parity symmetries, with a geometric heat-kernel correction for bosons. The calculation yields a dimensionless loading \( \Delta^* = 17.46 \), which maps to \( \Omega_\Lambda = \Delta^*/(8\pi) = 0.695 \pm 0.008_{\mathrm{th}} \) via a geometric normalization fixed by the causal diamond solid angle. Comparison with DESI 2024 + CMB data \( \Omega_\Lambda^{\mathrm{obs}} = 0.693 \pm 0.005 \) shows agreement within \( 0.2\sigma \). Every coefficient derives from Standard Model structure, spherical geometry, or thermal physics—no uncanceled UV divergences or fitted continuous parameters under assumptions M1–M4.