Semiclassical Determination of ΩΛ from Standard Model Horizon Loading

The cosmological constant problem—QFT vacuum energy exceeding observations by —remains unsolved without fine-tuning or anthropics. We present a semiclassical determination of 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 () block of a spherical horizon at radius , apply Kubo-Martin-Schwinger (KMS) thermal weighting at the Gibbons-Hawking temperature , 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 , which maps to via a geometric normalization fixed by the causal diamond solid angle. Comparison with DESI 2024 + CMB data shows agreement within . Every coefficient derives from Standard Model structure, spherical geometry, or thermal physics—no uncanceled UV divergences or fitted continuous parameters under assumptions M1–M4.