Vacuum Energy from Horizon Constraints

Vacuum energy is commonly treated as an independent parameter of the cosmological model. In the present framework it is instead described as a constitutive response associated with cosmological horizons. The construction is formulated on a kinematic reference geometry defined solely by conserved matter and radiation. The dimensionless gravitational admittance η=Sacc/SBHη=Sacc/SBH quantifies the fraction of horizon information dynamically accessible within the causal patch. Reflectionless matching of complementary horizon channels leads, within the proposed framework, to the equilibrium value η0=1/(2π)η0=1/(2π), with an independent consistency check from KMS periodicity. A scale-free constitutive relation, normalised by the Euclidean four-volume of the accessible domain, yields ΩΛ,0=2π/9≃0.698ΩΛ,0=2π/9≃0.698 without fitted cosmological parameters. Departures from stationarity relax through the fundamental scalar quasinormal mode of the de Sitter horizon, leading to the testable scaling w(z)+1∝(1+z)3/2w(z)+1∝(1+z)3/2. The deposit contains the main text and supplemental material.