Relativity of Reconstruction: Observer–Dependent Vacuum Energy in de Sitter Space

The cosmological constant problem is usually framed as a conflict between quantum field theory and gravity; here we argue instead that it reflects a misidentification of what vacuum energy means operationally. Quantum field theory assigns the vacuum a formally divergent energy density of order M₏₋^4, while cosmological observations infer an effective value of order H^2 M₏₋^2. The conventional assumption that these quantities describe the same global observable leads to the familiar discrepancy. We propose that vacuum energy is not a global scalar but an observer-dependent reconstruction conditioned by causal accessibility. Each observer, bounded by a cosmological or acceleration horizon, accesses only a restricted operator algebra of the global quantum state. The physically relevant vacuum energy is defined as the expectation value of the stress tensor reconstructed from this algebra via an admissible, positive, normalized reconstruction map, to which spacetime curvature responds through an observer-relative semiclassical Einstein equation. Recent results showing that gravitational dressing promotes horizon algebras to Type II crossed-product factors with observer-dependent entropy motivate this perspective; the present framework extends these insights beyond modular entropy to observer-relative stress tensors. Applied to de Sitter spacetime, the construction yields a universal effective vacuum energy scaling as\₄₅₅ H^4 + H^2 M₏₋^2, by geometric coarse-graining and horizon thermodynamics, radiatively stable, and requiring no fine tuning. We further show that any homogeneous and isotropic FRW projection of the framework necessarily lies, to first order, in a trivial-holonomy sector of the underlying reconstruction structure, rendering observer-relative effects degenerate with geometric redefinitions in standard cosmological observables. This clarifies why such effects are systematically suppressed in FRW-accessible data and implies a no-go result: observable deviations must arise from history-dependent or relational observables sensitive to horizon formation or entropy flow rather than from state-only FRW data.