Cosmological Perturbations and Observational Viability of the Coherential Dark Energy Sector RCD Hilltop Dark Energy Series — Paper V

Abstract. Papers III–IV of this series established a dynamically stabilized dark en- ergy model: the coherence field C sits at a hilltop maintained by vacuum backreaction from a Caldeira-Leggett bath, with all effective coefficients derived from a concrete Ohmic UV completion. Here we perturb this background and demonstrate that it is free of all standard pathologies. At the late-time attractor (Ċ₀ = 0, V’(C₀) = 0), the linearized perturbation equations simplify dramatically: the coherence displacement δC satisfies a standard damped massive-scalar equation with sound speed c²_s = 1 and no residual metric source, while the bath perturbation δS obeys a purely dissipa- tive equation δ̇S = −Γ_eff δS with no coupling to δC or the metric. The system passes all six pathology checks (ghost freedom, gradient stability, tachyonic stability, sublu- minality, isocurvature decay, and absence of anisotropic stress) with high confidence. The perturbed stress-energy tensor at the attractor yields δρ_DE = εδS, θ_DE = 0, and Π_DE = 0, giving effective parameters μ ≃ Σ ≃ 1 with corrections of O(εS_eq/ρ_DE) ≲ 1%. The model belongs to the class of smooth minimally coupled dark energy with w₀ ≃ −1 + 4εS_eq/v ≈ −0.987 and w_a ≃ 0 once transients decay. No dark energy clustering occurs on sub-Hubble scales. The stabilized hilltop survives perturbation: the model is observationally viable and will be tested by precision measurements of w₀ from DESI and Euclid.