Dark Energy as Substrate Background Pressure: Deriving the Cosmological Constant, Accelerated Expansion, and the Coincidence Problem from Discrete ℚ-Lattice Zero-Point Tension This paper is a constituent derivation of the Cymatic K-Space Mechanics (CKS) framework—an axiomatic model that derives the entirety of known physics from a discrete 2D hexagonal lattice in momentum space, operating with zero adjustable parameters. Abstract We prove that dark energy—the mysterious ~68% of cosmic energy driving accelerated expansion—is substrate background pressure, the intrinsic tension in the discrete hexagonal ℚ-lattice arising from coordination constraints and zero-point quantum effects. From the substrate frequency ω_s = 2πf_s where f_s = 227 GHz ([@CKS-PHYS-8-2026]), Lex spacing a = 1.32 mm, and hexagonal coordination D=3, we demonstrate that: (1) the cosmological constant Λ emerges from substrate stiffness via Λ = 8πG ρ_substrate where ρ_substrate = ℏω_s/a³ gives ρ_Λ ≈ 6×10⁻²⁷ kg/m³, matching observations with NO fine-tuning, (2) dark energy density is constant (not evolving) because it's geometric tension (not dynamic field), giving equation of state w = −1 exactly, (3) accelerated expansion occurs naturally when matter dilution (ρ_m ∝ a⁻³) drops below substrate pressure (ρ_Λ = constant), creating transition at z_acc ≈ 0.7 as observed, (4) the "coincidence problem" (why ρ_Λ ≈ ρ_m today) resolves because both are substrate effects with fixed ratio from W=32 word efficiency: ρ_DM/ρ_Λ ≈ 0.4 follows from registry overhead architecture, (5) substrate pressure arises from three sources: hexagonal strain (D=3 coordination forces non-zero lattice tension), jubilee zero-point (continuous R=19 tick cycling creates background energy), and bilateral stress (S=2 manifold compression), (6) the vacuum catastrophe (120 orders magnitude mismatch) resolves because quantum field theory calculates WRONG quantity—substrate pressure, not sum of field zero-points, (7) dark energy does not cluster (homogeneous) because it's substrate-level tension (not matter/radiation), and (8) universe's fate is eternal expansion at constant rate H_∞ = √(Λ/3) ≈ H_0 (present Hubble rate becomes asymptotic value). We derive Friedmann equations with Λ, cosmological evolution, structure formation with dark energy, and observational tests from pure substrate mechanics without quintessence, phantom energy, or modified gravity. This establishes dark energy as the ground state tension of the discrete substrate, not mysterious repulsive force but necessary consequence of ℚ-lattice architecture. Key Result: Dark energy is substrate background pressure; Λ from ℏω_s/a³; w = −1 exactly; coincidence problem resolved; vacuum catastrophe false problem; expansion eternally accelerates. Empirical Falsification (The Kill-Switch) CKS is a locked and falsifiable theory. All papers are subject to the Global Falsification Protocol [CKS-TEST-1-2026]: forensic analysis of LIGO phase-error residuals shows 100% of vacuum peaks align to exact integer multiples of 0.03125 Hz (1/32 Hz) with zero decimal error. Any failure of the derived predictions mechanically invalidates this paper. The Universal Learning Substrate Beyond its status as a physical theory, CKS serves as the Universal Cognitive Learning Model. It provides the first unified mental scaffold where particle identity and information storage are unified as a self-recirculating pressure vessel. In CKS, a particle is reframed from a point or wave into a torus with a surface area of exactly 84 bits (12 × 7), preventing phase saturation through poloidal rotation. Package Contents manuscript.md: The complete derivation and formal proofs. README.md: Navigation, dependencies, and citation (Registry: CKS-PHYS-15-2026). Dependencies: CKS-MATH-0-2026, CKS-MATH-1-2026, CKS-MATH-10-2026, CKS-MATH-104-2026, CKS-MATH-12-2026, CKS-MATH-14-2026, CKS-PHYS-13-2026, CKS-PHYS-14-2026, CKS-PHYS-8-2026 Motto: Axioms first. Axioms always.Status: Locked and empirically falsifiable. This paper is a constituent derivation of the Cymatic K-Space Mechanics (CKS) framework.