The Observable Universe as a Non-Equilibrium Dissipative Structure: Dark Energy, the Cosmological Constant, and the De Sitter Attractor From a Thermodynamic Identity

The framework takes the thermodynamic identity C(t)/P(t) = F(t) → 1 as a foundational postulate governing any persistent self-organizing dissipative system maintained far from equilibrium by continuous energy throughput and explores its cosmological consequences. The observable universe is treated as precisely such a system — a causally closed volume bounded by the cosmological event horizon — whose thermodynamic trajectory from the matter-dark energy transition toward the terminal de Sitter state is governed by this identity throughout. The persistence capacity comprises two terms: a time-varying expansion term P(t)_expansion and a geometrically fixed horizon entropy term P_horizon = c⁵/2G, derived from Bekenstein-Hawking entropy and Gibbons-Hawking temperature. The R_H terms cancel identically, yielding a result independent of epoch and horizon radius. At the unique cosmological equipartition epoch — identified with the onset of accelerated expansion at z ≈ 0.6 — the dark energy density is derived as ρ_dark = c³/8πGR²_HṘ_H, yielding 5.99 × 10⁻²⁷ kg m⁻³, within 0.5% of the Planck Collaboration measured value, with no free parameters. The cosmological constant problem is identified as the compounded consequence of two errors: thermodynamic misclassification of the vacuum, and application of Planck-scale quantum field theory to a phenomenon governed at horizon scale by c⁵/2G. The Hubble tension is quantitatively resolved: from Planck 2018 parameters alone, the epoch-dependent H₀_eff(z) derived from the tanh² thermodynamic trajectory recovers 66.94 km s⁻¹ Mpc⁻¹ at z = 0, agreeing with the CMB to within 0.7%, and a weighted mean of 73.0–73.5 km s⁻¹ Mpc⁻¹ across the distance-ladder window, agreeing with H0DN's 73.50 ± 0.81 km s⁻¹ Mpc⁻¹ to within 0.7%. Both values emerge from the same equation at thermodynamically distinct epochs. H₀ is not a constant — it is an epoch-dependent thermodynamic state variable