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 Pₕorizon = c⁵/2G, derived from Bekenstein-Hawking entropy and Gibbons-Hawking temperature. The RH 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 ρdark = c³/8πGR²HṘH, 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 H₀ₑff (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