A Thermodynamic Description of Cosmic Evolution from the Big Bang: An Explanation of Dark Energy Based on the Entropy of the Cosmological Horizon — Part III (Revised formulation based on an evolving effective entropic pressure)

This paper continues the thermodynamic–holographic framework developed in two previous works, in which the cosmological horizon and its degrees of freedom were proposed as the basis for a possible interpretation of dark energy. The present work introduces a revised energy-partition hypothesis, according to which the primordial Planck-scale energy associated with the horizon is distributed among matter, radiation, and the entropic sector. This leads to an analytical reduction factor for the effective entropic pressure, derived directly from the energy partition rather than introduced phenomenologically. The resulting model predicts the deceleration parameter q(z), which shows very good agreement with observational reconstructions over 0≤z≤1 redshift. The same formulation is then used to describe the main cosmological epochs, from matter–radiation equality and photon decoupling to the transition to accelerated expansion and the asymptotic future. The results provide a possible thermodynamic interpretation of dark energy and of the enormous dilution of the primordial Planck energy density during cosmic evolution.