A thermodynamic description of cosmic evolution from the Big Bang: an explanation of dark energy based on the entropy of the cosmological horizon — Part II

This paper is a continuation of a previous work on a thermodynamic–holographic interpretation of dark energy based on the entropy and evolving degrees of freedom of the cosmological horizon [1]. The present Part II develops the physical implications of that framework. Starting from the relation ρa=αρ/N, we derive the associated horizon energy Ua and establish a direct connection between energy, temperature, entropy, and horizon degrees of freedom. We then consider an entropic pressure pH associated with the variation of horizon entropy with volume, motivated by existing entropic-cosmology approaches. Without introducing ρa as an additional dark-energy component in the Friedmann equation, this effective horizon pressure leads to a dynamical contribution capable of producing accelerated expansion. In the simplest formulation, the resulting dynamics gives accelerated expansion already during radiation domination, although the acceleration is very weak when radiation dominates, and approaches q=−1/2in the matter-dominated regime. These results provide a possible physical mechanism linking the thermodynamic properties of the cosmological horizon to cosmic acceleration and further develop the interpretation of dark energy as an emergent horizon phenomenon.