Thermodynamic Horizon Cosmology with Variable Speed of Light: Transient Cosmic Acceleration and the Asymptotic Fate of the Universe

This repository contains a complete theoretical cosmology manuscript exploring the long-term evolution of the universe within a thermodynamic horizon framework with variable speed of light. Cosmic acceleration is interpreted as an emergent, non-fundamental phenomenon driven by the out-of-equilibrium thermodynamic state of the quantum vacuum and its associated horizon energy. The model predicts a prolonged but necessarily transient phase of acceleration, followed by asymptotic deceleration as the vacuum relaxes toward equilibrium. Several future scenarios are analyzed, including smooth eternal expansion without acceleration and extended but non-eternal accelerated phases. A final de Sitter state is excluded under well-defined physical assumptions, and the cosmological constant is recovered only as an effective description of the present epoch. The manuscript includes chapters and technical appendices addressing the mathematical formulation, thermodynamic interpretation, horizon dynamics, and comparison with standard cosmological scenarios.