Thermodynamic Origin of Exponential Cosmic Expansion in DK-RD2: A Newtonian-Relativistic Framework

This work presents a thermodynamic–relativistic interpretation of cosmic expansion within the DK-RD2 framework, addressing the physical origin of the accelerated expansion of the Universe without invoking a cosmological constant. In this approach, gravity is reformulated by promoting the Newtonian constant G0 to an effective coupling Gab(T,v), which depends on the thermodynamic and kinematic state of the cosmic medium. This formulation preserves the Newtonian energy structure while incorporating relativistic corrections through a dynamically evolving gravitational interaction. Starting from a classical Newtonian energy balance, a modified Friedmann-like equation is derived without requiring General Relativity explicitly or introducing additional vacuum energy components. The expansion dynamics are governed entirely by the evolution of the product Gab(T,v)·ρ. A central result of this work is that the system naturally evolves toward a late-time asymptotic regime characterized by a constant Hubble parameter, leading to exponential expansion. This behavior reproduces the key geometric features of de Sitter spacetime, but arises dynamically as an emergent state rather than from a fundamental cosmological constant. Furthermore, thermodynamic properties typically associated with de Sitter spacetime, such as horizon temperature and entropy, are reinterpreted as macroscopic consequences of the evolving thermal state of matter–energy. These results suggest that cosmic acceleration may be understood as a macroscopic thermodynamic phenomenon arising from the evolution of matter-energy, rather than as the effect of an intrinsic vacuum energy density. This work extends the DK-RD2 framework by providing a physical mechanism for the origin of exponential cosmic expansion, offering a conceptually grounded alternative to the standard ΛCDM paradigm. It builds upon the previously published DK-RD2 framework (Zenodo DOI: 10.5281/zenodo.15493629).