The Dark Sectors and the Hubble Tension as Entropy Phenomena: A Thermodynamic Extension of General Relativity at Cosmological Scales

Modern cosmology is characterized by two persistent conceptual challenges: the dominance ofthe so-called dark sector and the unresolved Hubble tension. Within the standard cosmologicalframework, dark matter and dark energy are introduced as independent components to reconcileGeneral Relativity with observations at galactic and cosmic scales, while the Hubble constant isassumed to be a single global parameter describing the expansion of the universe. Despite theirempirical success, these assumptions lack a clear physical interpretation and have led to growinginternal inconsistencies.In this qualitative and theoretical essay, we argue that both the dark sector and the Hubble tension originate from a common source: the incomplete incorporation of thermodynamiclaws—particularly entropy—into gravitational cosmology. General Relativity is shown to be afundamentally local theory of gravity, rigorously conserving energy–momentum in local spacetimeregions, but insufficient to describe the global evolution of an expanding, non-equilibrium universe.At galactic, cluster, and cosmic scales, the neglect of entropy production, entropy gradients, andhorizon thermodynamics manifests observationally as phenomena attributed to dark matter anddark energy.Within the Thermodynamic Relativistic Gravity - Classical General Relativity (TRG–CGR)framework, the dark sector is reinterpreted as an entropy sector: dark matter emerges as entropyinduced geometric response (“entropy matter”), while dark energy corresponds to entropy-drivencosmic acceleration (“entropy energy”). We further argue that cosmic geometry is fixed, whilethe spacetime metric evolves through matter–energy redistribution, entropy growth, and the thermodynamic arrow of time. This evolution naturally leads to a scale-dependent expansion rate,providing a conceptual resolution of the Hubble tension and related anomalies in cosmic age,structure growth, and gravitational lensing amplitude.This work does not propose new exotic substances or violate the well-tested local predictions ofGeneral Relativity. Instead, it offers a thermodynamically complete reinterpretation of cosmological phenomena, positioning the dark sector and the Hubble tension as emergent consequences ofentropy in an evolving universe. The essay aims to clarify the conceptual foundations of cosmologyand motivate future quantitative developments of thermodynamic gravity at large scales.