Universal Horizon Thermodynamics: Unifying Cosmic Acceleration and Black Hole Remnants via Boundary Geometry

We present a unified thermodynamic framework linking cosmic acceleration and black-hole remnants through a single geometric variable-the boundary surface tension . Within the boundary-geometry formulation of general relativity, acts as a universal thermodynamic potential governing all horizons. Its scale dependence, , connects the ultraviolet stability of black holes with the infrared de Sitter expansion. The renormalized boundary work introduces a finite thermal floor that halts Hawking evaporation at and drives the Universe toward equilibrium with . This dual description resolves the zero-temperature paradoxes of gravity and interprets both cosmic acceleration and black-hole endpoints as complementary relaxation processes toward a common boundary-determined equilibrium.