Cosmic Expansion Without Exterior Work: A Phenomenological Horizon-Thermodynamic Framework

This manuscript explores a phenomenological question rather than proposing a completed theory of dark energy or quantum gravity. If a cosmological horizon-bounded system has no thermodynamic exterior, in what sense should metric expansion be described as doing work? Standard cosmology successfully models late-time acceleration through an effective dark-energy sector, but that success does not by itself determine the most physically useful thermodynamic interpretation of the expansion. The central idea explored here is that, in the absence of an exterior reservoir, late-time cosmic acceleration may be better viewed not as ordinary mechanical work performed outward, but as an internal geometric response of a horizon-bounded system whose thermodynamic consistency must be maintained as its coarse-grained informational state evolves. In this perspective, dark-energy-like behavior is treated phenomenologically as the macroscopic signature of a constraint-like or relaxation-like response rather than as evidence for a literal externally driven expansion process. The paper does not claim a derivation from first principles, a fit to cosmological data, or a solution to the dark-energy problem. Its aim is narrower: to formulate a coherent horizon-thermodynamic framework, introduce a minimal effective scrambling sector, and show that such a framework can be discussed in a mathematically controlled way while preserving standard early-time background limits and allowing a stable late-time accelerating regime.