Unified Spacetime Fluid Cosmology: A Falsifiable Phenomenological Framework for Late-Time Cosmic Expansion and Structure Growth

Unified Spacetime Fluid (USF) cosmology is formulated as a falsifiable phenomenological framework for late-time cosmic expansion and structure growth. The central idea is to generalize the cosmological-constant sector into a spacetime-sector response while preserving the linear-GR limit. At the background level, the ΛCDM constant term is replaced by an effective geometric component Ω_S(a), with ΛCDM recovered as the constant limit. At the perturbative level, the framework introduces an effective spacetime-friction term X(a,k) in the matter growth equation while keeping μ = Σ = 1 and c_T = 1. This overview is intended to serve as a stable conceptual and notational reference. It summarizes the minimal USF…