COSMOS Track 3: A Unified Accessibility Geometry for the Cosmic Fluid Classification

We present the core mathematical foundation of COSMOS (Cosmological Ob-servable Sector Models) Track 3. The central postulate is that the energy density ofany cosmic component can be written as ρi(a) = Qi(a)S−1(a), where S(a) is a two-dimensional accessibility boundary and Qi(a) is a component-specific ”accessibilitycharge.” Assuming the accessible radius scales with the scale factor (S ∝ a2), thepure ”Bridge” state (constant Q) yields ρD ∝ a−2, corresponding to wD = −1/3and zero active gravitational mass (ρ + 3p = 0).