Inhomogeneous Spatial Backreaction and Differential Boundary Expansion as an Alternative to Dark Energy Cosmologies

Standard cosmological models rely on the Cosmological Principle—assuming a spatially homogeneous and isotropic Friedmann-Lemaître-Robertson-Walker (FLRW) metric—to infer the existence of a dark energy component (Λ ≈ 0.7) driving late-time cosmic acceleration. However, real-world structure formation creates a strongly two-phase inhomogeneous medium dominated by low-density voids (fvoid ≈ 70% - 80%) bounded by dense galaxy filaments. In this paper, we present a fluid-boundary interpretation of inhomogeneous backreaction, demonstrating that net macro-acceleration (Aₙₑₜ > 0) emerges naturally as a volume-weighted statistical consequence of structure formation without requiring a non-zero cosmological constant (Λ = 0). We formulate a two-phase algebraic model establishing a critical void volume threshold (fvoidᶜʳⁱᵗⁱᶜᵃˡ ≈ 71.5%) for acceleration onset. We then integrate the non-linear Lemaître-Tolman-Bondi (LTB) metric across a smooth hyperbolic tangent density profile to evaluate localized expansion differentials (Hᵣ vs H_⊥). Finally, we fit our inhomogeneous expansion profile against synthetic Type Ia Supernova luminosity distances (μ vs z), demonstrating a maximum magnitude variance of Δ μ ≤ 0.16~mag relative to standard Λ-CDM across z ∈ [0, 1.5]. Finally, we discuss the implications of scale-invariant fluid boundaries, suggesting that Planck-scale density limits (ρPlanck) may provide necessary non-singular boundary conditions for future inhomogeneous backreaction models.