The Physical Origin of the Refractive Field in Density Field Dynamics: Gravity as Electromagnetic Vacuum Loading

We develop the interpretation that gravity arises from electromagnetic energy loading of the quantum vacuum, treated as a physical storage medium with vacuum force scale K₀ = c⁴/(8πG). The dimensionless scalar field ψ measures the fractional loading, and the vacuum refractive index n = eᵠ follows uniquely from the multiplicative composition law via Cauchy's exponential functional equation. We interpret the full nonlinear DFD field equation as a stress–strain constitutive equilibrium of the vacuum medium, define gravitational strain and stress in analogy with continuum mechanics, and show that the vacuum exhibits reduced gravitational permittivity at low gradients — the mechanism behind flat rotation curves without dark matter. The constitutive split parameter κ = α/4 ≈ 1.82 × 10⁻³ is derived from gauge-emergence corrections to the optical metric. Newton's constant G = c⁴/(8πK₀) is identified as the inverse vacuum force scale, explaining both its universality and its smallness. The vacuum stiffness is topologically structured, with normalization set by H₀ through K₀ = ℏH₀²/(8πα⁵⁷c). Companion Python module (dfd_gravity.py, 35/35 validation tests passing) included. Parent theory: Density Field Dynamics (DFD) unified theory v3.2, https://doi.org/10.5281/zenodo.18066593Alpha derivation: https://doi.org/10.5281/zenodo.19178465