The Scalar Substrate Model (SSM): A Unied Eective Field Theory for Dark Matter, Dark Energy, and MOND-like Gravity

The Scalar Substrate Model (SSM) is a minimal effective field theory in which a single real scalar field — the substrate — aims to account for cold dark matter, the cosmological constant, and the modified low-acceleration (MOND-like) dynamics observed in galaxies. The kinetic sector is governed by a non-analytic function of the standard kinetic invariant, yielding a MOND regime at low gradients and kinetic screening (k-mouflage) at high gradients, thereby suppressing fifth-force effects in dense environments. A quadratic potential provides both a pressureless oscillating dark-matter-like component and an effective vacuum energy driving cosmic acceleration. The model contains four physical parameters: the MOND acceleration scale, the scalar mass, the vacuum energy density, and a dimensionless coupling to baryonic matter. Within the regime analysed in the paper, the theory appears ghost-free and causal, with subluminal propagation speeds. The work derives the energy-momentum tensor, Friedmann equations, and the static galactic field equation, recovering the MOND relation in the low-acceleration limit. Although detailed numerical simulations (CMB, rotation curves, lensing) and a complete post-Newtonian analysis remain to be performed, the SSM provides a unified and falsifiable exploratory framework for the dark sector.