Scalar-Tensor Hydrodynamics: A Modified Gravity Framework for Vacuum Viscosity and Cyclic Vacuum Cosmology

A theoretical proposal extending General Relativity by treating the vacuum as a dynamic scalar medium with its own stress-energy, pressure and dissipative response, rather than as a fixed geometric background plus a constant cosmological term. The formalism separates the Einstein geometric tensor from the vacuum viscous stress tensor, introduces a dynamical scalar-field sector, replaces a fixed cosmological constant with a field-dependent pressure contribution, and allows the effective gravitational coupling to depend on the local vacuum state. The matter sector is partitioned by a localization criterion derived from the excitation spectrum of the scalar field itself. The crossover is set by the medium's healing length rather than by a declared energy threshold, which recovers the electron rest energy as a calibration of that length rather than as an input. This is a proposal, not an established theory. The document states its own falsification conditions explicitly and carries an itemized research program, several items of which could terminate the framework.