A Unified Theory of Spacetime Excitation Based on a Dynamical Scalar Field and Its Explanation of Dark Matter and Dark Energy Phenomena

This preprint proposes a unified theoretical framework that explains both dark matter and dark energy as manifestations of a single classical scalar field ϕ, interpreted as an "excitation" of the spacetime ground state. The theory modifies general relativity via a minimal coupling between the scalar field and baryonic matter density. In the weak-field, galactic regime, it predicts an effective enhancement of Newtonian gravity—successfully fitting the rotation curve of the NGC 3198 galaxy with only two universal parameters: a characteristic length scale λ ≈ 4.2 kpc and a dimensionless coupling β ≈ 0.12. On cosmological scales, the same scalar field’s potential acts as dynamical dark energy, driving late-time cosmic acceleration. The model preserves the constancy of the speed of light and is compatible with special relativity. A linear version is presented, with a discussion of the necessity of a non-linear screening mechanism (e.g., chameleon-type) to satisfy solar system constraints. This work offers a promising path toward unifying the dark sector without invoking new particles or a cosmological constant.