Towards a Unified Dark Sector: The Scalar Unified Dark Adaptive Theory (SUDAT) Framework and its Cosmological Implications.

The \LambdaCDM cosmological model, while empirically successful, faces theoretical challenges including the cosmic coincidence problem, ontological disparity between dark matter and dark energy, and extreme fine-tuning requirements. We propose the Scalar Unified Dark Adaptive Theory (SUDAT) as a novel framework where a single canonical scalar field exhibits different behaviors in high and low-density environments through a baryon density-dependent potential V(\phi,\rho_{b}). The model introduces a density-dependent transition mechanism controlled by a critical density \rho_{crit}=m^{2}/(2\lambda V_{0}), with the scalar field exhibiting pressureless behavior (w_{\phi}\approx0) in high-density regions (\rho_{b}\gg\rho_{crit}) and negative pressure (w_{\phi}\approx-1) in low-density environments (\rho_{b}\ll\rho_{crit}). We derive the complete field equations from a Jordan-frame action with geometric coupling to baryonic density, ensuring energy-momentum conservation and covariance. We present preliminary observational tests against cosmological data and galactic rotation curves, while explicitly acknowledging significant theoretical limitations and challenges. Our analysis reveals that the model does not fully resolve the cosmic coincidence problem as initially claimed, but rather shifts fine-tuning requirements to the parameters defining \rho_{crit}. The framework requires substantial further development, particularly regarding stability analysis, UV completion, and comprehensive observational testing. This work presents SUDAT as a developing theoretical framework that merits further investigation but should not be considered a complete solution to dark sector challenges. All numerical implementations are publicly available at https://doi.org/10.5281/zenodo.17539070 for independent verification and extension.