This working paper introduces the Extended Finsler Timescape-Lapse (EFT-Lapse) Theory, a transformative framework that resolves the fundamental cosmological tension between different expansion measurements by treating the expansion rate as a direct consequence of inhomogeneous geometry and bidirectional causality. By anchoring the cosmological coupling to the minimal shear viscosity of the primordial plasma, the model achieves seamless agreement with local astronomical observations without the need for Dark Energy. As a preliminary theoretical blueprint, this document serves as a foundation for the scientific community to explore and refine a more rigorous, structure-dependent paradigm of spacetime. It is intended to inspire future studies into the geometric origins of cosmic acceleration and the potential for a fully unified, backreaction-based cosmology. ACKNOWLEDGMENTS AND AI DISCLOSURE The author acknowledges the use of AI tools (including Perplexity and Google Gemini) for drafting assistance, the derivation and formatting of mathematical equations, computational verification, and problem-solving. While these models provided support in refining formulations and verifying solutions, all theoretical conclusions and scientific insights were developed and critically evaluated by the author, who maintains full responsibility for the integrity of the work All theoretical content, ideas, scientific conclusions were developed and critically evaluated by A. Backmund LLM-EFT-Lapse Collaboration. This AI use is disclosed in the spirit of transparency and researcher responsibility as recommended by the European Commission’s “Living Guidelines on the Responsible Use of Generative AI in Research” (2025). The author gratefully acknowledges the provision of foundational datasets by the SH0ES, Planck, and Bayesian QGP collaborations, which were essential for the validation of this working paper

This working paper introduces the Extended Finsler Timescape–Lapse (EFT‑Lapse) theory as a unified, field-theoretic framework that attributes the Hubble-tension discrepancy to inhomogeneous cosmology, spacetime lapse modulation, and time-symmetric (CPT-consistent) bidirectional causality, without introducing an additional exotic dark-energy component.The approach models spatially varying proper-time vs. coordinate-time relations via a lapse function controlled by an ultra-light scalar field, and formulates forward- and backward-causal contributions as distinct channels to avoid apparent double counting of backreaction and local-void effects.It further presents a decomposition into several orthogonal solution modes and outlines testable observational signatures and technical addenda supporting mathematical rigor. As a hypothetical blueprint, this document serves as an open invitation for collaboration to refine its Finslerian field equations and test its specific predictions—including CMB birefringence and accelerated early-galaxy growth—aiming to inspire a transition from FLRW-averaging to a more rigorous, structure-dependent spacetime paradigm.