Boundary Driven Expansion Model

The Boundary Driven Expansion Model (BDEM) presents a geometric reinterpretation of cosmological expansion in which large-scale spacetime evolution is governed by constraints imposed at the causal boundary of the observable universe. Expansion is described as a geometric response of interior spacetime to boundary-imposed curvature conditions rather than as a process sourced by interior energy densities. The framework preserves empirically established observations including the redshift–distance relation, the structure of the cosmic microwave background, large-scale homogeneity and isotropy, gravitational lensing behaviour, and structure formation. Dark matter and dark energy are not introduced as independent dynamical sectors; instead, gravitational effects commonly attributed to unseen components are interpreted as consequences of boundary-determined curvature. The core geometric relation replaces the Friedmann density formulation with a boundary-conditioned curvature equation involving extrinsic curvature, induced boundary curvature, and an asymptotic geometric term. The model is formulated entirely within established relativistic differential geometry and does not modify local field equations. Supplement A explores the interpretive interface between boundary-generated spacetime and particle phenomenology, proposing a geometric compatibility framework for field persistence without altering the operational structure of the Standard Model. Supplement B demonstrates background-level acoustic consistency with Planck TT observations, including sound horizon placement, peak alignment, trough extraction, and diffusion-scale damping constraints. Supplement C examines the testability of boundary-conditioned expansion at both cosmological and analogue boundary-dominated scales. BDEM is presented as a testable geometric reformulation within standard relativistic formalism. Its distinctions from ΛCDM are observationally discriminable in principle and require further perturbative and numerical development for full comparative analysis. ORCID iD: 0009-0002-8739-7727