Quantum Substrate and Emergent Spacetime: A Complexity-Selection Framework for Resolving Foundational Physics Puzzles

We present a quantitative emergence framework in which a complexity-dependent scalar λ governs the transition from microscopic quantum reversibility to macroscopic classical spacetime. Empirical inputs from many-body localization, Krylov-complexity measurements, and entanglement-based gravity programs identify two robust thresholds that explain staged irreversibility in laboratory systems. The formalism λ (ρ, C, S) = ραCβ exp(−S/Scrit) combines energy density, organizational complexity, and symmetry-resolved entropy; calibrated exponents (α ≈ 0.41, β ≈ 0.70, Scrit ≈ 50 nats) reproduce experimental dual thresholds and export directly to cosmology. Big Bang regularization, the arrow of time, dark-sector phenomena, and the quantum measurement problem become calculable threshold crossings, while seventeen falsifiable predictions anchored in laboratory and astrophysical observables ensure Popperian accountability. The same bookkeeping extends cautiously to higher organizational layers, offering a disciplined template for cross-scale emergence studies.