Black Holes and Expanding Universes as Observer-Dependent Descriptions of the Same Geometry : A Proposal for Pure Geometric Duality

A proposal for geometric duality between black hole interiors and expanding cosmologies. Argues that event horizons create observer-complementary descriptions where "black hole" and "universe" are names for the same geometry viewed from opposite sides of a causal boundary. Based on signature flip at horizons, Kantowski-Sachs geometry, and horizon thermodynamics. 1+1=1.