Edward Witten, physicist at the Institute for Advanced Study, delivers this talk at the MPG-IAS-NTU Center kick-off conference in Wolfensohn Hall. He takes up two linked problems: what it means to describe an observer embedded inside de Sitter space, the geometry cosmologists use to model an accelerating, expanding universe, and how that bears on Hugh Everett's many-worlds interpretation of quantum mechanics. Witten works through the technical difficulties of defining an observer's entropy and algebra of observables in a spacetime with a cosmological horizon, then turns to what these results imply for how branching and probability should be understood in Everett's framework. The talk assumes graduate-level familiarity with quantum field theory and general relativity, moving through Witten's own recent work on von Neumann algebras in de Sitter space before connecting it back to foundational questions about measurement and observation in quantum theory.