Philip Broadbridge Australian Mathematical Sciences Institute and University of Melbourne Dark Energy States of a Scalar Field in an Unstable Universe Wednesday 7th March 14:05-14:55pm, Eastern Avenue Lecture Theatre. Observations now indicate an expanding and accelerating universe. The simplest compatible large-scale model is the spatially flat FLRW universe with positive cosmological constant. The Hamiltonian of a scalar massless Klein-Gordon field, minimally coupled to the space-time geometry, naturally decomposes into a light component with well-defined particle numbers, plus a dark component whose most elementary quantized subsystems have continuous spectrum unbounded below. At a sequence of discrete times, progressively shorter wavelength components are lost from the light domain to the dark domain.