Dark Energy and Condensate Stars: Casimir Energy in the Large

Vacuum fluctuations and the Casimir effect are considered in a cosmological setting. It is suggested that the dark energy, which recent observations suggest make up 73% of our universe, is vacuum energy due to a causal boundary effect at the cosmological horizon. After a discussion of the similarities and differences between material boundaries in flat spacetime and causal horizons in general relativity, a simple model with a purely vacuum energy de Sitter interior and Schwarzschild exterior, separated by a thin boundary layer is outlined. The boundary layer is a quantum transition region which replaces the event horizons of the classical de Sitter and Schwarzschild solutions, through which the vacuum energy changes.