  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.
