  Cosmic horizons arise in general relativity in the context of black holes and
in certain cosmologies. Classically, regions beyond a horizon are inaccessible
to causal observers. However, quantum mechanical correlations may exist across
horizons that may influence local observations. For the case of de Sitter
space, we show how a single particle excitation behind the horizon changes the
density matrix governing local observables. As compared to the vacuum state, we
calculate the change in the average energy and entropy per unit volume. This
illustrates what may be a generic property allowing some features of spacetime
beyond a horizon to be inferred.
