Over the Rainbow: Numerical Relativity beyond Scri+

This is a study of the behavior of wave equations in conformally compactified spacetimes suited to the use of computational boundaries beyond Scri+. There light cones may be adjusted for computational convenience and/or Scri+ may be approximated by a "proto-Scri" spacelike hypersurface just outside a de Sitter horizon. One expects a numerical implementation to excise the physically unnecessary universe somewhat beyond the outer horizon. As an entry level example I study forms of the Maxwell equations and causal relations for an outer boundary in that example.