 To what extent can observations determine the spacetime geometry of the universe directly? The question can be posed more precisely in terms of the region that is, in principle, accessible to an observer at a location in spacetime \(p\)—the causal past, \(J^-(p)\), of that point. This set includes all regions of spacetime from which signals traveling at or below the speed of light can reach \(p\). What can observations confined to \(J^-(p)\), assuming that GR is valid, reveal about the spacetime geometry of \(J^-(p)\) itself, and the rest of spacetime?