Can we know the global structure of spacetime?

Here, we briefly review the notion of observational indistinguishability within the context of classical general relativity. We settle a conjecture given by Malament [(1977). Observationally indistinguishable space-times. In J. Earman, C. Glymour, & J. Statchel (Eds.), Foundations of space–time theories. Minnesota studies in the philosophy of science (Vol. VIII, pp. 61–80). Minneapolis: University of Minnesota Press] concerning the subject and then strengthen the result considerably. The upshot is this: There seems to be a robust sense in which the global structure of every cosmological model is underdetermined.