  In this work we study maximal hypersurfaces in spatially open Generalized
Robertson-Walker spacetimes with Ricci-flat fiber by means of a generalized
maximum principle. In particular, under natural geometric and physical
assumptions we provide new uniqueness and non-existence results for complete
maximal hypersurfaces in spatially open Robertson-Walker spacetimes with flat
fiber. Moreover, our results are applied to relevant spacetimes as the steady
state spacetime, Einstein-de Sitter spacetime and radiation models.
