Some tense theorists think that the expansion of the universe can be used to define an absolute cosmic time that then serves to define the notion of absolute simultaneity required by tense theories. I show how Mellor's argument against this strategy fails. Clarifying the strategy leads to an exposition of the Robertson-Walker metric, which describes such expanding universes. I identify the real reasons why we should reject this way of defining absolute simultaneity and temporal becoming.
Keywords: expanding universe, cosmic time, simultaneity, Robertson-Walker metric