Spacelike hypersurfaces of constant higher order mean curvature in   generalized Robertson-Walker spacetimes
  In this paper we analyze the problem of uniqueness for spacelike hypersurfaces with constant higher order mean curvature in generalized Robertson-Walker spacetimes. We consider first the case of compact spacelike hypersurfaces, completing some previous results given in [2]. We next extend these results to the complete noncompact case. In that case, our approach is based on the use of a generalized version of the Omori-Yau maximum principle for trace type differential operators, recently given in [3]. 
Luis J. Alias, Debora Impera and Marco Rigoli
math.DG