Any compact spacelike hypersurface immersed in a doubly warped product spacetime I h × ρ P with nondecreasing warping factor ρ must be a spacelike slice, provided that the mean curvature satisfies H ≥ ρ ′ /hρ everywhere on the hypersurface. The conclusion also holds, under suitable assumptions on the immersion, when the hypersurface is complete and noncompact. A similar rigidity property is shown for compact hypersurfaces in spacetimes carrying a conformal, strictly expanding, timelike vector field.
The geometric settingg) be an isometric immersion between (connected) semi-Riemannian manifolds and denote with ∇ and ∇, respectively, the Levi-Civita connections for g and g. For any given point p ∈ M , there exist sufficiently small neighbourhoods U ⊆ M and U ⊆ M , respectively, of p and ψ(p), such that ψ(U ) ⊆ U , ψ| U : U → U is an embedding and U supports a nowhere vanishing vector field Z ∈ X(U ) satisfying g(Z ψ(q) , (dψ) q V ) = 0 for every q ∈ U , V ∈ T q M . We say that