Uniqueness of the non-singular family and characterisation of   cosmological models
  We prove that for an orthogonal spacetime metric separable in space and time in comoving coordinates, the requirements of perfect fluid and non-singularity single out the unique family of singularity free cosmological models. Further homogeneous models could only be Bianchi I or FLRW while inhomogeneous ones can be with or without a singularity. 
Naresh Dadhich and L.K. Patel