Nonperturbative gravito-magnetic fields
  In a cold matter universe, the linearized gravito-magnetic tensor field satisfies a transverse condition (vanishing divergence) when it is purely radiative. We show that in the nonlinear theory, it is no longer possible to maintain the transverse condition, since it leads to a non-terminating chain of integrability conditions. These conditions are highly restrictive, and are likely to hold only in models with special symmetries, such as the known Bianchi and $G_2$ examples. In models with realistic inhomogeneity, the gravito-magnetic field is necessarily non-transverse at second and higher order. 
Carlos Sopuerta (Jena), Roy Maartens (Portsmouth), George Ellis (Cape   Town), William Lesame (Pretoria)