Petrov classification of perturbed spacetimes: the Kasner example
  In this paper we consider vacuum Kasner spacetimes, focusing on those that can be parametrized as linear perturbations of the special Petrov type D case. For these quasi-D Kasner models we first investigate the modification to the principal null directions, then a Teukolsky Master Equation for fields of any spin, considering in particular the quasi-D models as curvature perturbations of the type D background. Considering the speciality index and the principal null directions and comparing the results for the exact solutions and those for the perturbative ones, this simple Kasner example allows us to clarify that perturbed spacetime do not retain in general the speciality character of the background. There are four distinct principal null directions, although they are not necessarily first order perturbations of the background principal null directions, as our example of the quasi-D Kasner models shows.   For the quasi-D Kasner models the use of a Teukolsky Master Equation, a classical tool for studying black hole perturbations, allows us to show, from a completely new point of view, the well known absence of gravitational waves in Kasner spacetimes. This result, used together with an explicit expression of the electric Weyl tensor in terms of Weyl scalars, provides an example of the fact that the presence of transverse curvature terms does not necessarily imply the presence of gravitational waves. 
Christian Cherubini, Donato Bini, Marco Bruni, Zoltan Perjes