Constructing exact perturbations of the standard cosmological models - art. no. 103515

In this paper we show a procedure to construct cosmological models which, a ccording to a covariant criterion, can be seen as exact (nonlinear) perturb ations of the standard Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmologi cal models. The special properties of this procedure will allow us to selec t some of the characteristics of the models and also to study in depth thei r main geometrical and physical features. In particular, the models are con formally stationary, which means that they are compatible with the existenc e of isotropic radiation, and the observers that would measure this isotrop y are rotating. Moreover, these models have two arbitrary functions (one of them is a complex function) which control their main properties, and in ge neral they do not have any isometry. We study two examples, focusing on the case when the underlying FLRW models are flat dust models. In these exampl es we compare our results with those of the linearized theory of perturbati ons about a FLRW background. [S0556-2821(99)08320-4].