In a typical inflation model the inflaton is completely out of the picture by the end of reheating and one is left with a SBB model set up with particular initial conditions. In realistic models the inhomogeneities are small at early times and can be treated in perturbation theory. Linear perturbation theory in the SBB has special properties that lead to a certain type of phase coherence in the perturbations. This coherence can be observed, especially in the microwave background anisotropies. Models where the perturbations are subject to only linear evolution at early times are call ``passive'' models.