### 4. COSMIC COHERENCE

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.