  We study several approaches for constructing a minimal model of Universe
evolution by matching different stages of scale factor laws. We discuss the
continuity in the transitions among the stages and the time variables involved.
We develop a way to modelize these transitions without loss of observational
predictability. The key is the scale factor and its connection with minimal but
well stablished observational information (transition times and expansion
ratii). This construction is clearly useful in metric perturbations type
computations, but it can be applied in whatever subject dealing with a
cosmology involving different evolution stages.
