  We shall discuss cosmological models in extended theories of gravitation. We
shall define a surface, called the model surface, in the space of observable
parameters which characterises families of theories. We also show how this
surface can be used to compare with observations. The model surface can
potentially be used to falsify whole families of models instead reasoning on a
single model basis as it is usually done by best fit arguments with
observations.

General Relativity and Quantum Cosmology