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8.4 Homogeneous models

There are still several open areas in homogeneous models, which later can be extended to inhomogeneous ones:
Conceptual issues:
 This has already been mentioned above. Isotropic models provide simpler settings to analyze, e.g., the physical inner product [12716224], observables, different interpretations of quantum aspects or the emergence of a classical world.

Effective equations:
 Even in isotropic models effective equations have not yet been derived completely. A general scheme exists, shown to be analogous to standard effective action techniques [73], but it remains to be applied completely to quantum cosmology. This will then lead to a complete set of correction terms and their ranges of validity and importance. Also the question of whether an effective action for quantum cosmology exists and what its form is can be addressed.

Matter systems:
 Matter systems provide a rich source of diverse scenarios, but a full analysis is yet to be done. This includes adding different kinds of fluids [163], fermions or anisotropy parameters (shear term).

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