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4.16 Inhomogeneous matter with isotropic quantum geometry

Inhomogeneous matter fields cannot be introduced directly to isotropic quantum geometry since, after the symmetry reduction, there is no space manifold left for the fields to live on. There are then two different routes to proceed: one can simply take the classical field Hamiltonian and introduce phenomenological modifications modeled on what happens to the isotropic Hamiltonian, or perform a mode decomposition of the matter fields and just work with the space-independent amplitudes. The latter is possible since the homogeneous geometry provides a background for the mode decomposition to be defined.

The basic question then, for the example of a scalar field, is how to replace the metric coefficient Ea Eb ∕∘ |detE-| i i in the gradient term of Equation (12View Equation). For the other terms, one can simply use the isotropic modification, which is taken directly from the quantization. For the gradient term, however, one does not have a quantum expression in this context and a modification can only be guessed. The problem arises because the inhomogeneous term involves inverse powers of E, while in the isotropic context the coefficient just reduces to ∘ --- |p|, which would not receive corrections at all. There is thus no obvious and unique way to find a suitable replacement.

A possible route would be to read off the corrections from the full quantum Hamiltonian, or at least from an inhomogeneous model, which requires a better knowledge of the reduction procedure. Alternatively, one can take a more phenomenological point of view and study the effects of different possible replacements. If the robustness of these effects to changes in the replacements is known, one can get a good picture of possible implications. So far, only initial steps have been taken (see [182Jump To The Next Citation Point] for scalar modes and [226Jump To The Next Citation Point] for tensor modes) and there is no complete program in this direction.

Another approximation of the inhomogeneous situation has been developed in [100Jump To The Next Citation Point] by patching isotropic quantum geometries together to support an inhomogeneous matter field. This can be used to study modified dispersion relations to the extent that the result agrees with preliminary calculations performed in the full theory [16834267268] even at a quantitative level. There is thus further evidence that symmetric models and their approximations can provide reliable insights into the full theory.

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