### 4.4 Isotropy: Effective densities and equations

The isotropic model is thus quantized in such a way that the operator has a discrete spectrum containing zero. This immediately leads to a problem since we need a quantization of in order to quantize a matter Hamiltonian such as (23) where not only the matter fields but also geometry are quantized. However, an operator with zero in the discrete part of its spectrum does not have a densely defined inverse and does not allow a direct quantization of .

This leads us to the first main effect of the loop quantization: It turns out that despite the non-existence of an inverse operator of one can quantize the classical to a well-defined operator. This is not just possible in the model but also in the full theory where it even has been defined first [193]. Classically, one can always write expressions in many equivalent ways, which usually result in different quantizations. In the case of , as discussed in Section 5.3, there is a general class of ways to rewrite it in a quantizable manner [41] which differ in details but have all the same important properties. This can be parameterized by a function [4750] which replaces the classical and strongly deviates from it for small while being very close at large . The parameters and specify quantization ambiguities resulting from different ways of rewriting. With the function

we have
which indeed fulfills for , but is finite with a peak around and approaches zero at in a manner
as it follows from . Some examples displaying characteristic properties are shown in Figure 9 in Section 5.3.

The matter Hamiltonian obtained in this manner will thus behave differently at small . At those scales also other quantum effects such as fluctuations can be important, but it is possible to isolate the effect implied by the modified density (25). We just need to choose a rather large value for the ambiguity parameter such that modifications become noticeable already in semiclassical regimes. This is mainly a technical tool to study the behavior of equations, but can also be used to find constraints on the allowed values of ambiguity parameters.

We can thus use classical equations of motion, which are corrected for quantum effects by using the effective matter Hamiltonian

(see Section 5.5 for details on effective equations). This matter Hamiltonian changes the classical constraint such that now
Since the constraint determines all equations of motion, they also change: We obtain the effective Friedmann equation from ,
and the effective Raychaudhuri equation from ,

Matter equations of motion follow similarly as

which can be combined to the effective Klein-Gordon equation
Further discussion for different forms of matter can be found in [186].