A difference to homogeneous models, however, is that the internal directions of a connection and a triad do
not need to be identical, which in homogeneous models with internal directions is the case as a
consequence of the Gauss constraint
. With inhomogeneous fields, now, the Gauss constraint
reads
As a consequence, is not conjugate to
, anymore, and instead the momentum of
is
[52
]. This seems to make a quantization more complicated since the momenta will be
quantized to simple flux operators, but do not directly determine the geometry such as the volume
. For this, one would need to know the angle
which depends on both
connections and triads. Moreover, it would not be obvious how to obtain a discrete volume spectrum since
then volume does not depend only on fluxes.
It turns out that there is a simple canonical transformation, which allows one to work with canonical
variables and
playing the role of momenta of
and
[75
]. This seems to be
undesirable, too, since now the connection variables are modified which play an important role for
holonomies. That these canonical variables are very natural, however, follows after one considers the
structure of spin connections and extrinsic curvature tensors in this model. The new canonical variables are
then simply given by
,
, i.e., proportional to extrinsic curvature
components. Thus, in the inhomogeneous model we simply replace connection components with extrinsic
curvature in homogeneous directions (note that
remains unchanged) while momenta remain
elementary triad components. This is part of a broader scheme which is also important for the Hamiltonian
constraint operator (Section 5.14).
With the polarization condition the kinematics of the quantum theory simplifies. Relevant
holonomies are given by along edges in the 1-dimensional manifold
and
in vertices with real
. Cylindrical functions depend on finitely many of those holonomies,
whose edges and vertices form a graph in the 1-dimensional manifold. Flux operators, i.e., quantized triad
components, act simply by
Since triad components have simple quantizations, one can directly combine them to get the volume operator and its spectrum
The labelsCommutators between holonomies and the volume operator will technically be similar to homogeneous models, except that there are more possibilities to combine different edges. Accordingly, one can easily compute all matrix elements of composite operators such as the Hamiltonian constraint. The result is only more cumbersome because there are more terms to keep track of. Again as in diagonal homogeneous cases, the triad representation exists and one can formulate the constraint equation there. Now, however, one has infinitely many coupled difference equations for the wave function since the lapse function is inhomogeneous (one obtains one difference equation for each vertex).
There are obvious differences to cases considered previously owing to inhomogeneity. For instance, each
edge label can take positive or negative values, or go through zero during evolution corresponding to the
fact that a spatial slice does not need to intersect the classical singularity everywhere. Also the structure of
coefficients of the difference equations, though qualitatively similar to homogeneous models, is changed
crucially in inhomogeneous models, mainly due to the volume eigenvalues (64). Now,
, say,
and thus
can be zero without volume eigenvalues in neighboring vertices having zero
volume.
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