Since, even for isotropic models, complete effective equations have been derived only in rare cases so far,
it will take longer to obtain complete effective equations for inhomogeneities. Nevertheless, the corrections
can be derived systematically, and several of the simpler terms have already been obtained along
the lines of [86]. Of advantage in this perturbative setting is the fact that, to linear order in
inhomogeneities, one can split the perturbations in different modes (scalar, vector and tensor) and
compute their corrections separately. For each mode, gauge invariant variables can be determined.
This is discussed for scalar modes in [84, 92], for vector modes in [90
] and for tensor modes
in [91
]. Such a procedure would not be available at higher orders, where cosmological observables
can be computed in a canonical scheme [157, 156], but are difficult to introduce into a loop
quantization.
Once effective equations for inhomogeneities have been obtained, valuable insights can be derived for fundamental, as well as phenomenological, questions. As already mentioned, effective constraints allow a simpler discussion of anomalies than the full quantum theory provides. Anomalies are a pressing issue in the presence of inhomogeneities since the constraint algebra is quite non-trivial. By analyzing whether anomaly freedom is possible in the presence of quantum corrections, one can get hints as to whether such quantizations can be anomaly free in a full setting. This usually leads to additional conditions on the corrections, by which one can reduce quantization ambiguities. Since closed equations of motion for gauge invariant perturbations are possible only in the absence of anomalies, this issue also has direct implications for phenomenology. Anomaly-free effective constraints and the equations of motion they generate can be formulated in terms of gauge invariant quantities only, where gauge invariance refers to gauge transformations generated by the effective constraints including corrections. In this way, a complete analysis can be performed and evaluated for cosmological effects.
Effective equations for inhomogeneous models also provide the means to analyze refinements of the
underlying discreteness as they are suggested by the full Hamiltonian constraint operators. In this context,
inhomogeneous considerations help to elucidate the role of certain auxiliary structures in homogeneous
models, which have often led to a considerable amount of confusion. The basic objects of inhomogeneous
states are edge holonomies of the form where
is the coordinate length of the edge and
is some integrated connection component. (This may not directly be a component of
due to
the path ordering involved in non-Abelian holonomies. Nevertheless, we can think of matrix
elements of holonomies as of this form for the present purpose.) Similarly, basic fluxes are given by
surfaces transversal to a single edge in the graph. These are the elementary objects on which
constructions of inhomogeneous operators are based, in contrast to homogeneous models, where only
the total space or a chosen box of size
, as in Section 4.2, is available to define edges for
holonomies and surfaces for fluxes. Such a box is one of the auxiliary structures appearing in the
definition of homogeneous models since there is no underlying graph to relate these objects to a
state.
If a graph is being refined during evolution (in volume as internal time, say), the parameter can be
thought of as being non-constant, but rather a function of volume. Note that the whole expression of a
holonomy is coordinate-independent since the product
comes from a scalar quantity. The refinement
can thus also be formulated in coordinate independent terms. For instance, if we are close to an isotropic
configuration, we can, as in Equation (22
), introduce the isotropic connection component
with coordinate size
of the above-mentioned box. Isotropic holonomies thus take the form
, where, for a nearly-regular graph with respect to the background geometry,
is the inverse cubic root of the number of vertices within the box. For a refining
model,
increases with volume. For instance, if
with the total geometrical volume
we have holonomies of the form
with
. This agrees with
the suggestion of [27], or
in the notation of Section 5.5 [66
], but is recognized
here only as one special case of possible refinements. Moreover, one can easily convince oneself
that a vertex number proportional to volume is not allowed by the dynamics of loop quantum
gravity: this would require the Hamiltonian constraint to create only new vertices but not change
the spin of edges. The other limiting case, where only spins change but no new vertices are
created, corresponds to the non-refining model, where
. A realistic refinement must
therefore lie between those behaviors, i.e.,
with
, if it follows a power
law.
One can also see that appears only when the exactly isotropic model is introduced (possibly as a
background for perturbations), but not in inhomogeneous models. Basic corrections are thus independent of
and the chosen box. They refer rather to sizes of the elementary discrete variables: local edge
holonomies and fluxes. This characterization is independent of any auxiliary structures but directly refers to
properties of the underlying state. One should certainly expect this, because it is the quantum
state, which determines the quantum geometry and its corresponding corrections to classical
behavior.
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