First the standard effective action is not a gauge invariant functional of its argument. For example if in a Yang–Mills theory one gauge-fixes the functional integral with an ordinary gauge fixing condition like , couples the Yang–Mills field to a source, and constructs the ordinary effective action, the resulting functional is not invariant under the gauge transformations of . Although physical quantities extracted from are expected to be gauge invariant, the noninvariance is cumbersome for renormalization purposes. The second problem is related to the fact that in a gauge theory a “coarse graining” based on a naive Fourier decomposition of is not gauge covariant and hence not physical. In fact, if one were to gauge transform a slowly varying with a parameter function with a fast -variation, a gauge field with a fast -variation would arise, which however still describes the same physics.

Both problems can be overcome by using the background field formalism. The background effective action generally is a gauge invariant functional of its argument (see Appendix B). The second problem is overcome by using the spectrum of a covariant differential operator built from the background field configuration to discrimate between slow modes (small eigenvalues) and fast modes (large eigenvalues) [187]. This sacrifices to some extent the intuition of a spatial coarse graining, but it produces a gauge invariant separation of modes. Applied to a non-gauge theory it amounts to expanding the field in terms of eigenfunctions of the (positive) operator and declaring its eigenmodes ‘long’ or ‘short’ wavelength depending on whether the corresponding is smaller or larger than a given .

This is the strategy adopted to define the effective average action for gravity [179]. In short: The effective average action for gravity is a variant of the background effective action described in Appendix B (see Equations (B.48, B.51)), where the bare action is modified by mode cutoff terms as in Appendix C, but with the mode cutoff defined via the spectrum of a covariant differential operator built from the background metric. For convenience we quickly recapitulate the main features of the background field technique here and then describe the modifications needed for the mode cutoff.

The initial bare action is assumed to be a reparameterization invariant functional of the metric . Infinitesimally the invariance reads , where is the Lie derivative of with respect to the vector field . The metric (later the integration variable in the functional integral) is decomposed into a background and a fluctuation , i.e. . The fluctuation field is then taken as the dynamical variable over which the functional integral is performed; it is not assumed to be small in some sense, no expansion in powers of is implied by the split. Note however that this linear split does not have a geometrical meaning in the space of geometries. The symmetry variation can be decomposed in two different ways

We shall refer to the first one as “genuine gauge transformations” and to the second one as the “background gauge transformations”. The background effective action is a functional of the expectation value of the fluctuation variable, the background metric , and the expectation values of the ghost fields , . Importantly is invariant under the background field transformations (4.2). So far one should think of the background geometry as being prescribed but of generic form; eventually it is adjusted self-consistently by a condition involving the full effective action (see Equation (2.48) and Appendix B).In the next step the initial bare action should be replaced by one involving a mode cutoff term. In the background field technique the mode cutoff should be done in a way that preserves the invariance under the background gauge transformations (4.2). We now first present the steps leading to the scale dependent effective average action in some detail and then present the FRGE for it. The functional integrals occuring are largely formal; for definiteness we consider the Euclidean variant where the integral over Riemannian geometries is intended. The precise definition of the generating functionals is not essential here, as they mainly serve to arrive at the gravitional FRGE. The latter provides a novel tool for investigating the gravitational renormalization flow.

We begin by introducing a scale dependent variant of the generating functional of the connected Greens functions. The cutoff scale is again denoted by , it has unit mass dimension, and no physics interpretation off hand. The defining relation for reads

Here the measure differs from the naive one, , by gauge fixing terms and an integration over ghost fields , , where the action for the latter is again modified by a mode-cutoff: The first term in the exponent is the gauge fixing term. The gauge fixing condition must be invariant under Equation (4.2), for the moment we may leave it unspecified. The second term is the Faddeev–Popov action for the ghosts obtained in the usual way: One applies a genuine gauge transformation (4.1) to and replaces the parameter by the ghost field . The integral over and then exponentiates the Fadeev–Popov determinant . This gauge fixing procedure has a somewhat perturbative flavor; large scale aspects of the space of geometries are not adequately taken into account. The terms and implement the mode cutoff in the gravity and the ghost sector, respectively. We shall specify them shortly. Finally we coupled in Equation (4.4) the ghosts to sources , for later use.The construction of the effective average action now parallels that in the scalar case. We quickly run through the relevant steps. The Legendre transform of at fixed is

As usual, if is differentiable with respect to the sources, the extremizing source configurations , , allow one to interpret , , as the expectation values of , , via Note that the expectation values defined through Equation (4.3) are in general both -dependent and source dependent. In Equation (4.6), by construction, the -dependence carried by cancels that carried by the extremizing source. Concretely the extremizing sources are constructed by assuming that has a series expansion in powers of the sources with -dependent coefficients; formal inversion of the series then gives a -dependent with the property that , and similarly for the ghost sources. The formal effective field equations dual to Equation (4.6) read As in the scalar case the effective average action differs from by the cutoff action with the expectation value fields inserted, Sometimes it is convenient to introduce , which is the expectation value of the original ‘quantum’ metric , and to regard as a functional of rather than , i.e. .Usually one is not interested in correlation functions involving Faddeev–Popov ghosts and it is sufficient to know the reduced functional

As indicated we shall simply write for its argument .The precise form of the gauge condition is inessential, only the invariance under Equation (4.2) is important. It ensures that the associated ghost action is invariant under Equation (4.2) and , . We shall ignore the problem of the global existence of gauge slices (“Gribov copies”), in accordance with the formal nature of the construction. For later reference let us briefly describe the most widely used gauge condition, the “background harmonic gauge” which reads

The covariant derivative involves the Christoffel symbols of the background metric. Note that of Equation (4.10) is linear in the quantum field . On a flat background with the condition reduces to the familiar harmonic gauge condition, . In Equation (4.10) is an arbitrary constant with the dimension of a mass. We shall set and interpret as the bare Newton constant. The ghost action for the gauge condition (4.10) is where and are the covariant derivatives associated with (as a short for ) and , respectively.The last ingredient in Equations (4.3, 4.4) to be specified are the mode cutoff terms, not present in the usual background effective action. Their precise form is arbitrary to some extent. Naturally they will be taken quadratic in the respective fields, with a kernel which is covariant under background gauge transformations. These requirements are met if

and the kernels , transform covariantly under . In addition they should effectively suppress covariant ‘momentum modes’ with ‘momenta’ . As mentioned earlier one way of defining such a covariant scale is via the spectrum of a covariant differential operator. Concretely the following choice will be used.Consider the Laplacian of the Riemannian background metric with being its torsionfree connection. We assume to be such that has a non-negative spectrum and a complete set of (tensorial) eigenfunctions. The spectral values of will then be functionals of and one can choose and such that only eigenmodes with spectral values ( being the mass dimension of the operator) enter the functional integral unsuppressed. Here one should think of the functional integral as being replaced by one over the (complete system of) eigenfunctions of , for a fixed . Concretely, for and we take expressions of the form

As indicated in Equation (4.14) the prefactors are different for the gravitational and the ghost cutoff. For the ghosts is a pure number, whereas for the metric fluctuation is a tensor constructed from the background metric . We shall discuss the choice of these prefactors later on.The essential ingredient in Equation (4.14) is a function interpolating smoothly between and ; for example

Its argument should be identified with the weighted spectral values of . One readily sees that then the exponentials in Equation (4.13) have the desired effect: They effectively suppress eigenmodes of with spectral values much smaller than , while modes with large compared to are unaffected. This also illustrates that a mode suppression can be defined covariantly using the background field formalism.This concludes the definition of the effective average action and its various specializations. We now present its key properties.

- The effective average action is invariant under background field diffeomorphisms where all its arguments transform as tensors of the corresponding rank. This is a direct consequence of the corresponding property of in Equation (4.3) where is the Lie derivative of the respective tensor type. Here Equation (4.16) is obtained as in Equation (B.47) of Appendix B, where the background covariance of the mode cutoff terms (4.13) is essential. Further the measure is assumed to be diffeomorphism invariant.
- It satisfies the functional integro-differential equation where (with and minus the first two terms in the exponent of Equation (4.4) and .
- The -dependence of the effective average action is governed by an exact FRGE. Following the same
lines as in the scalar case one arrives at [179]
Here denotes the Hessian of with respect to the dynamical fields , , at fixed
. It is a block matrix labeled by the fields , ,
(In the ghost sector the derivatives are understood as left derivatives.) Likewise, is a block
matrix with entries and . Performing the trace in the
position representation it includes an integration involving the background volume
element. For any cutoff which is qualitatively similar to Equation (4.14, 4.15) the traces on the
right-hand-side of Equation (4.19) are well convergent, both in the infrared and in the
ultraviolet. By virtue of the factor , the dominant contributions come from a narrow
band of generalized momenta centered around . Large momenta are exponentially
suppressed.
The conceptual status and the use of the gravitational FRGE (4.20) is the same in the scalar case discussed in Section 2.2. Its perturbative expansion should reproduce the traditional non-renormalizable cutoff dependencies starting from two-loops. In the context of the asymptotic safety scenario the hypothesis at stake is that in an exact treatment of the equation the cutoff dependencies entering through the initial data get reshuffled in a way compatible with asymptotic safety. The Criterion (FRGC1) for the existence of a genuine continuuum limit discussed in Section 2.3 also applies to Equation (4.20). In brief, provided a global solution of the FRGE (4.20) can be found (one which exists for all ), it can reasonably be identified with a renormalized effective average action constructed by other means. The intricacies of the renormalization process have been shifted to the search for fine-tuned initial functionals for which a global solution of Equation (4.20) exists. For such a global solution then is the full quantum effective action and is the fixed point action. As already noted in Section 2.3 the appropriate positivity requirement (FRGC2) remains to be formulated; one aspect of it concerns the choice of factors in Equation (4.13) and will be discussed below.

The background gauge invariance of expressed in Equation (4.16) is of great practical importance. It ensures that if the initial functional does not contain non-invariant terms, the flow implied by the above FRGE will not generate such terms. In contrast locality is not preserved of course; even if the initial functional is local the flow generates all sorts of terms, both local and nonlocal, compatible with the symmetries.

For the derivation of the flow equation it is important that the cutoff functionals in Equation (4.13) are quadratic in the fluctuation fields; only then a flow equation arises which contains only second functional derivatives of , but no higher ones. For example using a cutoff operator involving the Laplace operator of the full metric would result in prohibitively complicated flow equations which could hardly be used for practical computations.

For most purposes the reduced effective average action (4.9) is suffient and it is likewise background invariant, . Unfortunately does not satisfy an exact FRGE, basically because it contains too little information. The actual RG evolution has to be performed at the level of the functional . Only after the evolution one may set , . As a result, the actual theory space of Quantum Einstein Gravity in this setting consists of functionals of all four variables, , subject to the invariance condition (4.9). Since involves derivatives with respect to at fixed it is clear that the evolution cannot be formulated in terms of alone.

- approaches for the background effective action of Appendix B, since , vanish for . The limit can be infered from Equation (4.18) by the same reasoning as in the scalar case (see Appendix C). This gives Note that the bare initial functional includes the gauge fixing and ghost actions. At the level of the functional Equation (4.21) reduces to .
- The effective average action satisfies a functional BRST Ward identity which reflects the invariance of under the BRST transformations Here is an anti-commuting parameter. Since the mode cutoff action is not BRST invariant, the Ward identity differs from the standard one by terms involving , . For the explicit form of the identity we refer to [179].
- Initially the vertex (or 1-PI Greens) functions are given by multiple functional derivatives of
with respect to , at fixed and setting
after differentiation. The resulting multi-point functions are -dependent
functionals of the (-independent) . For extremizing sources obtained by formal series
inversion the condition (4.23) automatically switches off the sources in Equation (4.7); for the ghosts
this is consistent with having ghost number zero. Note that the one-point function
vanishes identically.
An equivalent set of vertex functions should in analogy to the Yang–Mills case [68, 42, 67] be obtained by differentiating with respect to . Specifically for one gets multipoint functions , with the shorthand (4.9). The solutions of

play an important role in the interpretation of the formalism (see Section4.2).The precise physics significance of the multipoint functions and remains to be understood. One would expect them to be related to S-matrix elements on a self-consistent background but, for example, an understanding of the correct infrared degrees of freedom is missing.

This concludes our summary of the key properties of the gravitational effective average action. Before turning to applications of this formalism, we discuss the significance and the proper choice of the factors in Equation (4.14), which is one aspect of the positivity issue (FRGC2) of Section 2.2. The significance of these factors is best illustrated in the scalar case. As discussed in Appendix C in scalar theories with more than one field it is important that all fields are cut off at the same . This is achieved by a cutoff function of the form (C.21) where is in general a matrix in field space. In the sector of modes with inverse propagator the matrix is diagonal with entries . In a scalar field theory these factors are automatically positive and the flow equations in the various truncations are well-defined.

In gravity the situation may be more subtle. First, consider the case where is some normal mode of and that it is an eigenfunction of with eigenvalue , where is a positive eigenvalue of some covariant kinetic operator, typically of the form -terms. If the situation is clear, and the rule discussed in the context of scalar theories applies: One chooses because this guarantees that for the low momentum modes the running inverse propagator becomes , exactly as it should be.

More tricky is the question how should be chosen if is negative. If one continues to use , the evolution equation is perfectly well defined because the inverse propagator never vanishes, and the traces of Equation (4.19) are not suffering from any infrared problems. In fact, if we write down the perturbative expansion for the functional trace, for instance, it is clear that all propagators are correctly cut off in the infrared, and that loop momenta smaller than are suppressed. On the other hand, if we set , then introduces a spurios singularity at , and the cutoff fails to make the theory infrared finite. This choice of is ruled out therefore. At first sight the choice might have appeared more natural because only if the factor is a damped exponentially which suppresses the low momentum modes in the usual way. For the other choice the factor is a growing exponential instead and, at least at first sight, seems to enhance rather than suppress the infrared modes. However, as suggested by the perturbative argument, this conclusion is too naive perhaps.

In all existing RG studies using this formalism the choice

has been adopted, and there is little doubt that, within those necessarily truncated calculations, this is the correct procedure. Besides the perturbative considerations above there are various arguments of a more general nature which support Equation (4.25):- At least formally the construction of the effective average action can be repeated for Lorentzian signature metrics. Then one deals with oscillating exponentials , and for arguments like the one leading to Equation (4.21) one has to employ the Riemann–Lebesgue lemma. Apart from the obvious substitutions, , , the evolution equation remains unaltered. For it has all the desired features, and seems not to pose any special problem, since for either sign leads to an IR suppression.
- For finite the Euclidean FRGE is perfectly well-defined even if , while the status of the Euclidean functional integral with its growing exponential seems problematic. In principle there exists the possibility of declaring the FRGE the primary object. If a global solution to it exists this functional might define a consistent quantum theory of gravity even though the functional integral per se does not exist. As noted in Section 3.4 the inclusion of ‘other desirable’ features might then be more difficult, though.
- It might be, and there exist indications in this direction [131, 132], that for the exact RG flow is always a positive operator (one with positive spectrum) along physically relevant RG trajectories. Then for all modes and the problem does not arise. If this contention is correct, the phenomenon which is known to occur in certain truncations would be an artifact of the approximations made.

In fact, as we shall discuss in more detail later on, the to date best truncation used for the investigation of asymptotic safety (“ truncation”) has only positive factors in the relevant regime. On the other hand, the simpler “Einstein–Hilbert truncation” has also negative ’s. If one applies the rule (4.25) to it, the Einstein–Hilbert truncation produces almost the same results as the truncations [131, 132]. This is a strong argument in favor of the rule.

In [133, 131] a slightly more general variant of the construction described here has been employed. In order to facilitate the calculation of the functional traces in the FRGE (4.19) it is helpful to employ a transverse-traceless (TT) decomposition of the metric: . Here is a transverse traceless tensor, a transverse vector, and and are scalars. In this framework it is natural to formulate the cutoff in terms of the component fields appearing in the TT decomposition . This cutoff is referred to as a cutoff of “type B”, in contradistinction to the “type A” cutoff described above, . Since covariant derivatives do not commute, the two cutoffs are not exactly equal even if they contain the same shape function . Thus, comparing type A and type B cutoffs is an additional possibility for checking scheme (in)dependence [133, 131].

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