In typical applications of the FRG the ultraviolet renormalization problem does not have to be addressed. In the context of the asymptotic safety scenario this is different. By definition the perturbative series in a field theory based on an asymptotically safe functional measure has a dependence on the UV cutoff which is not strictly renormalizable (see Section 1.3). The perturbative expansion of an FRGE must reproduce the structure of these divergencies. On the other hand in an exact treatment or based on different approximation techniques a reshuffling of the cutoff dependence is meant to occur which allows for a genuine continuum limit. We therefore outline here how the UV renormalization problem manifests itself in the framework of the functional flow equations. The goal will be to formulate a criterion for the plausible existence of a genuine continuum limit in parallel to the one above based on perturbative indicators.
Again we illustrate the relevant issues for a scalar quantum field theory on flat space. For definiteness
we consider here the flow equation for the effective average action , for other types
of FRGEs the discussion is similar though. The effective average action interpolates between
the bare action
and the above, initially regulated, effective action
, according to
The conventional effective action obeys a well-known functional integro-differential equation which
implicitly defines it (see Equation (B.8) below). Its counterpart for
reads
The precise form of the mode suppression is inessential. In the following we outline a variant which is
technically convenient. Here is a quadratic form in the fields defined in terms of a kernel
chosen such that both
and
define integral operators of trace-class on the function
space considered. We write
for the integral operator and
for its trace. The other properties of the kernel are best
described in Fourier space, where
acts as
, with
, the Fourier transform of
and similarly for the kernel (where we omit the
hat for notational simplicity). The UV cutoff
renders Euclidean momentum space compact
and Mercer’s theorem then provides simple sufficient conditions for an integral operator to be
trace-class [157
]. We thus take the kernel
to be smooth, symmetric in
, and such that
The presence of the extra scale allows one to convert Equation (2.8
) into a functional differential
equation [228
, 229
, 29
],
For finite cutoffs the trace of the right-hand-side of Equation (2.11
) will exist as the potentially
problematic high momentum parts are cut off. In slightly more technical terms, since the product of a
trace-class operator with a bounded operator is again trace-class, the trace in Equation (2.11
) is finite as
long as the inverse of
defines a bounded operator. For finite UV cutoff one sees
from the momentum space version of Equation (B.2
) in Section 3.4 that this will normally
be the case. The trace-class property of the mode cutoff operator (for which Equation (2.10
)
is a sufficient condition) also ensures that the trace in Equation (2.11
) can be evaluated in
any basis, the momentum space variant displayed in the second line is just one convenient
choice.
Importantly the FRGE (2.11) is independent of the bare action
, which enters only via the initial
condition
(for large
). In the FRGE approach the calculation of the functional integral for
is replaced by the task of integrating this RG equation from
, where the initial condition
is imposed, down to
, where the effective average action equals the ordinary effective
action
.
All this has been for a fixed UV cutoff . The removal of the cutoff is of course the central theme of
UV renormalization. In the FRG formulation one has to distinguish between two aspects: first, removal of
the explicit
dependence in the trace on the right-hand-side of Equation (2.11
), and second removal of
the UV cutoff in
itself, which was needed in order to make the original functional integral
well-defined.
The first aspect is unproblematic: The trace is manifestly finite as long as the inverse of
defines a bounded operator. If now
is independently known to have a finite and nontrivial limit as
, the explicit
dependence carried by the
term is harmless and the trace always exists.
Roughly this is because the derivative kernel
has support mostly on a thin shell around
, so that the (potentially problematic) large
behavior of the other factor is irrelevant
(cf. Appendix C.2).
The second aspect of course relates to the traditional UV renormalization problem. Since came
from a regularized functional integral it will develop the usual UV divergencies as one attempts
to send
to infinity. The remedy is to carefully adjust the bare action
– that is,
the initial condition for the FRGE (2.11
) – in such a way that functional integral – viz. the
solution of the FRGE – is asymptotically independent of
. Concretely this could be done by
fine-tuning the way how the parameters
in the expansion
depends on
. However the FRGE method in itself provides no means to find the proper
initial functional
. Identification of the fine-tuned
lies at the core of the UV
renormalization problem, irrespective of whether
is defined directly via the functional integral or
via the FRGE. Beyond perturbation theory the only known techniques to identify the proper
start directly from the functional integral and are ‘constructive’ in spirit (see [195
, 48]).
Unfortunately four-dimensional quantum field theories of interest are still beyond constructive
control.
One may also ask whether perhaps the cutoff-dependent FRGE (2.11) itself can be used
to show that a limit
exists. Indeed using other FRGEs and a perturbative
ansatz for the solution has lead to economic proofs of perturbative renormalizability, i.e. of
the existence of a formal continuum limit in the sense of Criterion (PTC1) discussed before
(see [200, 123
]). Unfortunately so far this could not be extended to construct a nonperturbative
continuum limit of fully fledged quantum field theories (see [145] for a recent review of such
constructive uses of FRGEs). For the time being one has to be content with the following if …then
statement:
If there exists a sequence of initial actions ,
, such that the solution
of the
FRGE (2.11
) remains finite as
, then the limit
has to obey the cut-off
independent FRGE
So far the positivity or unitarity requirement has not been discussed. From the (Osterwalder–Schrader
or Wightman) reconstruction theorems it is known how the unitarity of a quantum field theory on a flat
spacetime translates into nonlinear conditions on the multipoint functions. Since the latter
can be expressed in terms of the functional derivatives of , unitarity can in principle be
tested retroactively, and is expected to hold only in the limit
. Unfortunately this is a
very indirect and retroactive criterion. One of the roles of the bare action
is
to encode properties which are likely to ensure the desired properties of
. In
theories with massless degrees of freedom the
limit is nontrivial and the problem of
gaining computational control over the infrared physics should be separated from the UV aspects
of the continuum limit as much as possible. However the
limit is essential to probe
stability/positivity/unitarity.
One aspect of positivity is the convexity of the effective action. The functional equations (2.11, 2.12
) do
in itself “not know” that
is the Legendre transform of a convex functional and hence must be itself
convex. Convexity must therefore enter through the inital data and it will also put constraints on the
choice of the mode cutoffs. Good mode cutoffs are characterized by the fact that
has positive spectral values for all
(cf. Equation (C.14
)). If no blow-up occurs in the flow
the limit
will then also have non-negative spectrum. Of course this presupposes
again that the proper initial conditions have been identified and the role of the bare action is as
above.
For flat space quantum field theories one expects that must be local, i.e. a differential
polynomial of finite order in the fields so as to end up with an effective action
describing a local/microcausal unitary quantum field theory.
For convenient reference we summarize these conclusions in the following criterion:
Criterion (Continuum limit in the functional RG approach):
(FRGC1) A solution of the cutoff independent FRGE (2.12) which exists globally in
(for all
) can reasonably be identified with the continuum limit of the effective average action
constructed by other means. For such a solution
is the full quantum
effective action and
is the fixed point action.
(FRGC2) For a unitary relativistic quantum field theory positivity/unitarity must be tested retroactively
from the functional derivatives of .
We add some comments:
Since the FRGE (2.12) is a differential equation in
, an initial functional
has to be
specified for some
, to generate a local solution near
. The point is that for
‘almost all’ choices of
the local solution cannot be extended to all values of
.
Finding the rare initial functionals for which this is possible is the FRGE counterpart of the
UV renormalization problem. The existence of the
limit is itself not part of the UV
problem; in conventional quantum field theories the
limit is however essential to probe
unitarity/positivity/stability.
It is presently not known whether the above criterion can be converted into a theorem.
Suppose for a quantum field theory on the lattice (with lattice spacing ) the effective action
has been constructed nonperturbatively from a transfer operator satisfying reflection
positivity and that a continuum limit
is assumed to exist. Does it coincide with a
solution
of Equation (2.12
) satisfying the Criteria (FRGC1) and (FRGC2)? Note that
this is ‘only’ a matter of controlling the limit, for finite
also
will satisfy the flow
equation (2.11
).
For an application to quantum gravity one will initially only ask for Criterion (FRGC1),
perhaps with even only a partial understanding of the limit. As mentioned, the
limit should also be related to positivity issues. The proper positivity requirement replacing
Criterion (FRGC2) yet has to be found, however some constraint will certainly be needed. Concerning
Criterion (FRGC1) the premise in the if …then statement preceeding Equation (2.12
) has to be justified by
external means or taken as a working hypothesis. In principle one can also adopt the viewpoint
that the quantum gravity counterpart of Equation (2.12
) discussed in Section 4 simply defines
the effective action for quantum gravity whenever a solution meets Criterion (FRGC1). The
main drawback with this proposal is that it makes it difficult to include information concerning
Criterion (FRGC2). However difficult and roundabout a functional integral construction is, it
allows one to incorporate ‘other’ desirable features of the system in a relatively transparent
way.
We shall therefore also in the application to quantum gravity assume that a solution of the cutoff
independent FRGE (2.12
) satisfying Criterion (FRGC1) comes from an underlying functional integral.
This amounts to the assumption that the renormalization problem for
defined in terms of a
functional integral can be solved and that the limit
can be identified with
. This is of
course a rather strong hypothesis, however its self-consistency can be tested within the FRG
framework.
To this end one truncates the space of candidate continuum functionals to one where the
initial value problem for the flow equation (2.12
) can be solved in reasonably closed form. One can then by
‘direct inspection’ determine the initial data for which a global solution exists. Convexity of the truncated
can serve as guideline to identify good truncations. If the set of these initial
data forms a nontrivial unstable manifold of the fixed point
,
application of the above criterion suggests that
can approximately be identified with the
projection of the continuum limit
of some
computed by other means.
The identification can only be an approximate one because in the
evolution one first
truncates and then evolves in
, while in
one first evolves in
and
then truncates. Alternatively one can imagine to have replaced the original dynamics by some
‘hierarchical’ (for want of a better term) approximation implicitly defined by the property that
(see [75] for the relation between a hierarchical dynamics and the
local potential approximation). The existence of an UV fixed point with a nontrivial unstable
manifold for
can then be taken as witnessing the renormalizability of the ‘hierarchical’
dynamics.
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