Spectral asymptotics of Euclidean quantum gravity with diff-invariant boundary conditions

TL;DR

This paper investigates the spectral properties of Euclidean quantum gravity with boundary conditions invariant under diffeomorphisms, revealing that only scalar perturbation modes are affected by the lack of strong ellipticity, with implications for one-loop calculations.

Contribution

It demonstrates that, on the Euclidean four-ball, only the scalar sector of metric perturbations is impacted by the absence of strong ellipticity, and provides a new spectral identity ensuring regularity of zeta-function asymptotics.

Findings

Scalar perturbation modes are affected by non-ellipticity.

Three sectors of scalar perturbations remain elliptic.

A new spectral identity ensures regularity of zeta-function at the origin.

Abstract

A general method is known to exist for studying Abelian and non-Abelian gauge theories, as well as Euclidean quantum gravity, at one-loop level on manifolds with boundary. In the latter case, boundary conditions on metric perturbations h can be chosen to be completely invariant under infinitesimal diffeomorphisms, to preserve the invariance group of the theory and BRST symmetry. In the de Donder gauge, however, the resulting boundary-value problem for the Laplace type operator acting on h is known to be self-adjoint but not strongly elliptic. The latter is a technical condition ensuring that a unique smooth solution of the boundary-value problem exists, which implies, in turn, that the global heat-kernel asymptotics yielding one-loop divergences and one-loop effective action actually exists. The present paper shows that, on the Euclidean four-ball, only the scalar part of perturbative modes for quantum gravity are affected by the lack of strong ellipticity. Further evidence for lack of strong ellipticity, from an analytic point of view, is therefore obtained. Interestingly, three sectors of the scalar-perturbation problem remain elliptic, while lack of strong ellipticity is confined to the remaining fourth sector. The integral representation of the resulting zeta-function asymptotics is also obtained; this remains regular at the origin by virtue of a spectral identity here obtained for the first time.