Conformal geometry as a gauge theory of gravity: Covariant equations of motion & conservation laws

Abstract

We study Weyl conformal geometry as a general gauge theory of the Weyl group (of Poincaré and dilatations symmetries) in a manifestly Weyl gauge covariant formalism in which this geometry is automatically metric and physically relevant. This gives a realistic (quadratic) gauge theory of gravity, with Einstein–Hilbert gravity recovered in its spontaneously broken phase, motivating our interest in this geometry. For the most general action we compute the manifestly Weyl gauge covariant equations of motion and present the conservation laws for the energy–momentum tensor and Weyl gauge current. These laws are valid both in Weyl conformal geometry (with respect to the Weyl gauge covariant derivative) but also in the Riemannian geometry equivalent picture (with respect to its associated covariant derivative). This interesting result is a consequence of gauged diffeomorphism invariance of the former versus usual diffeomorphism invariance of the latter. These results are first derived in $d=4$ dimensions. We then successfully derive the conservation laws and equations of motion in Weyl conformal geometry in arbitrary $d$ dimensions, while maintaining manifest Weyl gauge invariance/covariance. The results are useful in physical applications with this symmetry.