# 7 Appendix: Conformal Rescalings And Curvature

We compile here some formulae which are helpful for performing conformal rescalings. Suppose we rescale the given metric to a new metric . Define . The Levi-Civita connection of the new metric is given in terms of the Levi-Civita connection of by its action on an arbitrary vectorfield ,

From the action on vectorfields we can obtain the action on tensors of arbitrary valence in the usual way.

Next we consider the curvature tensor. It is useful to split the Riemann tensor into several pieces which transform differently under conformal rescalings. We write

The tensor is, of course, Weyl's conformal tensor, characterised by the property of having the same symmetries as the Riemann tensor with all traces vanishing. The other piece, the tensor , can be uniquely expressed in terms of the Ricci tensor

The tensor is proportional to the trace-free part of the Ricci tensor, while is a multiple of the scalar curvature.

Under the conformal rescaling , the different parts of the curvature transform as follows:

Thus, the Weyl tensor is invariant under conformal rescalings. When is expressed entirely in terms of the transformed quantities we get the relation

from which we can deduce (note that the contractions are performed with the transformed metric)

Next consider the Bianchi identity . Inserting the decomposition (105) and taking appropriate traces allows us to write it as two equations,

 Conformal Infinity Jörg Frauendiener http://www.livingreviews.org/lrr-2000-4 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de