Codazzi tensors and their space-times, and Cotton gravity

We study the geometric properties of certain Codazzi tensors for their own sake, and for their appearance in the recent theory of Cotton gravity. We prove that a perfect-fluid tensor is Codazzi if and only if the metric is a generalized Stephani universe. A trace condition restricts it to a warped space-time, as proven by Merton and Derdzinski. We also give necessary and sufficient conditions for a space-time to host a current-flow Codazzi tensor. In particular, we study the static and spherically symmetric cases, which include the Nariai and Bertotti-Robinson metrics. The latter are a special case of Yang Pure space-times, together with spatially flat FRW space-times with constant curvature scalar. We apply these results to the recent Cotton gravity by Harada. The equations have the freedom of choosing a Codazzi tensor, that constrains the space-time where the theory is staged. The tensor, chosen in forms significative for physics, implies the form of the Ricci tensor, and the two specify the energy-momentum tensor, which is the source in Cotton gravity for the chosen metric. For example, the Stephani, Nariai and Bertotti-Robinson space-times solve Cotton gravity with physically sensible energy-momentum tensors. Finally, we discuss Cotton gravity in De Sitter space-times.
Comment: 19 pages, no figures. We added a discussion of Yang Pure spaces, with references. Eq.45 and the Ricci tensor in the proof of Prop.2.3 (this version) are now correct