Explicit Diffeomorphism Breaking, No-go Conditions, and the Gravity Sector of the SME

Abstract

A brief recap is given of issues that can occur when fixed nondynamical backgrounds that explicitly break diffeomorphism invariance are included in gravity theories. Implications for the Standard-Model Extension (SME) are summarized, including recent results showing how the SME can be modified to accommodate such fixed nondynamical backgrounds.

Comments: Proceedings of the Ninth Meeting on CPT and Lorentz Symmetry, Indiana University, Bloomington, May 2022