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.

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