Beyond general relativity: gravitational waves in non-minimally coupled theories

Abstract

Non-minimal couplings between matter and curvature tensors arise in many different contexts. Such couplings modify solutions of general relativity (GR) and therefore can be probed in various astrophysical systems. A particularly interesting scenario arises if dark matter experiences non-minimal couplings, as dark matter densities are expected to spike in the vicinity of binary black hole mergers. This gives a novel setting for simultaneously studying dark matter and (beyond) GR physics via observations of gravitational waves (GWs). In this work, we explore effects of various non-minimal couplings on GWs by working with a model-independent parameterization for left- and right-handed GW strains. We extend the parameterization proposed in \cite{Jenks:2023pmk,Daniel:2024lev} to include early-universe effects, and we write down the generic solution assuming slowly-varying matter fields. We then systematically apply our results to three models: Kalb-Ramond dark matter with dimension-four operators, axion-dilaton-Chern-Simons-Gauss-Bonnet dimension-five operators, and dimension-six couplings to a (dark) vector field.