On the Metric Propagator and Affine Modes of Extended Hybrid Metric--Palatini Gravity with Ricci--Squared Invariants

Abstract

We investigate an extended hybrid metric--Palatini theory defined by an arbitrary function $f(R,\mathcal{R},\hat{Q},Q,\mathcal{Q})$, where $R$ and $\mathcal{R}$ are the metric and Palatini scalar curvatures, and $Q=R_{\mu\nu}R^{\mu\nu}$, $\mathcal{Q}=\mathcal{R}_{(\mu\nu)}\mathcal{R}^{(\mu\nu)}$, and $\hat{Q}=R_{\mu\nu}\mathcal{R}^{(\mu\nu)}$ are the metric, Palatini, and mixed quadratic Ricci invariants, respectively. We derive the field equations and analyze the Minkowski weak--field limit. We obtain the metric propagator and identify the additional spin--$2$ and spin--$0$ poles that couple to the metric source. When these poles are finite, real, and simple, the spin-2 sector of the metric propagator contains two additional poles, at least one of which has a negative residue. Once the spin-2 sector is restricted to contain at most one additional pole, the absence of ghostlike metric-coupled excitations in the generic two-scalar branch requires eliminating the remaining spin--$2$ pole, whereas one healthy metric-coupled spin--$2$ pole may coexist with at most one healthy scalar pole when the scalar denominator becomes linear in $k^2$. We derive the corresponding algebraic conditions on the Minkowski-background derivatives of $f$ and specialize them to hybrid $f(R,\mathcal{R})$, mixed $f(R,\mathcal{Q})$, and purely Palatini $f(\mathcal{R},\mathcal{Q})$ theories, including the metric $f(R)$ and Palatini $f(\mathcal{R})$ limits. We further show that the complete linearized system can admit modes carried entirely by the independent connection that are absent from the metric propagator. These results therefore provide necessary flat-space stability criteria for the metric-coupled sector on the branches analyzed, supplemented by the analysis of purely affine modes.