Degenerate higher-order scalar-tensor theories in metric-affine gravity

TL;DR

This work extends DHOST theory to a metric-affine setting by formulating a general Palatini scalar-tensor action linear in curvature and quadratic in scalar second derivatives, and solving the algebraic distortion-tensor equation to obtain an effective metric theory. Imposing DHOST degeneracy collapses the Palatini quadratic sector to a Class Ia-like structure, initially controlled by two free functions $F_1$ and $F_2$, with all higher-order coefficients determined algebraically. Requiring luminal gravitational-wave propagation further constrains the theory to a one-function subfamily, fixing $F_2$ in terms of $F_1$ via a specific algebraic relation. The results provide a detailed, self-contained characterization of the quadratic Palatini Class Ia sector and establish how GW observations constrain the viable scalar-tensor dynamics in metric-affine geometry, setting the stage for cosmology and strong-field tests.

Abstract

We construct the metric-affine analogue of the quadratic degenerate higher-order scalar-tensor (DHOST) theories. We begin with a general metric-affine scalar-tensor action that is linear in curvature and contains all operators that are at most quadratic in the covariant second derivatives of the scalar field, ensuring that the connection enters only through curvature and through these second derivatives. Solving the connection equation by performing a full decomposition of the distortion tensor gives a closed-form effective metric theory. Imposing the standard metric DHOST degeneracy conditions then selects a Palatini Class Ia branch that is fully determined by two free functions in the original action. Analyzing the tensor sector shows that requiring gravitational waves to propagate at the speed of light further restricts the theory to a one-function family. These results provide a detailed and self-contained characterization of the quadratic metric-affine Class Ia sector within this operator basis and identify the theoretical conditions implied by gravitational wave observations.