Horndeski gravity and standard sirens

TL;DR

This paper investigates how Horndeski gravity affects gravitational wave propagation in realistic, inhomogeneous spacetimes, revealing that observable deviations depend on local effective Planck mass and confirming graviton conservation.

Contribution

It provides a comprehensive analysis of Horndeski gravitational waves in arbitrary backgrounds, including effects of inhomogeneities and screening mechanisms, which was not addressed in previous homogeneous studies.

Findings

Kinetic braiding produces a nonphysical longitudinal mode.

Conformal coupling alters the amplitude but not polarization of transverse modes.

Deviations from GR depend on local effective Planck mass at emission and reception.

Abstract

Standard sirens have been proposed as probes of alternative theories of gravity, such as Horndeski models. Hitherto, all studies have been conducted on a homogeneous-isotropic cosmological background, which is unable to consistently account for realistic distributions of matter, and for inhomogeneities in the Horndeski scalar field. Yet, the latter are essential for screening mechanisms. In this article, we comprehensively analyze the propagation of Horndeski gravitational waves in an arbitrary background spacetime and scalar field. We restrict to the class of theories in which gravitational waves propagate at light speed, and work in the geometric-optics regime. We find that kinetic braiding only produces a nonphysical longitudinal mode, whereas conformal coupling affects the amplitude of the standard transverse modes but not their polarization. We confirm that any observable deviation from general relativity depends on the local value of the effective Planck mass at emission and reception of the wave. This result is interpreted as the conservation of the number of gravitons.