Standard Sirens as a novel probe of dark energy

TL;DR

This paper explores how gravitational wave observations, combined with lunar laser ranging and pulsar data, can be used to test models of dark energy that modify gravity's strength over time.

Contribution

It clarifies the roles of different observational constraints on gravitational couplings and highlights the potential of future gravitational wave data to probe dark energy.

Findings

LLR constrains matter-matter gravitational coupling G_N(t)

Binary pulsars and standard sirens constrain G_{gw}(t)

Future gravitational wave data will best test G_{gw}(t) and dark energy models

Abstract

Cosmological models with a dynamical dark energy field typically lead to a modified propagation of gravitational waves via an effectively time-varying gravitational coupling $G(t)$. The local variation of this coupling between the time of emission and detection can be probed with standard sirens. Here we discuss the role that Lunar Laser Ranging (LLR) and binary pulsar constraints play in the prospects of constraining $G(t)$ with standard sirens. In particular, we argue that LLR constrains the matter-matter gravitational coupling $G_N(t)$, whereas binary pulsars and standard sirens constrain the quadratic kinetic gravity self-interaction $G_{gw}(t)$. Generically, these two couplings could be different in alternative cosmological models, in which case LLR constraints are irrelevant for standard sirens. We use the Hulse-Taylor pulsar data and show that observations are highly insensitive to time variations of $G_{gw}(t)$ yet highly sensitive to $G_N(t)$. We thus conclude that future gravitational waves data will become the best probe to test $G_{gw}(t)$, and will hence provide novel constraints on dynamical dark energy models.