Testing Scalar-Tensor Gravity with Gravitational-Wave Observations of Inspiralling Compact Binaries

TL;DR

This paper investigates how gravitational-wave observations of inspiralling compact binaries can test scalar-tensor gravity theories, especially Brans-Dicke theory, by analyzing energy loss due to dipole radiation and deriving bounds on the coupling constant.

Contribution

It provides a method to constrain Brans-Dicke theory using gravitational-wave data, potentially surpassing solar-system bounds for certain low-mass binary systems.

Findings

Bounds on ω_BD can exceed current solar-system limits for low-mass neutron-star/black-hole binaries.

For a 0.7 M_sun neutron star and 3 M_sun black hole, ω_BD ≈ 2000 is achievable.

Black hole binaries are indistinguishable from general relativity in gravitational-wave signals.

Abstract

Observations of gravitational waves from inspiralling compact binaries using laser-interferometric detectors can provide accurate measures of parameters of the source. They can also constrain alternative gravitation theories. We analyse inspiralling compact %binaries in the context of the scalar-tensor theory of Jordan, Fierz, Brans and Dicke, focussing on the effect on the inspiral of energy lost to dipole gravitational radiation, whose source is the gravitational self-binding energy of the inspiralling bodies. Using a matched-filter analysis we obtain a bound on the coupling constant $\omega_{\rm BD}$ of Brans-Dicke theory. For a neutron-star/black-hole binary, we find that the bound could exceed the current bound of $\omega_{\rm BD}>500$ from solar-system experiments, for sufficiently low-mass systems. For a $0.7 M_\odot$ neutron star and a $3 M_\odot$ black hole we find that a bound $\omega_{\rm BD} \approx 2000$ is achievable. The bound decreases with increasing black-hole mass. For binaries consisting of two neutron stars, the bound is less than 500 unless the stars' masses differ by more than about $0.5 M_\odot$. For two black holes, the behavior of the inspiralling binary is observationally indistinguishable from its behavior in general relativity. These bounds assume reasonable neutron-star equations of state and a detector signal-to-noise ratio of 10.