Testing scalar-tensor gravity with gravitational-wave observations of inspiralling compact binaries

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 analyze inspiralling compact binaries in the context of the scalar-tensor theory of Jordan, Fierz, Brans, and Dicke, focusing 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 ${\mathrm{\ensuremath{\omega}}}_{\mathrm{BD}}$ of Brans-Dicke theory. For a neutron-star--black-hole binary, we find that the bound could exceed the current bound of ${\mathrm{\ensuremath{\omega}}}_{\mathrm{BD}}$>500 from solar-system experiments, for sufficiently low-mass systems. For a 0.7${\mathit{M}}_{\mathrm{\ensuremath{\bigodot}}}$ neutron star and a 3${\mathit{M}}_{\mathrm{\ensuremath{\bigodot}}}$ black hole we find that a bound ${\mathrm{\ensuremath{\omega}}}_{\mathrm{BD}}$\ensuremath{\approxeq}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${\mathit{M}}_{\mathrm{\ensuremath{\bigodot}}}$. 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.