Testing scalar-tensor gravity using space gravitational-wave interferometers

We calculate the bounds which could be placed on scalar-tensor theories of gravity of the Jordan, Fierz, Brans and Dicke type by measurements of gravitational waveforms from neutron stars (NS) spiraling into massive black holes (MBH) using LISA, the proposed space laser interferometric observatory. Such observations may yield significantly more stringent bounds on the Brans-Dicke coupling parameter $\ensuremath{\omega}$ than are achievable from solar system or binary pulsar measurements. For NS-MBH inspirals, dipole gravitational radiation modifies the inspiral and generates an additional contribution to the phase evolution of the emitted gravitational waveform. Bounds on $\ensuremath{\omega}$ can therefore be found by using the technique of matched filtering. We compute the Fisher information matrix for a waveform accurate to second post-Newtonian order, including the effect of dipole radiation, filtered using a currently modeled noise curve for LISA, and determine the bounds on $\ensuremath{\omega}$ for several different NS-MBH canonical systems. For example, observations of a ${1.4M}_{\ensuremath{\bigodot}}$ NS inspiraling to a ${10}^{3}{M}_{\ensuremath{\bigodot}}$ MBH with a signal-to-noise ratio of 10 could yield a bound of $\ensuremath{\omega}g240000,$ substantialy greater than the current experimental bound of $\ensuremath{\omega}g3000.$