Testing alternative theories of gravity with space-based gravitational wave detectors

We use gravitational waves (GWs) from binary black holes (BBHs) and neutron stars inspiraling into intermediate-mass black holes to evaluate how accurately the future space-based GW detectors such as LISA, Taiji, and TianQin and their combined networks can determine source parameters and constrain alternative theories of gravity. We find that, compared with single detector, the detector network can greatly improve the estimation errors of source parameters, especially the sky localization, but the improvement of the constraint on the graviton mass m₆ and the Brans-Dicke coupling constant ₁₃ is small. We also discuss possible scalar modes existed in alternative theories of gravity, and we find the inclusion of the scalar mode has little effect on the constraints on source parameters, m₆, and ₁₃ and the parametrized amplitude A₁ of scalar modes are small. For the constraint on the graviton mass, we consider both the effects in the GW phase and the transfer function due to the mass of graviton. With the network of LISA, Taiji, and TianQin, we get the lower bound on the graviton Compton wavelength ₆1. 2410^20 m for BBHs with masses (10^6+10^7) M_, and A₁6. 1110^6 for neutron star--black hole binary with masses (1. 4+400) M_.