A Direct Probe of Mass Density near Inspiraling Binary Black Holes

Now that LIGO has revealed the existence of a large number of binary black holes (BHs), identifying their origin has emerged as an important challenge. Observed binary BHs might reside in more isolated regions of the galaxy or alternatively they might be emerging from dense environments such as galactic centers or globular clusters. In the latter case, their center of mass motion as well as their orbital parameters lead to potentially observable changes in the waveforms that would reflect their gravitational interactions with the surrounding matter. The gravitational wave signal would be distinguished by a net phase change or even a time-dependent Doppler shift arising from the orbital motion. We show that this time-dependence might be observable in future space gravitational wave detectors such as LISA that could provide direct information about the BH binary environments and otherwise invisible ambient mass.