Testing the multipole structure of compact binaries using gravitational wave observations

We propose a novel method to test the consistency of the multipole moments of compact binary systems with the predictions of general relativity (GR). The multipole moments of a compact binary system, known in terms of symmetric and trace-free tensors, are used to calculate the gravitational waveforms from compact binaries within the post-Newtonian (PN) formalism. For nonspinning compact binaries, we derive the gravitational wave phasing formula, in the frequency domain, parametrizing each PN order term in terms of the multipole moments which contribute to that order. Using GW observations, this parametrized multipolar phasing would allow us to derive the bounds on possible departures from the multipole structure of GR and hence constrain the parameter space of alternative theories of gravity. We compute the projected accuracies with which the second-generation ground-based detectors, such as the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO), the third-generation detectors such as the Einstein Telescope and Cosmic Explorer, as well as the space-based detector Laser Interferometer Space Antenna (LISA) will be able to measure these multipole parameters. We find that while Advanced LIGO can measure the first two or three multipole coefficients with good accuracy, Cosmic Explorer and the Einstein Telescope may be able to measure the first four multipole coefficients which enter the phasing formula. Intermediate-mass-ratio inspirals, with mass ratios of several tens, in the frequency band of the planned space-based LISA mission should be able to measure all seven multipole coefficients which appear in the 3.5PN phasing formula. Our finding highlights the importance of this class of sources for probing the strong-field gravity regime. The proposed test will facilitate the first probe of the multipolar structure of Einstein's general relativity.