It is expected that the orbital planes of gravitational-wave (GW) sources are isotropically distributed. However, both physical and technical factors, such as alternate theories of gravity with birefringence, catalog contamination, and search algorithm limitations, could result in inferring a non-isotropic distribution. Showing that the inferred astrophysical distribution of the orbital orientations is indeed isotropic can thus be used to rule out some violations of general relativity, as a null test about the purity of the GW catalog sample, and as a check that selection effects are being properly accounted for. We augment the default mass/spins/redshift model used by the LIGO-Virgo-KAGRA Collaboration in their most recent analysis to also measure the astrophysical distribution of orbital orientations. We show that the 69 binary black holes in GWTC-3 are consistent with having random orbital orientations. The inferred distribution is highly symmetric around π/2, with skewness Spost=0.01−0.17+0.17. Meanwhile, the median of the inferred distribution has a Jensen-Shannon divergence of 1.4×10−4 bits when compared to the expected isotropic distribution.
@article{arxiv.2204.00968,
title = {The orientations of the binary black holes in GWTC-3},
author = {Salvatore Vitale and Sylvia Biscoveanu and Colm Talbot},
journal= {arXiv preprint arXiv:2204.00968},
year = {2022}
}