The orbits of binary black holes could be eccentric, but in practice they’re not. At least when we observe them, and that’s because of a relativistic effect that circularizes the orbit. Even if astrophysics formed black holes eccentric, relativity makes them circular when we observe them with gravitational-wave interferometers. But we’re interested in the astrophysics back then! What we find here is that the tiny residual eccentricity at detection can be crucial. Even eccentricities that are so small that we cannot tell them apart from circular can mess up the astrophysical inference. Unfortunately, this is a new systematic error that needs to be taken into account: inferring the “formation channel” of binary black holes might be even harder than we thought.

Giulia Fumagalli, Isobel Romero-Shaw, Davide Gerosa, Viola De Renzis, Konstantinos Kritos, Aleksandra Olejak.
Physical Review D 110 (2024) 063012.
arXiv:2405.14945 [astro-ph.HE].