Systematic challenges for future gravitational wave measurements of precessing binary black holes

The properties of precessing, coalescing binary black holes are presently inferred through comparison with two approximate models of compact binary coalescence. In this work we show that these two models often disagree substantially when binaries have modestly large spins (a ≳ 0.4) and modest mass ratios (q ≳ 2). We demonstratethese disagreements using standard figures ofmeritandthe parametersinferred for recent detections of binary black holes. By comparing to numerical relativity, we confirm that these disagreements reflect systematic errors. We provide concrete examples to demonstrate that these systematic errors can significantly impact inferences about astrophysically significant binary parameters. For the immediate future, parameter inference for binary black holes should be performed with multiple models (including numerical relativity), and carefully validated by performing inference under controlled circumstances with similar synthetic events.