# When will we observe binary black holes precessing?

## TL;DR

This paper introduces a new method to identify and quantify orbital precession in gravitational-wave signals from binary black holes, which could improve detection and understanding of black-hole spin distributions.

## Contribution

The authors develop the precession signal-to-noise ratio ($
ho_p$), enabling quick identification and measurement of precession in gravitational-wave data without intensive computations.

## Key findings

- Precession is likely observable in about 4% of detections under certain spin distributions.
- $
ho_p$ effectively distinguishes precessing signals from non-precessing ones.
- The method has broad applications in gravitational-wave data analysis and astrophysical inference.

## Abstract

After eleven gravitational-wave detections from compact-binary mergers, we are yet to observe the striking general-relativistic phenomenon of orbital precession. Measurements of precession would provide valuable insights into the distribution of black-hole spins, and therefore into astrophysical binary formation mechanisms. Using our recent two-harmonic approximation of precessing-binary signals~\cite{Fairhurst:2019_2harm}, we introduce the ``precession signal-to-noise ratio'', $\rho_p$. We demonstrate that this can be used to clearly identify whether precession was measured in an observation (by comparison with both current detections and simulated signals), and can immediately quantify the measurability of precession in a given signal, which currently requires computationally expensive parameter-estimation studies. $\rho_p$ has numerous potential applications to signal searches, source-property measurements, and population studies. We give one example: assuming one possible astrophysical spin distribution, we predict that precession has a one in $\sim 25$ chance of being observed in any detection.