Spin Orientations of Binary Black Holes at their Formation

One of the important parameters that can provide clues about the formation of binary black hole (BBH) mergers detected by ground-based gravitational wave detectors such as LIGO and Virgo are the spin orientations of the black holes. Specifically, the spin tilts, i.e., the angles between each black hole’s spin vector and the binary’s orbital angular momentum vector can change due to precessional effects as the black holes evolve from a large separation to their merger. The tilts one infers by comparing the signal in the detectors’ sensitive band with theoretical waveforms can thus be significantly different from when the binary originally formed. These tilts at the binary’s formation are well approximated in many cases by evolving the BBH backwards in time to a formally infinite separation. Using the tilts at infinity also places all binaries on an equal footing in analyzing their population properties. In this project, we perform parameter estimation for simulated BBH events and investigate the differences between the tilts one infers directly close to merger and those obtained by evolving back to infinite separation. The simulated observations we consider are selected such that their configurations show particularly large differences in their orientations close to merger and at infinity. While these differences may be buried in the statistical noise at current detection sensitivities, we show that in future plus-era (A+ and Virgo+) detectors, they can be easily distinguished in our inferred distributions.