# Distinguishing Spin-Aligned and Isotropic Black Hole Populations With Gravitational Waves

## TL;DR

This paper analyzes gravitational wave data from binary black hole mergers to distinguish between spin orientations, finding current evidence slightly favors isotropic distributions, with future data expected to provide stronger confirmation.

## Contribution

It introduces a statistical method to differentiate black hole spin distributions using gravitational wave measurements, highlighting the potential to confirm formation scenarios.

## Key findings

- Current data shows a 2.4σ preference for isotropic spins.
- Adding more detections increases confidence beyond 5σ.
- Future observations will decisively confirm or refute the spin distribution hypothesis.

## Abstract

The first direct detections of gravitational waves from merging binary black holes open a unique window into the binary black hole formation environment. One promising environmental signature is the angular distribution of the black hole spins; systems formed through dynamical interactions among already-compact objects are expected to have isotropic spin orientations whereas binaries formed from pairs of stars born together are more likely to have spins preferentially aligned with the binary orbital angular momentum. We consider existing gravitational wave measurements of the binary effective spin, the best-measured combination of spin parameters, in the four likely binary black hole detections GW150914, LVT151012, GW151226, and GW170104. If binary black hole spin magnitudes extend to high values we show that the data exhibit a $2.4\sigma$ ($0.015$ odds ratio) preference for an isotropic angular distribution over an aligned one. By considering the effect of 10 additional detections, we show that such an augmented data set would enable in most cases a preference stronger than $5\sigma$ ($2.9 \times 10^{-7}$ odds ratio). The existing preference for either an isotropic spin distribution or low spin magnitudes for the observed systems will be confirmed (or overturned) confidently in the near future.