Symmetry Breaking in Black Hole Binary Mergers from Primordial Binaries in Dense Star Clusters
It has been well established that globular clusters are a promising environment for producing binary black hole (BBH) mergers, such as those detected by the LIGO-Virgo-KAGRA collaboration. Since globular clusters are isotropic environments, it is expected that the distribution of effective spin $\chi_\mathrm{eff}$ is isotropic. Recent analyses of the BBH merger population properties shows that the $\chi_\mathrm{eff}$ distribution is skewed towards positive values, indicating that the spins prefer to be aligned with the angular momentum. In this talk, I will argue that this finding is not inconsistent with a cluster origin. As in the field, it is expected that BH progenitors in clusters are mostly born in primordial binaries, so that BBHs formed from primordial binaries will have spins initially aligned with the orbital angular momentum. Subsequent evolution of the spin orientations due to strong gravitational scattering (i.e. binary--single encounters) may be modelled as a discrete random walk on the unit sphere. As I will show both analytically and with a large suite of scattering experiments, the bias towards spin-orbit alignment can survive many encounters. I will also show preliminary results of $\chi_\mathrm{eff}$ distributions from Monte Carlo simulations of dense star clusters.
