Maia A. S. Martinez | Searcharxiv

arXiv2026

Getting Tilted: Random Walk of Binary Black Hole Spin-Orbit Alignment in Dense Star Clusters

It is commonly assumed that the spin-orbit angles of binary black holes (BBHs) originating from dense stellar environments rapidly converge to an isotropic distribution following a number of strong gravitational encounters. We challenge this assumption by modeling the evolution of the BBH orbital angular momentum orientation through successive binary--single encounters as a random walk on the unit sphere, yielding an exact solution for the orientation distribution after $n$ encounters and a closed-form expression for the number of encounters required to reach isotropy. To characterize the step distribution, we conduct a large suite of Newtonian point-particle scattering experiments with an equal-mass binary, varying the mass of the single, and obtain semi-analytic expressions for both the mean step size and the full distribution of steps. Applying these results to BBHs with initially aligned spins, as may arise from the evolution of primordial massive binaries, we find that spin-orbit alignment can survive several strong encounters before being erased. This has direct implications for the slight trend toward spin-orbit alignment reported in GWTC-5.0 as well as for the retention of hierarchical merger products, such as the components of GW231123.

astro-ph.HE↗

arXiv2026

Black hole mergers from dense star clusters with realistic binary populations

We present a suite of 24 full-lifetime simulations of dense star clusters with the Cluster Monte Carlo (CMC) code, featuring updated input physics and a realistic distribution of initial binary systems. The latter encompasses a mass-dependent binary fraction, period distribution, and eccentricity distribution based on observations of well-studied stellar populations in the Solar neighborhood and nearby star-forming regions. We predict the cosmic rate, masses, and spins of binary black hole (BBH) mergers formed through dynamical assembly, primordial binary evolution, and hierarchical mergers within dense clusters. As with previous model grids with fewer binaries, dynamically assembled first-generation (1G) mergers dominate the rate of cluster-derived mergers, and the total merger rate is consistent with that inferred from LIGO-Virgo-KAGRA observations as of GWTC-5.0. Our models naturally reproduce key features of the inferred BBH population, including the broken-power-law behavior of the primary BH mass spectrum for $m_1 \gtrsim 20 M_\odot$, the shallower (steeper) slope of the secondary mass spectrum relative to the primary for $m_2 \lesssim 10 M_\odot$ ($m_2 \gtrsim 30 M_\odot$), and the shape of the mass-ratio distribution in the low- and high-mass domains. We predict broad distributions of the spin parameters $χ_{\mathrm{eff}}$ and $χ_{\mathrm{p}}$, consistent with previous studies of dynamical assembly in clusters. The merger rate from primordial binary systems within clusters is a small fraction of the total; however, their merger products are frequently involved in subsequent hierarchical mergers, with the result that the hierarchical merger rate evolves more steeply than the 1G dynamical merger rate with redshift.

astro-ph.GA↗