Spin induction from scattering of two spinning black holes in dense clusters

• Published in: PHYSICS OF THE DARK UNIVERSE 2025-02, Vol.47, p.101776, Article 101776
• Main Authors: Rodríguez-Monteverde, Jorge L., Jaraba, Santiago, García-Bellido, Juan
• Format: Article
• Language: English
• Subjects: Close hyperbolic encounters; Dark matter; Gravitational waves; Primordial black holes
• ISSN: 2212-6864; 2212-6864
• DOI: 10.1016/j.dark.2024.101776

In this paper, we use numerical relativity to study the spin induction effect within close hyperbolic encounters of initially spinning black holes. We review the initially non-spinning case and explore the cases of initially aligned, anti-aligned and orthogonal spins with respect to the orbital angular momentum L→. We find that, for a given initial effective spin, the black hole with a smaller initial spin acquires a greater spin-up than the other black hole after the interaction. We study three different scenarios regarding initial effective spin (χeff=−0.1,0.0,0.1), using three different scattering angles in order to obtain maximally spin-inducing scenarios. We also find that the final effective spin-ups with respect to the initial spins are well-fitted by a parabola. For spins orthogonal to L→, we observe that the black hole spins precess, and that the induced spin in the z-direction depends quadratically on the value of the initial spins. These phenomena suggest that dense black hole clusters present a rich spin dynamics, where black hole spins may acquire non-trivial distributions.