Universality, maximum radiation and absorption in high-energy collisions of black holes with spin

TL;DR

This study investigates how black hole spins influence high-energy collisions, revealing that only part of the kinetic energy is radiated as gravitational waves, with significant energy absorption by the black holes themselves.

Contribution

It demonstrates that in ultrarelativistic black hole collisions, the energy radiated is limited and black holes absorb a substantial portion, challenging previous assumptions about energy emission at high energies.

Findings

Approximately 60% of kinetic energy is radiated at gamma=2.49.

Black hole spins significantly change during collisions.

Extrapolation suggests about half of the energy can be emitted as gravitational waves.

Abstract

We explore the impact of black hole spins on the dynamics of high-energy black hole collisions. We report results from numerical simulations with gamma-factors up to 2.49 and dimensionless spin parameter +0.85, +0.6, 0, -0.6, -0.85. We find that the scattering threshold becomes independent of spin at large center-of-mass energies, confirming previous conjectures that structure does not matter in ultrarelativistic collisions. It has further been argued that in this limit all of the kinetic energy of the system may be radiated by fine tuning the impact parameter to threshold. On the contrary, we find that only about 60% of the kinetic energy is radiated for gamma=2.49. By monitoring apparent horizons before and after scattering events we show that the "missing energy" is absorbed by the individual black holes in the encounter, and moreover the individual black-hole spins change significantly. We support this conclusion with perturbative calculations. An extrapolation of our results to the limit gamma->infinity suggests that about half of the center-of-mass energy of the system can be emitted in gravitational radiation, while the rest must be converted into rest-mass and spin energy.