Ultrarelativistic black hole formation

TL;DR

This study numerically investigates ultrarelativistic head-on collisions of equal mass fluid spheres, demonstrating black hole formation at high boosts and analyzing the dynamics and gravitational wave emissions near the threshold.

Contribution

It provides the first detailed numerical analysis of black hole formation in ultrarelativistic fluid collisions, revealing the threshold behavior and horizon dynamics.

Findings

Black holes form at sufficiently high boosts in ultrarelativistic collisions.

Near the threshold, two apparent horizons form and merge, indicating complex horizon dynamics.

Gravitational radiation reaches high luminosities, carrying significant energy.

Abstract

We study the ultrarelativistic head-on collision of equal mass particles, modeled as self-gravitating fluid spheres, by numerically solving the coupled Einstein-hydrodynamic equations. We focus on cases well within the kinetic energy dominated regime, where between 88-92% ($\gamma=8$ to 12) of the initial net energy of the spacetime resides in the translation kinetic energy of the particles. We find that for sufficiently large boosts, black hole formation occurs. Moreover, near yet above the threshold of black hole formation, the collision initially leads to the formation of two distinct apparent horizons that subsequently merge. We argue that this can be understood in terms of a focusing effect, where one boosted particle acts as a gravitational lens on the other and vice versa, and that this is further responsible for the threshold being lower (by a factor of a few) compared to simple hoop conjecture estimates. Cases slightly below threshold result in complete disruption of the model particles. The gravitational radiation emitted when black holes form reaches luminosities of 0.014 $c^5/G$, carrying $16\pm2%$ of the total energy.