Entropy production and entropic attractors in black hole fusion and fission

TL;DR

This paper investigates how entropy is generated during black hole fusion and fission processes, revealing that fission produces less entropy and that intermediate states act as attractors, with implications for black hole stability and charge diffusion.

Contribution

It introduces a large D effective theory analysis of black hole entropy production in dynamical processes, highlighting the role of attractor states and extending stability studies of new black hole phases.

Findings

Black hole fusion is highly irreversible with significant entropy production.

Fission generates less entropy and follows a pattern similar to black string decay.

Intermediate quasi-thermalized states act as attractors, narrowing the outcome parameters.

Abstract

We study how black hole entropy is generated and the role it plays in several highly dynamical processes: the decay of unstable black strings and ultraspinning black holes; the fusion of two rotating black holes; and the subsequent fission of the merged system into two black holes that fly apart (which can occur in dimension $D\geq 6$, with a mild violation of cosmic censorship). Our approach uses the effective theory of black holes at $D\to\infty$, but we expect our main conclusions to hold at finite $D$. Black hole fusion is highly irreversible, while fission, which follows the pattern of the decay of black strings, generates comparatively less entropy. In $2\to 1\to 2$ black hole collisions an intermediate, quasi-thermalized state forms that then fissions. This intermediate state erases much of the memory of the initial states and acts as an attractor funneling the evolution of the collision towards a small subset of outgoing parameters, which is narrower the closer the total angular momentum is to the critical value for fission. Entropy maximization provides a very good guide for predicting the final outgoing states. Along our study, we clarify how entropy production and irreversibility appear in the large $D$ effective theory. We also extend the study of the stability of new black hole phases (black bars and dumbbells). Finally, we discuss entropy production through charge diffusion in collisions of charged black holes.