# Phase space mixing of a Vlasov gas in the exterior of a Kerr black hole

## TL;DR

This paper investigates the long-term behavior of a collisionless gas around a Kerr black hole, showing that macroscopic observables converge to a stationary state under certain conditions, thus extending the understanding of kinetic gases in curved spacetime.

## Contribution

It introduces a novel analysis of phase space mixing for a Vlasov gas in Kerr spacetime using action-angle variables, proving convergence to equilibrium and validating the strong Jeans theorem.

## Key findings

- Macroscopic observables converge to a stationary state.
- The strong Jeans theorem is validated in this setting.
- The non-degeneracy condition on frequencies is proven to hold.

## Abstract

We study the dynamics of a collisionless kinetic gas whose particles follow future-directed timelike and spatially bound geodesics in the exterior of a sub-extremal Kerr black hole spacetime. Based on the use of generalized action-angle variables, we analyze the large time asymptotic behavior of macroscopic observables associated with the gas. We show that, as long as the fundamental frequencies of the system satisfy a suitable non-degeneracy condition, these macroscopic observables converge in time to the corresponding observables determined from an averaged distribution function. In particular, this implies that the final state is characterized by a distribution function which is invariant with respect to the full symmetry group of the system, that is, it is stationary, axisymmetric and Poisson-commutes with the integral of motion associated with the Carter constant. As a corollary of our result, we demonstrate the validity of the strong Jeans theorem in our setting, stating that the distribution function belonging to a stationary state must be a function which is independent of the generalized angle variables. An analogous theorem in which the assumption of stationarity is replaced with the requirement of invariance with respect to the Carter flow is also proven. Finally, we prove that the aforementioned non-degeneracy condition holds. This is achieved by providing suitable asymptotic expansions for the energy and Carter constant in terms of action variables for orbits having sufficiently large radii, and by exploiting the analytic dependency of the fundamental frequencies on the integrals of motion.