A probability theory for non-equilibrium gravitational systems

TL;DR

This paper develops a new probability framework using dynamical invariants to describe the evolution of collisionless, non-equilibrium gravitational systems under time-dependent forces, avoiding maximum-entropy assumptions.

Contribution

It introduces a novel formalism connecting diffusion in integral-of-motion space with collisionless relaxation, applicable to systems with evolving potentials and derived without ergodicity assumptions.

Findings

Analytical solutions match N-body simulations in near-adiabatic regimes.

Diffusion coefficients relate to virial quantities in time-varying potentials.

The formalism generalizes to potentials with changing symmetries.

Abstract

This paper uses dynamical invariants to describe the evolution of collisionless systems subject to time-dependent gravitational forces without resorting to maximum-entropy probabilities. We show that collisionless relaxation can be viewed as a special type of diffusion process in the integral-of-motion space. In time-varying potentials with a fixed spatial symmetry the diffusion coefficients are closely related to virial quantities, such as the specific moment of inertia, the virial factor and the mean kinetic and potential energy of microcanonical particle ensembles. The non-equilibrium distribution function (DF) is found by convolving the initial DF with the Green function that solves Einstein's equation for freely diffusing particles. Such a convolution also yields a natural solution to the Fokker-Planck equations in the energy space. Our mathematical formalism can be generalized to potentials with a time-varying symmetry, where diffusion extends over multiple dimensions of the integral-of-motion space. The new probability theory is in many ways analogous to stochastic calculus, with two significant differences: (i) the equations of motion that govern the trajectories of particles are fully deterministic, and (ii) the diffusion coefficients can be derived self-consistently from microcanonical phase-space averages without relying on ergodicity assumptions. For illustration we follow the cold collapse of $N$-body models in a time-dependent logarithmic potential. Comparison between the analytical and numerical results shows excellent agreement in regions where the potential evolution does not depart too strongly from the adiabatic regime.