Gravitational clustering: an overview

TL;DR

This paper reviews gravitational clustering, comparing finite and infinite systems, and discusses the dynamics of relaxation, mean-field approaches, and the impact of fluctuations on modeling self-gravitating systems.

Contribution

It provides an overview of gravitational clustering, analyzing differences between finite and infinite systems, and examines the role of fluctuations in mean-field modeling.

Findings

Collective relaxation leads to quasi-stationary states.

Fluctuations influence the validity of collision-less approximations.

Differences exist between finite and infinite gravitational systems.

Abstract

We discuss the differences and analogies of gravitational clustering in finite and infinite systems. The process of collective, or violent, relaxation leading to the formation of quasi-stationary states is one of the distinguished features in the dynamics of self-gravitating systems. This occurs, in different conditions, both in a finite than in an infinite system, the latter embedded in a static or in an expanding background. We then discuss, by considering some simple and paradigmatic examples, the problems related to the definition of a mean-field approach to gravitational clustering, focusing on role of discrete fluctuations. The effect of these fluctuations is a basic issue to be clarified to establish the range of scales and times in which a collision-less approximation may describe the evolution of a self-gravitating system and for the theoretical modeling of the non-linear phase.