Total Collisions in the N-Body Shape Space

TL;DR

This paper analyzes total collision singularities in the gravitational N-body problem within shape space, revealing that these singularities are essential and cannot be evolved through, similar to the big bang in cosmology.

Contribution

It provides a detailed analysis of total collision points on shape space, showing their essential singularity nature and drawing parallels with cosmological big bang singularities.

Findings

Total collision points are essential singularities on shape space.

Solutions cannot be extended through total collision points.

Modifications to the dynamics may regularize these singularities.

Abstract

We discuss the total collision singularities of the gravitational N-body problem on shape space. Shape space is the relational configuration space of the system obtained by quotienting ordinary configuration space with respect to the similarity group of total translations, rotations, and scalings. For the zero-energy gravitating N-body system, the dynamics on shape space can be constructed explicitly and the points of total collision, which are the points of central configuration and zero shape momenta, can be analyzed in detail. It turns out that, even on shape space where scale is not part of the description, the equations of motion diverge at (and only at) the points of total collision. We construct and study the stratified total-collision manifold and show that, at the points of total collision on shape space, the singularity is essential. There is, thus, no way to evolve the solutions through these points. This mirrors closely the big bang singularity of general relativity, where the homogeneous-but-not-isotropic cosmological model of Bianchi IX shows an essential singularity at the big bang. A simple modification of the general-relativistic model (the addition of a stiff matter field) changes the system into one whose shape-dynamical description allows for a deterministic evolution through the singularity. We suspect that, similarly, some modification of the dynamics would be required in order to regularize the total collision singularity of the N-body model.