Statistical Properties of Cosmological Billiards

TL;DR

This paper refines the understanding of the statistical behavior of cosmological billiards near singularities by analyzing unquotiented dynamics, revealing new correlations and invariant measures, and exploring connections to Kac-Moody symmetries.

Contribution

It introduces a detailed analysis of unquotiented cosmological billiard dynamics, uncovering new phenomena and invariant measures, and clarifies their relation to symmetry groups and Kac-Moody structures.

Findings

Discovered correlations between successive billiard corners.

Constructed a non-normalizable invariant measure on the Kasner circle.

Clarified the relation between unquotiented Bianchi IX dynamics and billiard models.

Abstract

Belinski, Khalatnikov and Lifshitz (BKL) pioneered the study of the statistical properties of the never-ending oscillatory behavior (among successive Kasner epochs) of the geometry near a space-like singularity. We show how the use of a "cosmological billiard" description allows one to refine and deepen the understanding of these statistical properties. Contrary to previous treatments, we do not quotient the dynamics by its discrete symmetry group (of order 6), thereby uncovering new phenomena, such as correlations between the successive billiard corners in which the oscillations take place. Starting from the general integral invariants of Hamiltonian systems, we show how to construct invariant measures for various projections of the cosmological-billiard dynamics. In particular, we exhibit, for the first time, a (non-normalizable) invariant measure on the "Kasner circle" which parametrizes the exponents of successive Kasner epochs. Finally, we discuss the relation between: (i) the unquotiented dynamics of the Bianchi IX (a, b, c or mixmaster) model; (ii) its quotienting by the group of permutations of (a, b, c); and (iii) the billiard dynamics that arose in recent studies suggesting the hidden presence of Kac-Moody symmetries in cosmological billiards.