Cosmological singularities from high matter density without global topological assumptions

TL;DR

This paper proves a new cosmological singularity theorem for spacetimes with high matter density in the past, without relying on global topological assumptions, and explores its implications under additional conditions.

Contribution

It introduces a singularity theorem based on a 'past null focusing' condition that does not require global topological assumptions, expanding the understanding of cosmological singularities.

Findings

All null geodesics with future endpoints develop conjugate points if past complete.

High matter density conditions imply past null focusing, leading to singularity formation.

Under additional convergence conditions, all timelike geodesics are past incomplete.

Abstract

Cosmological singularity theorems such as that of Hawking and Penrose assume local curvature conditions as well as global ones like the existence of a compact (achronal) slice. Here, we prove a new singularity theorem for chronological spacetimes that satisfy what we call a `past null focusing' condition. Such a condition forces all null geodesics with future endpoint to develop a pair of conjugate points if past complete. By the Einstein field equations, such a condition will be satisfied if the density of matter fields remains sufficiently high towards the past of the spacetime, as may be expected in certain cosmological scenarios. The theorem obtained doesn't make starting assumptions about the spacetime's topology, such as the existence of a compact achronal slice, and if in addition to a `past null focusing' condition we assume the timelike convergence condition, then further consequences pertaining to the existence of CMC foliations and the character of the singularity are obtained. With the addition of the timelike convergence condition, we obtain the conclusion that all timelike geodesics are past incomplete, which is much stronger than the usual single incomplete non-spacelike geodesic.