Influence of geometry variations on the gravitational focusing of timelike geodesic congruences

TL;DR

This paper develops a framework to analyze how geometric variations influence the focusing of timelike geodesic congruences, especially during gravitational contraction, using conformal variations and applications to cosmological models.

Contribution

It introduces a set of equations describing the linear response of geodesic convergence to arbitrary geometry variations, including conformal cases, and applies this to gravitational contraction and cosmological models.

Findings

Geometry variations affect geodesic focusing during contraction.

Negative conformal functions increase contraction rate.

Application to FRW model illustrates singularity formation insights.

Abstract

We derive a set of equations describing the linear response of the convergence properties of a geodesic congruence to arbitrary geometry variations. It is a combination of equations describing the deviations from the standard Raychaudhuri-type equations due to the geodesic shifts and an equation describing the geodesic shifts due to the geometry variations. In this framework, the geometry variations, which can be chosen arbitrarily, serve as probes to investigate the gravitational contraction processes from various angles. We apply the obtained framework to the case of conformal geometry variations, characterized by an arbitrary function $f(x)$, and see that the formulas get simplified to a great extent. We investigate the response of the convergence properties of geodesics in the latest phase of gravitational contractions by restricting the class of conformal geometry variations to the one satisfying the strong energy condition. We then find out that in the final stage, $f$ and $ {\bf D}\cdot {\bf D} f$ control the overall contraction behavior and that the contraction rate gets larger when $f$ is negative and $|f|$ is so large as to overwhelm $|{\bf D} \cdot {\bf D} f|$. (Here ${\bf D} \cdot {\bf D}$ is the Laplacian operator on the spatial hypersurfaces orthogonal to the geodesic congruence in concern.) To get more concrete insights, we also apply the framework to the time-reversed Friedmann-Robertson-Walker model as the simplest case of the singularity formations.