Repulsive regions in Lema\^itre-Tolman-Bondi gravitational collapse

TL;DR

This paper identifies regions of repulsive gravity in inhomogeneous Lemaître-Tolman-Bondi spacetimes, analyzes their effects on gravitational collapse, and explores the conditions under which black holes or naked singularities form, with potential astrophysical implications.

Contribution

It introduces an invariant method to detect repulsive gravity regions and examines their influence on collapse dynamics and black hole formation in Lemaître-Tolman-Bondi models.

Findings

Repulsive gravity regions are identified via curvature eigenvalues.

Black holes with dominant repulsive effects are possible.

Repulsive effects may be linked to gamma-ray burst progenitors.

Abstract

We show that in the inhomogeneous Lema{\^i}tre-Tolman-Bondi space-time there are specific regions in which repulsive gravity exists. To find these regions, we use an invariant definition of repulsive gravity based upon the behavior of the curvature eigenvalues. In addition, we analyze the effects of repulsive gravity on the dynamics of the gravitational collapse. In particular, we investigate the collapse in the case of the parabolic solution for the effective scale factor of the Lema{\^i}tre-Tolman-Bondi metric, corresponding to the marginally bound case. Exploring the corresponding cut-offs at which gravity becomes repulsive, we notice that black holes with dominant repulsive effects are not excluded \emph{a priori}. Indeed, we demonstrate that the collapse leads, in general, to the formation of a central naked singularity; however, for particular values of the free parameters entering the model, black holes with dominant repulsive gravity can exist. We show that the expected physical process is not modified as the marginally bound condition is dropped out. Moreover, we show that this is true independently of the hypothesis that the energy-momentum tensor is built up in terms of pressureless matter. Further, we demonstrate that geodesic deviations can depend on the sign of the curvature eigenvalues. Finally, we give an astrophysical interpretation of black holes with dominant repulsive gravity. Indeed, we argue that compact objects with dominant repulsive gravity could be interpreted as progenitors of Gamma Ray Bursts.