Matching Radial Geodesics in Two Schwarzschild Spacetimes (e.g. Black-to-White Hole Transition) or Schwarzschild and de Sitter Spacetimes (e.g. Interior of a Non-singular Black Hole)

TL;DR

This paper investigates the trajectories of radial geodesics across thin shells connecting Schwarzschild, de Sitter, and black-to-white hole spacetimes, revealing limitations in global conformal coordinates due to degeneracies at horizons.

Contribution

It extends previous work by analyzing geodesic matching in spacetimes joined via static spacelike thin shells, highlighting the non-existence of a universal conformal coordinate system in such scenarios.

Findings

Coordinate transformations relate geodesics across shells.

Degeneracy occurs at horizons in the chosen coordinate system.

Global conformal coordinates generally do not exist for these spacetimes.

Abstract

In this article, we study the trajectory equations of the bounded radial geodesics in the generalized black-to-white hole bounce with mass difference and the Schwarzschild-to-de Sitter transition approximated by the thin-shell formalism. We first review the trajectories equations of the general radial geodesics in Kruskal-Szekeres (like) coordinates of the Schwarzschild and de Sitter spacetimes, respectively. We then demonstrate how one relates the radial geodesics on each side of the shell by correctly choosing the constants of integration after performing the two coordinate transformations mentioned in our previous work, arXiv:2302.04923. We next show that the coordinate system used in the resulting Penrose diagram has no illness at the thin shell but instead creates a degeneracy between the timelike geodesics and null geodesics at the event horizons where the second transformation is applied. Due to this problem, we conclude that a global conformal coordinate chart for the spacetime connected via a static spacelike thin shell in general does not exist, except for some special cases. Since this is an extension of our work, arXiv:2302.04923, we focus on Schwarzschild and de Sitter spacetimes, though the method should be applicable to any cut-and-pasted spacetime connected with a static spacelike thin shell.