Effective black-to-white hole bounces: The cost of surgery

Abstract

We investigate possible geometries allowing transitions from a black hole to a white hole spacetime, by placing a space-like thin shell between them. Such proposals have been advanced recently to account for singularity-resolution in black-hole spacetimes. This space-like shell can be extended to be outside the event horizon and, thereby, reproduce some of the features of these proposals. On the other hand, if the space-like shell is confined fully within the horizon, then it results in a bounce near a space-like singularity inside the black hole. For both cases, the null energy condition is necessarily violated, at least effectively, due to the introduction of quantum effects. If the shell, with a non-trivial negative tension, extends beyond the event horizon, then one can see effects of quantum gravity modifications even outside the horizon as a cost of such a space-like surgery. Naturally, one needs to consider whether these types of manufactured spacetimes violate any known laws of nature, allowing for reasonable assumptions. After critically comparing our results with several models in the literature, we reiterate a new way to avoid such black-hole singularities without leaving a white-hole remnant via a quantum bounce.