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takyon writes:

Viewpoint: Black Hole Evolution Traced Out with Loop Quantum Gravity

[Pennsylvania State University researchers] have shown that loop quantum gravity—a candidate theory for providing a quantum-mechanical description of gravity—predicts that spacetime continues across the center of the hole into a new region that exists in the future and has the geometry of the interior of a white hole. A white hole is the time-reversed image of a black hole: in it, matter can only move outwards. The passage "across the center" into a future region is counterintuitive; it is possible thanks to the strong distortion of the spacetime geometry inside the hole that is allowed by general relativity. This result supports a hypothesis under investigation by numerous research groups: the future of all black holes may be to convert into a real white hole, from which the matter that has fallen inside can bounce out. However, existing theories have not been able to fully show a way for this bounce to happen. That loop quantum gravity manages to do it is an indication that this theory has ripened enough to tackle real-world situations.

[...] During the last few years, a number of research groups have applied loop theory to explore the evolution of black holes. These efforts are building a compelling picture based on a black-to-white-hole transition scenario, which can be summarized as follows. At the center of the black hole, space and time do not end in a singularity, but continue across a short transition region where the Einstein equations are violated by quantum effects. From this region, space and time emerge with the structure of a white hole interior, a possibility suggested in the 1930s by physicist John Lighton Synge. As the hole's center evolves, its external surface, or "horizon," slowly shrinks because of the emission of radiation—a phenomenon first described by Stephen Hawking. This shrinkage continues until the horizon reaches the Planck size (the characteristic scale of quantum gravity) or earlier, at which point a quantum transition ("quantum tunneling") happens at the horizon, turning it into the horizon of a white hole. Thanks to the peculiar distorted relativistic geometry, the white hole interior born at the center joins the white horizon, completing the formation of the white hole.

If true, it would still be difficult to exploit this transition for time travel since you would likely be destroyed by tidal forces.

Quantum Transfiguration of Kruskal Black Holes (DOI: 10.1103/PhysRevLett.121.241301) (DX)

Quantum extension of the Kruskal spacetime (DOI: 10.1103/PhysRevD.98.126003) (DX)

Original Submission