In A Dec 10 2018 article at physics.aps.org, Carlo Rovelli reports on
theoretical work by Ashtekar and Olmedo at Pennsylvania State University
and Singh at Louisiana State University that purportedly shows that
loop quantum gravity—a theory that extends general relativity by quantizing spacetime—
predicts that black holes evolve into white holes.
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Originally Posted by Rovelli. physics.apps.org, Dec 10 2018
Black holes are remarkable entities. On the one hand, they have now become familiar astrophysical objects that have been observed in large numbers and in many ways: we have evidence of stellar-mass holes dancing around with a companion star, of gigantic holes at the center of galaxies pulling in spiraling disks of matter, and of black hole pairs merging in a spray of gravitational waves. All of this is beautifully accounted for by Einstein’s century-old theory of general relativity. Yet, on the other hand, black holes remain highly mysterious. We see matter falling into them, but we are in the dark about what happens to this matter when it reaches the center of the hole.
Abhay Ashtekar and Javier Olmedo at Pennsylvania State University in University Park and Parampreet Singh at Louisiana State University, Baton Rouge, have taken a step toward answering this question [1]. They 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.
The reason why we are in the dark about aspects of black hole physics is that quantum phenomena dominate at the center and in the future of these objects. Classical general relativity predicts that a black hole lives forever and that its center is a “singularity” where space and time end. These predictions are not realistic because they disregard quantum effects. To tackle these effects we need a quantum theory of gravity. We don’t yet have consensus on such a theory, but we have candidates, some of which are now reaching the point of allowing actual calculations on the quantum behavior of black holes. Loop quantum gravity, which has a clean conceptual structure and a well-defined mathematical formulation based on representing the fabric of space as a spin network that evolves in time, is one such theory.
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Article here :-
https://physics.aps.org/articles/v11/127
