Once Before Time - Martin Bojowald [114]
In contrast to traversal of the high-curvature region around the classical singularity, a precise analysis of all details of temporal changes as they play out among the elementary constituents of discrete space-time is necessary at the would-be branching point. During a branching off to a daughter universe, space-time must, after all, separate between two neighboring space-atoms, connected initially but no longer at the following time step—after the singularity. At the present stage, a complete answer is difficult owing to the incompleteness and mathematical complexity of the theory. Yet there are indications of what the answer should look like once a complete and consistent theory becomes available: Namely, there is no branching off into a daughter universe; instead, the interior will be united with the exterior after the horizon evaporates.
Then the collapsed matter will reappear, rent asunder—possibly in an explosion widely visible from afar in the universe. The region of extremely high energy density and curvature around the classical singularity suddenly becomes accessible from outside, which is bound to represent a powerful cosmic event. Information loss such as Hawking initially feared does not occur, since with matter the stored information would also return. It may be highly deformed and difficult to decipher, for it will have had to traverse the inner region of high curvature, but in principle it is still there. And this is all that is required for the consistency of the theoretical framework.
Preliminary demonstrations of this behavior rely on characteristic properties of atomic space-time dynamics as it can be seen in loop quantum gravity. There might indeed be a rending of neighboring space-atoms where curvature is very high and quantum aspects dominate; the predecessor space-time could split into two separate regions. Depending on the exact form of the dynamics, which is yet to be found, there are two possibilities: The briefly separated parts directly reconnect in the next discrete time step, or they remain divorced forever.
Only in the second case can a branching off into a daughter universe occur. But the problem is that the point-in-time singularity of the black hole is spatially extended, just as the usual center of a star is not spatially but temporally extended. For a splitting off of the black hole interior into a daughter universe, it would be necessary for space-time to tear only at one end of the classical singularity—at one side of the high curvature region in general relativity, the right endpoint in our figures—but to remain intact elsewhere. Otherwise, the interior would indeed separate at the horizon and immediately disintegrate into numerous completely fragmented space-atoms: a cosmic miscarriage, in no way producing a daughter universe.
Since the high-curvature region is subject to the laws of quantum gravity not just at its edge but over its whole extension, a tearing at the edge comes just as easily as a ripping off anywhere in the interior. There is, after all, extremely high curvature everywhere, together with strongly quantum behavior; no reason exists for a special treatment at the edge. No longer does the theory have to decide between a branching off to a daughter universe and an opening up of the black hole back into the original space-time, but instead, and more dangerously, it must decide between the latter case and the interior’s complete disintegration.
How can one rule out a disastrous disintegration? Fortunately,