Once Before Time - Martin Bojowald [107]
Precisely how to extend the classical picture can be determined only by means of a theory of quantum gravity, for this governs (or so one expects) all events at the singularity of general relativity. A final clarification of the question remains open, but one can already use some of the known properties of quantum theories of gravity to speculate; and the most exciting speculation is, of course, the more spectacular possibility mentioned above: the creation of a daughter universe.
Theorists have repeatedly aimed their thoughts at this issue, but Lee Smolin, one of the founders of loop quantum gravity, has approached it with particular creativity. He assumes that this daughter universe indeed grows up to be a whole big universe like ours, and even that our universe was born in this way from an ancestor. Such birthing events are to be distinguished from a bounce as in cosmology, a simple turnaround in the expansion of a single universe. Through black holes, one universe would at some point split off into two. And if this is repeated, one obtains a strongly branched space-time of decidedly complex structure. Every black hole in the cosmos can give birth to its own daughter universe, and each daughter universe can, through the black holes it contains, produce granddaughter universes, and so on.
27. Alternatives of a black hole with prevented singularity. Left: Behind the singularity there is a daughter universe without contact with the previous black hole exterior. (The hashed region marks an overlap in the visualization where, however, no contact between the space-times exists.) Right: The classical singularity is merely surrounded by a high-curvature region (drawn wavily), penetrated in quantum gravity. Here, the diagram no longer differs significantly from that of a common compact object as in figure 22.
Another difference between universe birthing and the cosmic bounce is the fact that a discussion of daughter universes necessitates a theoretical viewpoint outside space-time itself. As internal observers, which we really all are, one can choose only between remaining with the mother universe, outside all black holes, or devotedly throwing oneself into one favorite daughter universe. One can never oversee the entire space-time as an observable and physical object; instead, when toying with several separate daughter universes, one would take a metaphysical standpoint outside any observability whatsoever, even in principle. Already in cosmology we saw immense difficulties in watching the universe before the big bang, although in that case it was possible at least in principle. But viewing several daughter universes at the same time is fundamentally impossible.
Here, Smolin’s important insight enters the game. He argues that there could nonetheless be observable consequences of such repeated branching-off processes. One merely has to make one more assumption, namely, that during each splitoff, parameters of the universe such as constants of nature change slightly. Concrete reasons do not exist, but this is up to quantum gravity to decide; we shall thus accept the assumption for now. (In string theory, many “constants of nature” are established as values taken by fields that may vary over long durations, providing a possible scenario for such changes to be combined with Smolin’s idea.)
Smolin goes on