Once Before Time - Martin Bojowald [58]
There is a prehistory of the universe before the big bang, with space and time as they cannot be seen in general relativity. As Dirac’s combination of special relativity and quantum theory had led to a new world of antimatter, a combination of general relativity and quantum theory provides a new world of space and time. This universe before the big bang is shrouded by a veil, penetrated not by the equations of general relativity but by those of loop quantum cosmology. Before the big bang, the universe was shrinking; it collapsed to smaller sizes under its own weight, eventually giving rise to the hot dense phase of the big bang. The transition itself can be analyzed only with quantum theories of gravity. Limits of the classical theory are transcended by a more encompassing theory, one that takes into account quantum theoretical properties. The big bang singularity turns out to be a limit of the language in which it was first formulated; it is not a limit of the world.
How is this possible? How can quantum physics prevent the universal collapse to a single point and the corresponding unbounded rise of temperature? How can gravity, after it had once recklessly caused the collapse of its own embodiment, space-time, repent? As in the stability problem of atoms, solved by quantum mechanics, the big bang singularity is prevented by new repulsive forces in loop quantum cosmology, counteracting the collapse caused by the purely attractive classical gravity. The collapse of a contracting universe, when it is small enough to bring quantum theory noticeably into the game, is first slowed down, completely stopped after some time, and then turned into expansion. One can roughly imagine a part of our universe’s history thus: Before the big bang it was contracting, opposite from the currently realized expansion. By collapsing, it became ever smaller and hotter, entering the big bang phase. Quantum effects were then prevailing and the slowdown as well as the rebound into the now visible expanding universe came about.
Details of this history, such as precise properties of the universe before the big bang, are much more difficult to come by. Did it look like our known expanding branch, except for the reversal of expansion? Did the reversed expansion have further consequences, such as a turnaround of one’s perception of time, causing one to remember the future and predict the past? Was it, anyway, so classical in its space-time structure long before the big bang, even at the large volumes it might possibly have had, that we can compare it with the present form of space-time? Or were quantum effects perhaps predominant even for space and time, such that the volume, though contracting on average, was subject to stronger fluctuations and jumpiness? Did it allow conditions that would make life possible? And where did this collapsing universe come from, and where is our expanding branch heading? Will the current expansion one day turn into a collapse, such that we will, like our preceding universe, move toward another big bang for the next world? Did the universe before the big bang come from such a recollapse of an older universe, one that rose from an ancestral grand big bang? Is there an infinite succession of such big bangs and intermittent expanding and contracting phases of the universe?
To answer these questions, one must enter much more deeply into the theory. Most of them cannot yet be solved reliably, and many such questions will probably never be reasonably treated by physical theories. Rather, they border on philosophy. What is known so far will be related in the course of this book.
THE PREVENTED SINGULARITY: REPELLING A WHOLE UNIVERSE
In the mathematical language of loop quantum gravity, a universe collapsing as a result of overpowering gravitational