Once Before Time - Martin Bojowald [134]
A series of partially cyclic views was proposed independently, with a certain behavior of matter or quantum theory postulated but not derived to be active during the big bang phase. In the 1930s, while the Great Depression reminded everyone that economic growth cannot go on forever, Richard Tolman was the first physicist to investigate the possibility of cyclic universes, with expansion followed by contraction and so on. To the present day, his studies, especially of the role of growing disorder and entropy in a cyclic universe, inspire ongoing research. After Tolman and the Great Depression, the ensuing decades—the 1940s through 1960s—were the heyday of Keynesianism, the theory that government intervention can spur economies to be ever expanding. During those linear years, the expanding big bang model was established in cosmology, with first results on cosmic background radiation published by Alpher and Herman and its eventual discovery by Penzias and Wilson. Hopes of an endlessly expanding economy were dashed by the oil crisis of the 1970s; but in 1979, physicists including Mario Novello and José Salím, and independently V. Melnikov and S. Orlov, proposed concrete mechanisms by which the universe could bounce back after a cosmic depression. Those ideas were used and extended in the work of Ruth Durrer and Joachim Laukenmann as well as Patrick Peter and Nelson Pinto-Neto in the 1990s and onward, providing entire cosmic scenarios in which structure can emerge without need for the negative pressure that must be present in an inflationary phase. Most of these investigations, however, were confined to the simplest isotropic universes and may, due to the required special properties, not prove valid generally. String theory has also found ways to construct cyclic pictures, as propagated by Gabriele Veneziano and more recently by Paul Steinhardt and Neil Turok, without, however, yet being fully supported. At the time of this writing, strong interest is again being shown in those fateful cyclic models—and again the world is trembling in financial crisis.
With the rise of loop quantum gravity, a new and different possibility arose to tackle this problem. Though it is still incomplete, we now had a quantum theory of gravity independent of the assumption of cosmic isotropy. Finding concrete solutions remained extremely complicated, but loop quantum cosmology soon made it possible, starting in 2000, to find the consequences of quantum theory for cosmological models. As already described, this indeed eliminates the big bang singularity by specific effects such as the discreteness of time as is typical for a combination of general relativity and quantum theory. To be sure, the desire to eliminate the singularity did not mark the beginning of loop quantum gravity, or of loop quantum cosmology either. It resulted from a long chain of mathematical constructions and calculations that made the nonsingular behavior visible only after several years of research. It is a consequence of the theory, seen independently of possible culturally influenced prejudices on the part of the participating researchers.3 Here, as so often is the case, the unbribable nature of mathematics is of crucial importance.
More details and intuitive aspects of what replaces the classical singularity, as we attempt to bail out a crashing universe, can be found in the earlier chapter on loop quantum cosmology. Now, some of the possibilities of this mechanism will be introduced to show how it may, as its details are further investigated, lead to new worldviews extending the classical big bang model to a prehistory before the big bang. It is not yet clear which of the diverse phenomena will dominate the process, or in what ratios all the different quantum effects arise; some of them have been analyzed by Ghanashyam Date with Kinjal Banerjee and Golam Hossain, others with crucial contributions from Aureliano Skirzewski. No complete picture is yet available, but