Inflationary Big Bang Cosmology

With the introduction of the “inflationary Big Bang” scenario by Alan Guth and colleagues in the 1970s and further developments in this direction in the 1980s, these problems were basically solved. According to inflation, the extremely early universe (roughly the Planck time 10^-43 seconds) expands extremely rapidly, then quickly settles down to the expansion rates of the standard Big Bang model. During inflation, countless domains may arise, separating the overall universe into huge portions of spacetime in which the natural constants and even the specific laws of physics can vary. The effect of inflation on the problem of t=0, however, is fascinating. In some inflationary cosmologies, the Hawking-Penrose theorems don’t apply during the inflationary epoch. In these cosmologies we may never know whether or not an essential singularity exists, even if it does. Recently, attempts have been made to unify quantum physics and gravity and apply the results to cosmology. Proposals by Hawking and Hartle, Linde, Isham, Guth, Hawking and Turek, and others, are still in a speculative stage, but there are already some indications of what different quantum cosmologies might look like, including models with or without an initial singularity (‘eternal inflation’), with open or closed domains embedded in an open or a closed mega-universe, and so on. In most quantum cosmologies, our universe is just a part of an eternally expanding, infinitely complex megauniverse. Quantum cosmology, however, is a highly speculative field. Theories involving quantum gravity, which underlie quantum cosmology, are notoriously hard to test empirically, and they lift the philosophical issues already associated with quantum mechanics to a much more complex level since the domain is now ‘the universe’.

Contributed by: Robert Russell