Cyclic universe theory

Article By:

Steinhardt, Paul J. Princeton Center for Theoretical Science and Department of Physics, Princeton University, Princeton, New Jersey.

Last reviewed:November 2019

DOI:https://doi.org/10.1036/1097-8542.176017

The cyclic universe theory is a model of cosmic evolution according to which the universe undergoes endless cycles of expansion and cooling, each beginning with a “big bang” and ending in a “big crunch”. The theory is based on three underlying notions: First, the big bang is not the beginning of space or time, but rather a moment when gravitational energy and other forms of energy are transformed into new matter and radiation and a new period of expansion and cooling begins. Second, the bangs have occurred periodically in the past and will continue periodically in the future, repeating perhaps once every 10¹² years. Third, the sequence of events that set the large-scale structure of the universe that we observe today took place during a long period of slow contraction before the bang; and the events that will occur over the next 10¹² years will set the large-scale structure for the cycle to come. Although the cyclic model differs radically from the conventional big bang–inflationary picture in terms of the physical processes that shape the universe and the whole outlook on cosmic history, both theories match all current observations with the same degree of precision. However, the two pictures differ in their predictions of primordial gravitational waves and the fine-scale statistical distribution of matter; experiments over the next decade will test these predictions and determine which picture survives.

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