Cosmology, Cyclic

Cyclic cosmology refers to the concept that the cosmos undergoes continuous cycles of evolution. Time itself has generally been defined by the beliefs regarding the specific nature of the cycles.

Cycles of destruction and renewal have been a recurrent theme in mythologies and religions, inspired by such natural changes as day and night, the seasons, and birth and death. In the ancient world, the concepts of linear and cyclical time were already being debated, but nearly every culture recognized the existence of cycles and had its own beliefs about when they would occur. The concept of time was generally defined by the cycles of nature. For example, the duration of cycles in Hindu cosmology were related to periods of the life of Brahma, the god of creation.

The Greek philosopher Heraclitus was the first known person to address the inevitability of change and the need to understand its nature. Western thought generally favored cyclical cosmology up until the time that Christian interpretations of the Book of Genesis became influential, though examples of cyclical theories continued to appear. In the 18th century, James Hutton, considered the founder of modern geology, referred to the recurrent destruction and renewal of the “great geological cycle.” In the late 19th century, German philosopher Friedrich Nietzsche developed the theory that, given the limitless nature of time, events must inevitably recur. Various cyclic models that failed to withstand the test of time were proposed in the 1920s and 1930s by cosmologist Richard C. Tolman and others.

Most recently, cosmologists Paul J. Steinhardt and Neil Turok have introduced the cyclic model [Page 224]of the universe as an alternative to the big bang theory, which they have described as having been generally dominant during the past 40 years.

The original big bang theory involves a singularity that occurred about 14 billion years ago, creating the universe from an infinitesimally tiny area of space with a density and temperature that were nearly infinite. Since then, the universe has continued expanding and cooling. A more recent version of the theory includes the concept of inflation, or the brief, accelerated expansion that occurred immediately after the big bang.

Steinhardt and Turok's cyclic model had its beginnings in the model of the ekpyrotic universe, which the two had developed earlier with other cosmologists. The word ekpyrosis is a Greek word for conflagration, referring to an ancient cosmo-logical model wherein the universe was created in a burst of fire.

The cyclic model also includes a big bang, as well as a “big crunch” at the end of a cycle. But since the model indicates that the universe has no beginning or end, it should now be viewed as a bridge to a past of endlessly repeating cycles. When a big bang occurs, it creates heat and radiation at a finite temperature and then cools to form galaxies and stars. The bang represents a transition from a contracting phase to an expanding phase where matter spreads out to become a nearly perfect vacuum. The interval between big bangs is believed to be about a trillion years, far longer than the 14 billion years predicted by the inflationary model.

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