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 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 cosmological 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.
Steinhardt and Turok describe the concept of an endless universe as still in its infancy, but they believe the model might be able to explain some points not included in the big bang theory. Those points involve extra dimensions, branes, and dark energy.
The string theory in physics refers to particles as waves on strings and was conceived as a way to reconcile quantum mechanics with general relativity. M-theory, which unites all five existing string theories, allows for the existence of additional dimensions. The three dimensions that expanded and contracted in the older theory now should be seen as expanding and stretching from one big bang to the next. As the stretching rate slows, an additional dimension is squeezed to nearly zero size before it begins expanding again.
Branes (short for mathematical membrane) in string theory consist of surfaces of varying dimensions that can move through space. In the cyclic model, our universe is referred to as a braneworld. It is separated by only a small gap from, and collides at regular intervals with, a second braneworld that we cannot observe. Matter and radiation are confined to branes, which can always be stretched. The space between the two eventually shrinks to zero when the big crunch occurs, but energy doesn’t become concentrated as described in the original big bang theory.
Though no specific role for dark energy had been established in the inflationary model, it plays three essential roles in the cyclic model. First, it speeds up the rate of expansion of the universe, allowing its components to spread out. Branes eventually stretch to a near-vacuum condition, resulting in an equal distribution of energy and smoothing out wrinkles so that each cycle begins with the same basic physical conditions. Second, dark energy serves as a stabilizing force by regulating the speed of branes and acting as a shock absorber to ensure that brane collisions don’t become completely random. Finally, dark energy has the ability to shut itself off. As the energy draws branes together, it gradually changes from positive to negative. This allows the branes to speed up and for the additional dimension to contract so that the next cycle can take place.
Steinhardt and Turok point out that the before and after of the cycles can be described in a way that follows the laws of physics and is justified by current astronomical observations.
The cyclic model has been described as more optimistic than earlier models. The universe remains essentially the same. Every part evolves through a series of regular cycles, starting with a bang and ending with a crunch. The existence of dark energy is important in helping to keep the cycles on track.
Steinhardt and Turok emphasize that the cyclic model is still only a theory that should be discussed and tested by the scientific community along with alternatives. They express optimism that recent advances in astronomical observation, along with laboratory experiments, will lead to the identification of the best model to explain our universe within the next two decades.
See also Big Bang Theory; Big Crunch Theory; Cosmogony; Eternal Recurrence; Heraclitus; Nietzsche, Friedrich; Plotinus; Time, Cyclical; Universe, Closed or Open; Universe, Contracting or Expanding
Kragh, H. S. (2007). Conceptions of cosmos: From myths to the accelerating universe: A history of cosmology. Oxford, UK: Oxford University Press.
Kragh, H. S. (1996). Cosmology and controversy. Princeton, NJ: Princeton University Press.
Steinhardt, P. J., & Turok, N. (2007). Endless universe: Beyond the big bang. New York: Doubleday.