The Cyclic Universe: Endless Cosmic Cycles
The cyclic universe theory proposes that the cosmos undergoes an infinite series of expansions and contractions rather than originating from a singular Big Bang event that marks a definitive beginning. In its simplest historical form, the oscillating universe model—first suggested by Richard Tolman in the 1930s—describes a universe that alternately expands under its own momentum until gravitational attraction eventually halts and reverses this expansion, leading to a “Big Crunch” followed by another expansion phase. While Tolman’s early model encountered thermodynamic issues (entropy accumulation after each cycle), modern cyclic models address these challenges with novel mechanisms that reset or redistribute entropy.
One prominent contemporary variant is Roger Penrose’s Conformal Cyclic Cosmology (CCC). In CCC, each aeon (cycle) ends in an exponentially expanding, cold, and diffuse state that can be mathematically “conformally rescaled” to serve as the low-entropy starting point of the next aeon. Penrose’s idea effectively maps the infinite future of one cosmic epoch onto the fiery birth of the next, bypassing singularities. Researchers studying CCC search for imprints from a prior aeon in the cosmic microwave background—potential concentric rings of anomalous temperature variations that could be relic signals of massive events (like supermassive black hole collisions) in a preceding cycle.
Another framework, the ekpyrotic model, arises from string theory and brane cosmology. In this picture, our four-dimensional universe exists on a “brane” within a higher-dimensional bulk. Collisions between our brane and a parallel brane trigger periodic bounces, each event analogous to a Big Bang. Between collisions, the universe undergoes a quasi-static contraction that smooths and flattens itself, solving the horizon and flatness problems without requiring a separate inflationary phase. Ekpyrotic scenarios predict distinctive signatures in primordial gravitational waves and the cosmic microwave background polarization, offering avenues for experimental tests.
A related approach, the “cyclic inflation” model, merges inflationary dynamics with a cyclic background. Each cycle features a brief inflationary expansion that seeds large-scale structure, followed by reheating and eventual contraction. Entropy is diluted via mechanisms like phantom energy or a decaying scalar field, allowing cycles to persist indefinitely without runaway entropy.
Critics of cyclic cosmologies point to unresolved challenges—such as achieving a truly nonsingular bounce in a quantum-gravitational framework and ensuring that each cycle resets rather than accumulates destructive irregularities. Nevertheless, advances in quantum gravity, string theory, and observational cosmology continue to refine these proposals. Future probes—like next-generation gravitational wave observatories and high-resolution cosmic microwave background experiments—may detect telltale signatures that distinguish cyclic predictions from one-time Big Bang inflationary models.
By positing an unending procession of cosmic epochs, cyclic universe theories offer a striking alternative to the conventional conception of a finite-age universe. They raise profound questions about time, entropy, and the meaning of “beginning”—and remind us that, in modern cosmology, what we perceive as the ultimate origin might simply be the latest in an eternal chain of cosmic rebirths.
