Cyclic Models Of The UniverseEdit
Cyclic models of the universe are a family of theoretical frameworks in cosmology that describe the cosmos as undergoing repeated epochs of expansion and contraction, or other repeating cycles, rather than arising from a single, unique beginning. Proponents argue that these models can connect the physics of the very early universe to the late-time evolution we observe today, offering alternatives to a one-shot Big Bang and to a universe that simply expands forever. The ideas are diverse, but they share a common aim: to explain how cosmic history might be unbounded in time while still producing the structure and radiation we detect in the present era.
In broad terms, cyclic proposals replace or augment the standard narrative of a singular origin with a sequence of events in which each cycle resets, or nearly resets, the conditions of the cosmos for the next. Different schools of thought implement this reset in different ways. Some rely on high-energy processes in higher dimensions or on novel gravity theories to generate a bounce that transitions from contraction to expansion. Others, such as conformal cyclic cosmology, appeal to a conformal rescaling of spacetime that carries information from one aeon to the next. A number of these models also aim to explain observed features in the cosmic microwave background Big Bang radiation and the distribution of large-scale structure without relying entirely on a brief inflationary period Inflation (cosmology).
Conceptual foundations
Cyclic models are characterized by an underlying assumption that the laws of physics remain applicable across cycles, with the mechanism of the cycle transition determining how one epoch gives way to the next. This often involves gravity studied within a framework that goes beyond classical general relativity, incorporating ideas from quantum gravity or string theory. See General Relativity and Quantum gravity for foundational context.
A central challenge is the management of entropy. According to the second law of thermodynamics, entropy tends to grow, which, in a naive cyclic picture, could imply progressively longer and more dilute cycles. Proponents have proposed several ways to address this, ranging from entropy shedding to the notion that each cycle effectively resets certain thermodynamic conditions. For background on the thermodynamic considerations, see Entropy.
The generation and evolution of primordial perturbations—the tiny fluctuations that grow into galaxies and clusters—are a focal point in evaluating cyclic models. Different cyclic proposals offer different mechanisms for producing a nearly scale-invariant spectrum of perturbations, a hallmark that cosmologists associate with observational data from the cosmic microwave background. See Cosmology and Planck (satellite) for the observational touchstones.
Notable theoretical frameworks
The ekpyrotic/cyclic universe
The ekpyrotic scenario originated in ideas about brane cosmology in higher dimensions and was developed into a cyclic framework by researchers such as Paul Steinhardt and Neil Turok. In this picture, a collision between higher-dimensional branes within a compactified space triggers a hot, expanding phase that resembles our observed cosmos, and a subsequent process leads to a contraction and a new bounce. The model emphasizes a slow, ultra-stiff equation of state during contraction to produce the observed uniformity and perturbation spectrum. See Ekpyrotic Universe for the primary exposition, and Brane cosmology for the higher-dimensional background.
Conformal cyclic cosmology (CCC)
Roger Penrose has proposed conformal cyclic cosmology, in which the distant future of one aeon becomes conformally equivalent to the beginning of the next. In CCC, successive aeons are related by a conformal rescaling of spacetime that allows information to pass from one cycle to the next, effectively erasing some of the distinctions between cycles while preserving the geometric and energetic structure needed to seed the next expansion. See Conformal cyclic cosmology.
Loop quantum cosmology and other bounce models
Other lines of inquiry explore a bounce arising from quantum gravity effects, such as in Loop quantum cosmology. In these approaches, quantum geometric properties replace classical singularities with a high-energy bounce, offering a mechanism to connect a contracting phase to a subsequent expansion. See Loop quantum cosmology for the technical development and its relationship to broader quantum gravity research.
Other approaches and variants
Additional cyclic or oscillatory ideas have appeared in the literature, blending elements from classical cosmology, particle physics, and higher-dimensional theories. The field remains diverse, with competing proposals about the exact mechanism of the cycle transition, the treatment of entropy, and the compatibility with observational data. See also Cosmology and Brane cosmology for related frameworks.
Observational status and controversies
Supporters argue that cyclic models can address questions about initial conditions, the nature of time, and the origin of large-scale structure while avoiding a singular creation event. They contend that certain observational signatures—such as specific patterns in the cosmic microwave background or relic gravitational waves—could, in principle, distinguish cyclic scenarios from a purely inflationary history. See Cosmic microwave background and Gravitational waves for the standard observational touchstones, and Inflation (cosmology) for the main competing paradigm.
Critics emphasize that many cyclic proposals depend on speculative or untested physics—such as higher dimensions, brane collisions, or particular quantum gravity effects—that lie beyond the current experimental reach. They point to the difficulty of reproducing precise observational features without resorting to fine-tuning, and to potential difficulties in explaining entropy growth across cycles. In particular, the question of how an aeon-to-aeon transition would consistently shape the spectrum of primordial perturbations and the absence or presence of detectable gravitational-wave signatures remains a point of contention. See Entropy, Planck (satellite), and BICEP2 for relevant observational discussions.
The debate touches on broader methodological questions in cosmology: to what extent should speculative high-energy physics be used to solve foundational problems about the universe, and how testable must a theory be to merit the label of a viable cosmological model? Proponents argue that cyclic and bounce scenarios offer concrete, falsifiable predictions and a path to unifying early- and late-time cosmology, while critics stress the need for clear, experiment-driven criteria before these ideas gain wide acceptance. See Philosophy of science and Scientific method for broader context.