Cyclic CosmologyEdit

Cyclic cosmology refers to a family of cosmological models in which the universe undergoes repeated cycles of expansion and contraction, or successive cosmological epochs that resemble a new start after a previous one ends. Rather than a single event that marks the origin of all the structure we see today, these theories embrace an ongoing rhythm of cosmic life. While the broad idea has ancient philosophical roots, modern variants are grounded in contemporary physics, especially ideas coming from general relativity, quantum gravity, and high-energy theory. See cyclic cosmology for the umbrella concept and how it relates to other large-scale models such as cosmology and Big Bang cosmology.

Cyclic ideas come in several flavors, with two of the most influential families being the ekpyrotic/cyclic model and conformal cyclic cosmology. In the ekpyrotic/cyclic approach, the cycle is driven by dynamics in higher-dimensional spaces where our visible universe is a brane that interacts with another brane. The collision and separation of branes set the stage for a sequence of bounces that replace a singular beginning with a quasi-regular pattern of expansion and contraction. In Penrose’s conformal cyclic cosmology, the far future of one cycle is conformally rescaled into a new big bang, letting successive aeons share a common geometric structure despite vast changes in the matter content. See ekpyrotic model and conformal cyclic cosmology for more on these approaches. Observational considerations connect cyclic proposals to data from the Cosmic microwave background and other cosmological probes like the Planck (space observatory) measurements.

History and development - Early ideas: The notion that the cosmos could undergo repeated cycles predates modern physics, but formal treatments began in the early 20th century. Ralph H. Tolman explored a periodic universe in which each cycle would differ from the last due to entropy production, raising the entropy bar higher with every pass. This classical challenge to the idea of an eternal, cycling universe remains central to how modern proposals address the thermodynamic arrow of time. See Ralph H. Tolman and thermodynamics in cosmology for the technical context. - Modern reinventions: The 1990s and early 2000s saw renewed interest in cyclic schemes tied to concrete physical frameworks. The ekpyrotic/cyclic model, developed by Steinhardt, Turok, and collaborators, ties cycles to brane dynamics in higher dimensions and to a particular contracting phase that smooths and flattens spacetime before a bounce to expansion. See ekpyrotic model for details on the mechanism and its motivations. - Alternative cyclic path: The conformal cyclic cosmology (CCC) proposed by Roger Penrose offers a different route: instead of a bounce driven by high-energy physics, the succeeding aeon is constructed by a conformal rescaling that erases the distinct matter content of the previous era. See conformal cyclic cosmology for a discussion of its core ideas and claims.

Core concepts and models - Ekpyrotic/cyclic model: This framework posits a slow contraction phase driven by a stiff equation of state that smooths out inhomogeneities and anisotropies before a bounce injects expansion. The cycle then repeats, with each new expansion phase starting from a near-homogeneous state. The bounce relies on physics beyond the Standard Model, often invoked from string-inspired or higher-dimensional theories (for example, brane dynamics). Proponents emphasize the model’s potential to address the initial-condition problem and to offer a natural mechanism for generating the large-scale structure observed in the universe. See ekpyrotic model and brane cosmology for related ideas. - Conformal cyclic cosmology: In CCC, the late-time universe of one aeon becomes the early-time universe of the next through conformal rescaling that removes the distinction between big-bang matter and the late-time radiation-dominated state. The Weyl curvature hypothesis plays a central role in arguing that each aeon can be stitched to the next in a way that preserves finite geometry. Observationally, CCC has been associated with claims of subtle, large-scale patterns in the Cosmic microwave background—patterns that remain subjects of debate in the observational community. See conformal cyclic cosmology and Weyl curvature hypothesis. - Key technical challenges: Across all cyclic proposals, major hurdles include the thermodynamic entropy problem (how to prevent ever-accumulating disorder from spoiling future cycles), the details of the bounce or conformal matching, and the ability to make falsifiable predictions that can be tested against data. See entropy and cosmological bounce for broader discussions of these issues.

Observational status and empirical status - Data and predictions: Standard cosmology with a period of inflation remains the dominant paradigm because it provides a simple, predictive framework for the primordial fluctuations that seed cosmic structure and for the observed features of the CMB. Cyclic models aim to address questions inflation does not fully resolve, such as the deep origin of initial conditions and the fate of entropy across cycles, while offering distinct predictions about signatures in the CMB or gravitational-wave backgrounds. See cosmological inflation and cosmic microwave background for context. - Tests and tensions: The ekpyrotic/cyclic framework and CCC make different claims about observable imprints, including possible non-Gaussianities, specific patterns in the CMB, or unique gravitational-wave spectra. So far, data from missions such as Planck (space observatory) have not provided definitive, unambiguous confirmation of cyclic-specific predictions, and many features attributed to cycles remain controversial or contested within the scientific community. See discussions of Planck data and CMB anomalies for the state of play. - Entropy and the arrow of time in observations: A persistent challenge for cyclic models is reconciling the second law of thermodynamics with repeated cycles. Several versions propose mechanisms to reset or shed excess entropy between aeons, but these proposals require physics beyond the Standard Model and remain actively debated among theorists.

Controversies and debates - Scientific debate: The primary contention centers on testability and naturalness. Skeptics argue that cyclic models often depend on speculative physics—extra dimensions, branes, or conformal rescalings—that lie far outside direct experimental reach, making them difficult to falsify. Proponents counter that cosmology must grapple with questions the standard hot big bang model and inflation do not fully answer, such as the origin of low-entropy initial conditions or the fundamental nature of time, and that exploring these ideas helps push the boundaries of theoretical physics. - Entropy and the problem of cycles: Tolman’s historical observation about entropy increasing with each cycle remains a central diagnostic. Different cyclic proposals offer different remedies—shedding entropy into a hidden sector, or resetting conditions through conformal matching—yet critics point out that these fixes add layers of new assumptions and potentially new fine-tuning. - Predictive power and data: The strength of any cosmological theory lies in its predictive power. Inflation has a broad and robust predictive track record, while cyclic models have more constrained or model-specific predictions. Ongoing and future observations—such as precise measurements of primordial gravitational waves, non-Gaussian statistics, and large-scale CMB features—will continue to test these ideas. - From a strategic viewpoint in physics discourse: Critics often emphasize a preference for minimalism and empirical grounding, arguing that speculative frameworks should not be granted equal weight to theories with strong observational anchors. Advocates stress that cosmology must entertain radical possibilities to address foundational questions about the origin, fate, and fundamental laws of the universe.

Terminology and related concepts - Cosmology: The study of the origin, evolution, and large-scale structure of the universe, typically incorporating general relativity and quantum theory at extreme scales. See cosmology. - Bounce:** A transition from contraction to expansion in some cyclic models, replacing a singular Big Bang with a smooth or quantum-gravity-mediated transition. See cosmological bounce. - Brane cosmology: Scenarios in which our universe is a lower-dimensional surface embedded in a higher-dimensional space, with dynamics that can drive cyclic behavior in some models. See brane cosmology. - Weyl curvature hypothesis: A proposal about initial conditions in cosmology that ties geometric properties of spacetime to entropy and the arrow of time; a central element in Concealment-type cyclic theories. See Weyl curvature hypothesis. - Conformal symmetry and conformal compactification: Mathematical tools used in several cyclic proposals to relate distant epochs in a way that preserves angle structure while erasing certain physical details. See conformal symmetry and conformal cyclic cosmology.

See also - cosmology - Big Bang - cosmological inflation - Planck (space observatory) - Cosmic microwave background - ekpyrotic model - brane cosmology - conformal cyclic cosmology - Weyl curvature hypothesis - entropy