Cyclic UniverseEdit
The cyclic universe refers to a family of cosmological models in which the cosmos undergoes a sequence of expansive and re-contractive phases, or otherwise experiences repeated, self-contained cycles that trace an overall history of time without a single beginning. While the standard picture of a hot Big Bang followed by cosmic expansion and eventual cooling remains the prevailing framework for describing our universe, cyclic approaches offer alternative mechanisms for addressing foundational questions such as the origin of large-scale structure, the observed flatness and homogeneity, and the nature of time's arrow. Over the decades, several distinct proposals have emerged, each tying together ideas from general relativity, quantum gravity, and high-energy theory in order to circumvent or revise some of the assumptions of a single-origin cosmology.
Cyclic cosmologies are part of a broader dialogue in cosmology about how the universe handles initial conditions, entropy, and ultimate fate. They are built to be testable within the constraints of modern physics and to connect with established pillars like the Big Bang paradigm, the observed cosmic microwave background, and the late-time acceleration driven by dark energy. In parallel with mainstream models, cyclic scenarios engage with deep questions about how the cosmos could evolve through many aeons or cycles, and what observational fingerprints such histories would leave behind.
Models and History
Ekpyrotic and cyclic models
One influential family of cyclic theories arises from ideas in higher-dimensional physics and string theory. In the ekpyrotic framework, a collision between higher-dimensional objects (often described as branes) in a surrounding bulk space can trigger a hot, dense state that resembles a Big Bang, with the subsequent expansion followed by contraction and a bounce into a new cycle. Proponents argue that this class of models can address the horizon and flatness problems without relying solely on a period of slow-roll inflation. The ekpyrotic/cyclic picture ties cosmology to concepts from string theory and M-theory, and it relies on specific dynamical mechanisms to smooth and flatten the universe across cycles. Observational implications include potential differences in the spectrum of primordial gravitational waves and in non-Gaussian features of the cosmic microwave background.
Conformal cyclic cosmology
Roger Penrose has advocated a distinct cyclic idea in which successive "aeons" of the universe are connected through conformal rescaling rather than through a physical bounce. In this conformal cyclic cosmology, the distant future of one aeon becomes the Big Bang of the next after the spacetime geometry loses its intrinsic scale. Supporters highlight how this approach reframes questions about the arrow of time and initial conditions, and they point to proposed observational hints in the low-multipole structure of the cosmic microwave background as potential evidence. Critics emphasize that the interpretation of such patterns is subtle and that the predicted signatures are not yet definitive tests.
Other cyclic proposals
A number of researchers have explored alternative routes to cyclicity, including models that use exotic forms of energy or non-standard gravitational dynamics to generate occasional bounces or turnovers in the expansion rate. Some of these frameworks connect to ongoing discussions about the ultimate fate of the universe, whether through a turnaround, a bounce, or a conformal transition between aeons. As with the ekpyrotic and CCC approaches, these ideas seek to address questions about entropy, horizon problems, and the fine-tuning of cosmological parameters in ways that differ from traditional inflationary thinking.
Loop quantum cosmology and quantum bounces
A closely related line of inquiry emerges from quantum gravity, where non-perturbative effects can modify the behavior of spacetime at very high densities. In loop quantum cosmology and related approaches, the classical Big Bang singularity is replaced by a quantum bounce, connecting a contracting phase to an expanding one. While not every loop-based model is explicitly cyclic, the presence of bounces opens the door to cyclic interpretations and fosters dialogue between quantum gravity and cosmology. These ideas have become part of the broader discussion about how quantum effects might shape the early and late-time evolution of the universe.
Key issues, debates, and status
Entropy and the second law of thermodynamics
A central challenge for cyclic models is how to contend with entropy. In many traditional cyclic visions, entropy would accumulate cycle after cycle, increasing disorder and potentially altering the conditions of subsequent cycles. Some cyclic proposals propose mechanisms to reset or dilute entropy, to dilute its cumulative effect, or to reinterpret entropy in a way that remains compatible with repeated cycles. The feasibility and naturalness of these mechanisms remain active topics of debate within the field. For historians of science, this echoes the classic questions raised by early cyclic ideas, notably by researchers who studied how thermodynamics would operate in a universe that never truly ends.
The need for new physics and energy conditions
Other concerns focus on the theoretical ingredients required by cyclic models. Some scenarios require forms of energy that violate traditional energy conditions or rely on features of higher-dimensional spaces that are difficult to test experimentally. Critics argue that such requirements can amount to fine-tuning or speculative extrapolation beyond current empirical support. Proponents maintain that these ingredients are natural within the broader landscape of high-energy theory, where many models invoke novel fields or geometries to achieve desired cosmological behavior.
Comparison with inflation and ΛCDM
Inflationary cosmology—often embedded in the ΛCDM framework via a period of rapid expansion—remains the most thoroughly tested and widely supported paradigm for explaining the smoothness, flatness, and spectrum of primordial fluctuations observed in the cosmic microwave background. Cyclic models must account for the same observational successes and, in many cases, propose distinct predictions, such as specific patterns in gravitational waves or features in the temperature anisotropy of the CMB. The degree to which cyclic frameworks can reproduce all successful inflationary predictions without introducing new tensions is a major axis of contemporary scrutiny.
Observational prospects
Advances in precision cosmology—planetary-scale surveys, CMB polarization measurements, and searches for primordial gravitational waves—offer channels to test cyclic ideas. A key challenge is identifying predictions that are robustly distinguishable from standard inflationary expectations. So far, data have not produced a smoking-gun signal for cyclic cosmologies, but proponents argue that upcoming observations could reveal subtle signatures, such as unique non-Gaussian patterns, or particular correlations in the CMB that would be hard to reconcile with a single Big Bang plus inflation story.