Cyclic universe
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Cyclic universe | |
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| Type | Cosmological framework |
| Field | Cosmology; Theoretical physics |
| Core idea | The universe undergoes repeated phases of expansion and contraction |
| Assumptions | Cosmic evolution is not temporally finite |
| Status | Theoretically developed; empirically unconfirmed |
| Related | Bounce cosmology; Big Bang; Entropy; Early universe |
Cyclic universe models propose that the universe does not evolve from a single beginning toward a final end, but instead undergoes a sequence of expanding and contracting phases. Each cycle may include periods resembling a Big Bang–like expansion followed by gradual contraction and renewal.
These models offer an alternative to singular-origin cosmologies.
Definition
In a cyclic universe, cosmic history is composed of multiple epochs or cycles, each characterized by expansion, cooling, and structure formation, followed by contraction or transformation into the next cycle.
Time extends across cycles rather than originating at a single boundary.
Motivation
Standard cosmology predicts an initial singularity and potentially a terminal state such as heat death. Cyclic models aim to avoid these endpoints by replacing them with repeating or regenerative dynamics.
They also address questions about cosmic initial conditions.
Historical background
Ideas of cyclic cosmology predate modern physics and appear in early relativistic models. Contemporary cyclic proposals are grounded in relativistic and quantum frameworks rather than philosophical recurrence alone.
Modern models differ substantially from earlier oscillatory universes.
Entropy challenge
A central difficulty for cyclic cosmology is entropy accumulation. In classical thermodynamics, entropy increases from cycle to cycle, eventually preventing further cycles.
Resolving or avoiding entropy buildup is a major theoretical challenge.
Modern cyclic mechanisms
Some models propose mechanisms that dilute or reset entropy between cycles, such as exponential expansion phases or dimensional transitions. Others posit that each cycle begins under new effective conditions.
These mechanisms remain speculative.
Relation to bounce cosmology
Cyclic models often incorporate a bounce cosmology at the transition between contraction and expansion. Not all bounce models are cyclic, but cyclic models typically require bounces.
The bounce replaces a singular transition.
Observational considerations
Cyclic cosmologies must reproduce observed features such as the cosmic microwave background spectrum and large-scale structure. Proposed signatures distinguishing cycles are subtle and currently unconfirmed.
Empirical discrimination remains difficult.
Time and causality
Cyclic models raise questions about temporal ordering and causality across cycles. Whether time is continuous through transitions or redefined each cycle depends on the framework.
Philosophical implications are significant.
Misconceptions
Cyclic universe models do not imply exact repetition of events. Cycles may differ in duration, structure, or physical parameters.
They also do not imply eternal recurrence in a deterministic sense.
Limits and uncertainty
Cyclic models rely on physics beyond well-tested regimes, particularly near bounce transitions. Their viability depends on unknown high-energy dynamics.
No cyclic model is currently confirmed.
Status
The cyclic universe is a serious but speculative alternative to singular cosmology. It motivates exploration of entropy, time, and cosmic boundary conditions.
Its significance lies in challenging assumptions about cosmic beginnings and endings.