Bounce cosmology
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Bounce cosmology | |
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| Type | Cosmological framework |
| Field | Cosmology; Theoretical physics; Quantum gravity |
| Core idea | The universe undergoes a contraction followed by expansion |
| Assumptions | Classical singularities are replaced by non-singular dynamics |
| Status | Theoretically developed; empirically unconfirmed |
| Related | Big Bang; Quantum cosmology; Initial singularity; Early universe |
Bounce cosmology is a class of cosmological models in which the universe does not begin with a singular origin but instead transitions from a prior contracting phase into the current expanding phase. The transition point is referred to as a “bounce,” replacing the classical Big Bang singularity.
These models aim to address limitations of singular cosmology.
Definition
In bounce cosmology, cosmic expansion is preceded by contraction. At high density or curvature, new physical effects intervene, preventing divergence and producing a smooth or controlled transition to expansion.
Time extends through the bounce rather than terminating at a boundary.
Motivation
Classical cosmology predicts an initial singularity where physical laws break down. Bounce models seek to preserve predictability by replacing singular behavior with finite, well-defined dynamics.
They also offer alternative explanations for early-universe conditions.
Types of bounce models
Several distinct mechanisms produce bounces, including:
Quantum-gravity-induced bounces
Effective modifications to gravitational dynamics
Matter fields with unusual equations of state
These approaches differ in assumptions and mathematical structure.
Relation to quantum gravity
Many bounce scenarios arise from attempts to incorporate quantum effects into cosmology. Quantum corrections can generate repulsive behavior at extreme densities, counteracting gravitational collapse.
Such effects are model-dependent.
Cosmological perturbations
Bounce models must account for observed primordial fluctuations. Some scenarios generate scale-invariant perturbations during contraction rather than inflation.
Matching observational data is a key challenge.
Arrow of time
A bounce raises questions about temporal direction. Some models posit a single arrow of time extending through the bounce, while others treat the bounce as a symmetry point.
The treatment of entropy is central.
Comparison with inflation
Bounce cosmologies are often presented as alternatives to inflationary cosmology. While inflation explains observed features through rapid expansion, bounce models rely on pre-expansion dynamics.
Each framework faces unresolved issues.
Singularities and avoidance
Not all bounce models successfully eliminate singular behavior. Some shift singularities to different variables or require fine-tuned conditions.
True singularity resolution remains uncertain.
Misconceptions
Bounce cosmology does not imply cyclic repetition by default. Some models feature a single bounce rather than repeated cycles.
It also does not imply eternal past time.
Limits and uncertainty
Bounce models rely on physics beyond well-tested regimes. Their predictions depend on speculative assumptions about high-energy behavior.
Observational discrimination from inflation remains difficult.
Status
Bounce cosmology is an active area of theoretical research exploring alternatives to singular cosmic origins. While promising conceptually, it lacks direct empirical support.
Its importance lies in probing the limits of classical cosmology.