Closed universe theory
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Closed Universe Theory: Insights and Implications
Introduction to Closed Universe Theory
The concept of a closed universe has intrigued cosmologists for decades. A closed universe is one where space is finite and unbounded, curving back on itself. This theory has significant implications for the origin, evolution, and ultimate fate of the universe.
Low-Density Closed Universe
Recent studies suggest that a closed universe can exist even with a low density of nonrelativistic matter. In certain scalar field theories, matter with an equation of state ( p = -\rho/3 ) can lead to a closed universe where the energy density decreases with the scale factor ( a ) of the universe . This model remains viable and could be tested with upcoming observations.
Big Bounce and Closed Universe Dynamics
The Einstein-Cartan theory of gravity introduces the concept of a "big bounce," replacing the traditional Big Bang with a nonsingular transition from contraction to expansion. This theory posits that a closed universe can only exist if the product of the scale factor and temperature exceeds a specific threshold. During inflation, this product must increase further to allow the universe to reach dark-energy acceleration .
Planck Evidence and Cosmological Crisis
The Planck Legacy 2018 data has provided compelling evidence for a closed universe, showing a preference for positive curvature at more than the 99% confidence level. This positive curvature explains the enhanced lensing amplitude observed in the cosmic microwave background (CMB) power spectra. The assumption of a flat universe could mask a cosmological crisis, as it leads to inconsistencies in the observed properties of the universe .
Thermodynamics and the Fate of a Closed Universe
A closed universe that began with low entropy in an unstable de Sitter phase will not experience a bounce during contraction. Instead, it will undergo a "final crunch," where the universe contracts irreversibly, leading to a final reheating phase. This process suggests that a closed universe lives only once, starting with a whimper and ending with a bang .
Pulsating Universe Model
Another model proposes a closed, pulsating universe that begins and ends with point singularities. This model addresses the mathematical difficulties of singularities by viewing the universe as a dynamically stabilized system that rebounds at a minimum expansion parameter value .
Gravitational Waves and Topology
Gravitational wave metrics in closed universes yield solutions to Einstein's vacuum field equations, resulting in universes with either three-sphere topology ( S^3 ) or wormhole (hypertorus) topology ( S^1 \times S^2 ). These solutions provide new insights into the structure and dynamics of closed universes .
String Theory and Closed Universe
String theory offers a model of a closed, inhomogeneous universe that expands and recollapses, avoiding cosmic censorship violations. This model suggests that the universe recollapses just before a naked singularity forms, maintaining consistency with cosmic censorship principles .
Quantum Gravity in Closed Universes
In the context of quantum gravity, particularly in 2 + 1 dimensions, a closed universe simplifies the complexities of general relativity. Despite the lack of local degrees of freedom, many conceptual challenges of quantizing gravity remain, providing a rich area for further research .
Conclusion
The closed universe theory presents a fascinating and complex picture of our cosmos. From low-density models and big bounce dynamics to evidence from the Planck data and string theory implications, the concept of a closed universe continues to evolve. Future observations and theoretical advancements will be crucial in resolving the current ambiguities and deepening our understanding of the universe's true nature.
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