The cyclic universe theory
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The Cyclic Universe Theory: An Overview
Introduction to the Cyclic Universe Theory
The cyclic universe theory proposes that the universe undergoes endless cycles of expansion and contraction, rather than having a single beginning and end. This theory addresses several cosmological issues that the traditional Big Bang model does not, such as the homogeneity, isotropy, and flatness problems.
Ekpyrotic and Cyclic Cosmology
Ekpyrotic Phase and Brane Collisions
The cyclic universe theory often incorporates the ekpyrotic scenario, where the universe undergoes ultra-slow contraction before a non-singular bounce leads to a new cycle of expansion. This phase is characterized by a high equation of state, resolving standard cosmological puzzles and generating a nearly scale-invariant spectrum of density perturbations . The big bang in this model is described as a collision of branes, suggesting that time does not begin with the big bang but continues through cycles.
Cosmic Evolution and M Theory
Incorporating concepts from M theory, the cyclic model suggests that the universe undergoes an endless sequence of cosmic epochs, each beginning with a "big bang" and ending with a "big crunch." The transition between these phases is facilitated by cosmic acceleration, which ensures the flat and vacuous initial conditions required for the next cycle. This model also addresses the entropy problem by restoring the universe to a vacuum state before each big crunch, allowing the cycle to repeat indefinitely.
Variations and Extensions of the Cyclic Universe
Cyclic Multiverses and Varying Constants
Some models extend the cyclic universe concept to multiverses, where individual universes within the multiverse undergo their own cycles. These models often involve varying fundamental constants, such as the gravitational constant, to regularize singularities and maintain non-singular bounces. This approach allows for the possibility of universes exchanging properties at points of maximum expansion, a concept also explored in quantum cosmology.
Modified Gravity and Higher Order Curvature Terms
Another variation involves modified gravity theories, where higher order curvature terms are added to the Lagrangian density for gravity. This approach does not require new ingredients like extra dimensions or phantom energy. Instead, it focuses on the general properties of each phase of the cycle, including the cosmological bounce and the low curvature phase. Analytical and numerical solutions confirm the viability of cyclic solutions within this framework.
Dynamic Properties and Observables
Model-Independent Properties
Studies on the dynamic properties of cyclic cosmologies aim to understand the universal characteristics of these models. By treating the cyclic universe as a dynamic system, researchers can apply theorems from dynamic systems analysis to ensure the existence of cyclic solutions under certain conditions. This approach helps in identifying the general requirements for cyclic solutions across different modified gravity theories.
Causal Set Theory
In causal set theory, cycles of cosmic expansion and collapse are modeled using causal sets with "breaks" and "posts." This framework allows for the definition of cyclic dynamics as random walks on a novel tree, providing a new perspective on the observables in these models. The physical interpretation of these observables offers insights into the cyclic nature of the universe.
Conclusion
The cyclic universe theory presents a compelling alternative to the traditional Big Bang model, addressing several long-standing cosmological issues. By incorporating concepts from ekpyrotic cosmology, M theory, and modified gravity, this theory offers a robust framework for understanding the universe's endless cycles of expansion and contraction. As research continues, the cyclic universe theory may provide deeper insights into the fundamental nature of our cosmos.
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