Cyclic theory of the universe
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Cyclic Theory of the Universe: A Comprehensive Overview
Introduction to Cyclic Universe Models
The cyclic theory of the universe proposes that the cosmos undergoes endless cycles of expansion and contraction, avoiding the singularities associated with the traditional Big Bang model. This theory addresses several cosmological issues and offers an alternative to the inflationary model.
Ekpyrotic and Cyclic Cosmology
Ekpyrotic Phase and Brane Collisions
In ekpyrotic and cyclic cosmologies, the universe's cycles are driven by the collision of branes, which are higher-dimensional objects. The big bang is not the beginning of time but a transition phase following a period of slow contraction known as the ekpyrotic phase. This phase resolves standard cosmological puzzles and generates a nearly scale-invariant spectrum of cosmological perturbations with significant non-Gaussian components . The dark energy observed today is reinterpreted as a small attractive force between our brane and a parallel one, eventually leading to a new ekpyrotic phase and subsequent brane collision .
Classical Bounce and Periodic Oscillations
Combining intervals of ekpyrotic contraction with a non-singular classical bounce leads to a novel cyclic theory where the Hubble parameter, energy density, and temperature oscillate periodically. However, the scale factor grows exponentially from one cycle to the next. This model resolves homogeneity, isotropy, flatness, and monopole problems while generating a nearly scale-invariant spectrum of density perturbations .
Cosmic Evolution and Multiverse Implications
Big Bang to Big Crunch Cycles
A cyclic model based on the ekpyrotic scenario and M theory suggests that the universe undergoes endless sequences of cosmic epochs, starting with a big bang and ending with a big crunch. The expansion phase includes periods of radiation and matter domination followed by cosmic acceleration, which is crucial for establishing the flat and vacuous initial conditions required for ekpyrosis. This acceleration phase removes entropy, black holes, and other debris, ensuring the cycle can repeat .
Cyclic Multiverses
The concept of cyclic multiverses extends the idea of regularizing singularities through varying fundamental constants. This model includes oscillating and non-singular mass density and pressure, regulated by a varying gravitational constant. It proposes a multiverse containing cyclic individual universes with either growing or decreasing entropy, though the net entropy remains constant. This framework allows for the possibility of exchanging universes at the point of maximum expansion .
Observables and Perturbations in Cyclic Cosmology
Causal Set Theory and Cyclic Dynamics
In causal set theory, cycles of cosmic expansion and collapse are modeled by causal sets with breaks and posts. Cyclic dynamics are characterized by random walks on a novel tree, offering a new framework for defining these dynamics. Observables in these models are interpreted as statements about unlabelled stems with a single maximal element .
Processing Cosmological Perturbations
The evolution of cosmological fluctuations from one cycle to the next leads to a reddening of the spectrum, opening new ways to generate a scale-invariant spectrum of curvature perturbations. However, the large increase in the amplitude of fluctuations quickly leads to a breakdown of the linear theory, suggesting that a cyclic universe cannot be truly cyclic when linearized cosmological perturbations are included .
Modified Gravity and Cyclic Models
Higher Order Curvature Terms
A general model of an eternal cyclic universe can be proposed by incorporating quantum gravity corrections through higher-order curvature terms in the Lagrangian density for gravity. This model does not require new ingredients like extra dimensions or phantom energy. The evolution of the universe in this framework includes phases such as cosmological bounce, low curvature phase, destruction of bounded systems, and contracting phase .
Dynamic Properties and Modified Gravity Theories
Cyclic cosmologies can be treated as dynamic systems, and the existence of cyclic solutions can be ensured under certain conditions on the field equations. This approach can be applied to various frameworks of modified gravity theories, such as $f(R)$ gravity, dynamic dark energy, and $f(T)$ gravity, providing examples of cyclic cosmologies and discussing their basic properties .
Conclusion
The cyclic theory of the universe offers a compelling alternative to the traditional Big Bang model, addressing several cosmological issues and providing a framework for understanding the universe's perpetual cycles. By incorporating concepts from ekpyrotic scenarios, M theory, and modified gravity, cyclic models present a rich and diverse landscape for exploring the cosmos's dynamic nature.
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