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TOPIC: Cyclic Universe
Blobrana


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RE: Cyclic Universe
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Theorists Propose a New Method to Probe the Beginning of the Universe

The most widely accepted theoretical scenario for the beginning of the universe is inflation, which predicts that the universe expanded at an exponential rate in the first fleeting fraction of a second. However a number of alternative scenarios have been suggested, some predicting a Big Crunch preceding the Big Bang. The trick is to find measurements that can distinguish between these scenarios.
One promising source of information about the universe's beginning is the cosmic microwave background (CMB) - the remnant glow of the Big Bang that pervades all of space. This glow appears smooth and uniform at first, but upon closer inspection varies by small amounts. Those variations come from quantum fluctuations present at the birth of the universe that have been stretched as the universe expanded.
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Blobrana


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Penrose Conformal Cyclic Cosmology
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Title: Possible Duality of CBE and Penrose CCC Cyclic Cosmologies
Author: Paul H. Frampton

In a cyclic entropy model in which the extroverse is jettisoned at turnaround with a Come Back Empty assumption, we address matching of the contraction scale factor f(t_T)a(t) to the expansion scale factor a(t), where f(t_T) is the ratio at turnaround of the introverse to extroverse radii. Such matching is necessary for infinite cyclicity and fixes the CBE period at ~2.6 Ty. We then compare such a CBE model with alternative cyclic cosmologies including Penrose Conformal Cyclic Cosmology of period 10100 y, and speculate that the CBE model may be related to the CCC model by a highly nontrivial isomorphism.

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Blobrana


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The End
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Title: 'The End'
Author: Nemanja Kaloper, Antonio Padilla

Recently we proposed a mechanism for sequestering the Standard Model vacuum energy that predicts that the universe will collapse. Here we present a simple mechanism for bringing about this collapse, employing a scalar field whose potential is linear and becomes negative, providing the negative energy density required to end the expansion. The slope of the potential is chosen to allow for the expansion to last until the current Hubble time, about 1010 years, to accommodate our universe. Crucially, this choice is technically natural due to a shift symmetry. Moreover, vacuum energy sequestering selects radiatively stable initial conditions for the collapse, which guarantee that immediately before the turnaround the universe is dominated by the linear potential which drives an epoch of accelerated expansion for at least an efold. Thus a single, technically natural choice for the slope ensures that the collapse is imminent and is preceded by the current stage of cosmic acceleration, giving a new answer to the 'Why Now?' problem.

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Blobrana


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RE: Cyclic Universe
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Title: Cyclic universe from a new chameleon scalar field
Author: Changjun Gao, Youjun Lu, You-Gen Shen

We explore a cyclic universe by introducing a new chameleon scalar field. In the original version of chameleon scalar field, the mass of the chameleon scalar depends on the environment, specifically on the ambient matter density. But in this new version, the ambient energy density determines not its mass but its kinetic energy which is achieved by the Lagrange multiplier field. We find the new chameleon scalar is dominant both in the very early universe and in the far future of the universe such that a cyclic universe is found. In this model of universe, there are infinite cycles of expansion and contraction. Different from the inflationary universe, the corresponding cosmic space-time is geometrically complete and quantum stable. But similar to the Cyclic Model, the flatness problem, the horizon problem and the large scale structure of the universe can be explained in this cyclic universe.

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Blobrana


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Title: Past and future of some universes
Authors: David Polarski (Lab. Charles Coulomb, Universite Montpellier 2)

We consider a class of toy models where a spatially flat universe is filled with a perfect fluid. The dynamics is found exactly for all these models. In one family, the perfect fluid is of the phantom type and we find that the universe is first contracting and then expanding while the dynamics is always accelerated. In a second family, the universe is first in an accelerated expansion stage, then in a decelerated expansion stage until it reaches a turning point after which it contracts in a decelerated way (increasing contraction rate) followed by another accelerated stage (decreasing contraction rate). We also consider the possibility to embed this perfect fluid in a realistic cosmology. The first family cannot be viable in a conventional big bang universe and requires a rebound in the very early universe. The second family is viable in the range 0<1+w_{DE,0}\lesssim 0.09 for a spatially closed universe with a curvature satisfying current bounds. Though many of the models in this family cannot be distinguished today from a universe dominated by a cosmological constant, the present accelerated expansion is transient and these universes will reach a turning point in the future before entering a contraction phase.

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Blobrana


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Title: Was There a Beginning?
Authors: Leonard Susskind

In this note I respond to Vilenkin's claim that there must have been a beginning.

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Blobrana


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Title: Did the universe have a beginning?
Authors: Audrey Mithani, Alexander Vilenkin

We discuss three candidate scenarios which seem to allow the possibility that the universe could have existed forever with no initial singularity: eternal inflation, cyclic evolution, and the emergent universe. The first two of these scenarios are geodesically incomplete to the past, and thus cannot describe a universe without a beginning. The third, although it is stable with respect to classical perturbations, can collapse quantum mechanically, and therefore cannot have an eternal past.

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Blobrana


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Title: Cosmological Hysteresis and the Cyclic Universe
Authors: Varun Sahni, Aleksey Toporensky

A Universe filled with a homogeneous scalar field exhibits 'Cosmological hysteresis'. Cosmological hysteresis is caused by the asymmetry in the equation of state during expansion and contraction. This asymmetry results in the formation of a hysteresis loop: \oint pdV, whose value can be non-vanishing during each oscillatory cycle. For flat potentials, a negative value of the hysteresis loop leads to the increase in amplitude of consecutive cycles and to a universe with older and larger successive cycles. Such a universe appears to possess an arrow of time even though entropy production is absent and all of the equations respect time-reversal symmetry ! Cosmological hysteresis appears to be widespread and exists for a large class of scalar field potentials and mechanisms for making the universe bounce. For steep potentials, the value of the hysteresis loop can be positive as well as negative. The expansion factor in this case displays quasi-periodic behaviour in which successive cycles can be both larger as well as smaller than previous ones. This quasi-regular pattern resembles the phenomenon of BEATS displayed by acoustic systems. Remarkably, the expression relating the increase/decrease in oscillatory cycles to the quantum of hysteresis appears to be model independent. The cyclic scenario is extended to spatially anisotropic models and it is shown that the anisotropy density decreases during successive cycles if the hysteresis loop is negative.

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Ed ~ Indeed, time is probably independent to entropy, and possibly derives from the asymmetric boundary conditions. 



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Blobrana


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Title: Oscillating universe in the DGP braneworld
Authors: Kaituo Zhang, Puxun Wu, Hongwei Yu

With a method in which the Friedmann equation is written in a form such that evolution of the scale factor can be treated as that of a particle in a "potential", we classify all possible cosmic evolutions in the DGP braneworld scenario with the dark radiation term retained. By assuming that the energy component is pressureless matter, radiation or vacuum energy, respectively, we find that in the matter or vacuum energy dominated case, the scale factor has a minimum value a_0. In the matter dominated case, the big bang singularity can be avoided in some special circumstances, and there may exist an oscillating universe or a bouncing one. If the cosmic scale factor is in the oscillating region initially, the universe may undergo an oscillation. After a number of oscillations, it may evolve to the bounce point through quantum tunnelling and then expand. However, if the universe contracts initially from an infinite scale, it can turn around and then expand forever. In the vacuum energy dominated case, there exists a stable Einstein static state to avoid the big bang singularity. However, in certain circumstances in the matter or vacuum energy dominated case, a new kind of singularity may occur at a_0 as a result of the discontinuity of the scale factor. In the radiation dominated case, the universe may originate from the big bang singularity, but a bouncing universe which avoids this singularity is also possible.

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Blobrana


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Big-bounce cosmology
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Title: Big-bounce cosmology with spinor-torsion coupling
Authors: Nikodem J. Poplawski

The Einstein-Cartan-Sciama-Kibble theory of gravity removes the constraint of general relativity that the affine connection be symmetric by regarding its antisymmetric part, the torsion tensor, as a dynamical variable. The minimal coupling between the torsion tensor and Dirac spinors leads to gravitational repulsion in fermionic matter at extremely high densities even without approximating it as a spin fluid. We show that such a repulsion replaces the unphysical big-bang singularity with a nonsingular big bounce that follows a contracting phase of the Universe. This scenario also naturally explains why the Universe today appears spatially flat, homogeneous and isotropic.

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