Cosmological theory without singularities

Brandenberger, Robert H.; Mukhanov, Viatcheslav; Sornborger, Andrew

doi:10.1103/physrevd.48.1629

Cited by 230 publications

(298 citation statements)

References 30 publications

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“…From the form of the deformed Hubble rate and scale factor in Eq. (12), it can be seen that the values of the parameter α crucially determine the finite time singularity structure of the theory. Particularly, according to the values of α, if α > 1, we now demonstrate that a Type I [55], or so called Big Rip singularity can occur at t s , since both the Hubble rate and the scale factor diverge at t = t s .…”

Section: The Qualitative Features Of the Deformed Matter Bounce Smentioning

confidence: 99%

Deformed matter bounce with dark energy epoch

Odintsov

Oikonomou

2016

Phys. Rev. D

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We extend the Loop Quantum Cosmology matter bounce scenario in order to include a dark energy era, which ends abruptly at a Rip singularity where the scale factor and the Hubble rate diverge. In the "deformed matter bounce scenario", the Universe is contracting from an initial non-causal matter dominated era until it reaches a minimal radius. After that it expands in a decelerating way, until at late times, where it expands in an accelerating way, thus the model is described by a dark energy era that follows the matter dominated era. Depending on the choice of the free parameters of the model, the dark energy era is quintessential like which follows the matter domination era, and eventually it crosses the phantom divide line and becomes phantom. At the end of the dark energy era, a Rip singularity exists, where the scale factor and Hubble rate diverge, however the physical system cannot reach the singularity, since the effective energy density and pressure become complex. This indicates two things, firstly that the ordinary Loop Quantum Cosmology matter bounce evolution stops, thus ending the infinite repetition of the ordinary matter bounce scenario. Secondly, the fact that both the pressure and the density become complex indicates probably that the description of the cosmic evolution within the theoretical context of Loop Quantum Cosmology, ceases to describe the physics of the system and possibly a more fundamental theory of quantum gravity is needed near the would be Rip singularity. We describe the qualitative features of the model and we also investigate how this cosmology could be realized by a viscous fluid in the context of Loop Quantum Cosmology. In addition to this, we show how this deformed model can be realized by a canonical scalar field filled Universe, in the context of Loop Quantum Cosmology. Finally, we demonstrate how the model can be generated by a vacuum F (R) gravity.

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Section: The Qualitative Features Of the Deformed Matter Bounce Smentioning

confidence: 99%

Deformed matter bounce with dark energy epoch

Odintsov

Oikonomou

2016

Phys. Rev. D

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“…Historically, the first models included a scalar mode in addition to the metric (a tensor) of General Relativity; since these scalar tensor theories are equivalent to ordinary GR with a modified matter content, we do not consider them separately. More complicated theories have been suggested, where terms are added to render the cosmological evolution explicitly singularity-free [35]. Such models, which are appropriate to describe a bouncing phase, can be expressed as…”

Section: Modified Gravitymentioning

confidence: 99%

“…A seemingly viable alternative, which also provides a GR-compatible solution to the singularity problem, relies on a nonsingular bouncing cosmology [35,36], whereby an initially contracting phase connects with the currently expanding one through some minimal scale factor (and hence a vanishing Hubble rate). These models have a history that predates inflationary solutions by many decades, as they were proposed shortly after the first observations of the expansion [26,37] by Tolman [38] and Lemaître [39,40] (see also [41] for a more modern viewpoint concerning Tolman's cyclic approach): at this time, the expansion appeared to imply that Einstein's theory of gravity was doomed to fail, as the scale factor reaches infinitesimally small values, such that the Universe emerges from a primordial singularity.…”

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confidence: 99%

A critical review of classical bouncing cosmologies

2015

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Given the proliferation of bouncing models in recent years, we gather and critically assess these proposals in a comprehensive review. The PLANCK data shows an unmistakably red, quasi scaleinvariant, purely adiabatic primordial power spectrum and no primary non-Gaussianities. While these observations are consistent with inflationary predictions, bouncing cosmologies aspire to provide an alternative framework to explain them. Such models face many problems, both of the purely theoretical kind, such as the necessity of violating the NEC and instabilities, and at the cosmological application level, as exemplified by the possible presence of shear. We provide a pedagogical introduction to these problems and also assess the fitness of different proposals with respect to the data. For example, many models predict a slightly blue spectrum and must be fine-tuned to generate a red spectral index; as a side effect, large non-Gaussianities often result.We highlight several promising attempts to violate the NEC without introducing dangerous instabilities at the classical and/or quantum level. If primordial gravitational waves are observed, certain bouncing cosmologies, such as the cyclic scenario, are in trouble, while others remain valid. We conclude that, while most bouncing cosmologies are far from providing an alternative to the inflationary paradigm, a handful of interesting proposals have surfaced, which warrant further research. The constraints and lessons learned as laid out in this review might guide future research.

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“…This is the main difference from other schemes that constrain curvature invariants and metric related functions. Indeed, in the unimodular gravity proposal, for instance, the determinant of the metric is non-dynamical and the cosmological constant is shown to be an integration constant [25,26,27]; in the limiting curvature proposal, the value of the curvature invariants are bound from above so to avoid singularities [28,29]. Another, curvature-type principle arises in the context of the field theory of closed strings, where minimal area metrics are proposed to solve the problem of generating all Riemann surfaces [30].…”

Section: Discussion and Outlookmentioning

confidence: 99%

A Curvature Principle for the interaction between universes

Bertolami¹

2008

Gen Relativ Gravit

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We propose a Curvature Principle to describe the dynamics of interacting universes in a multi-universe scenario and show, in the context of a simplified model, how interaction drives the cosmological constant of one of the universes toward a vanishingly small value. We also conjecture on how the proposed Curvature Principle suggests a solution for the entropy paradox of a universe where the cosmological constant vanishes. * Essay selected for an honorable mention by the Gravity Research

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Cosmological theory without singularities

Cited by 230 publications

References 30 publications

Deformed matter bounce with dark energy epoch

Deformed matter bounce with dark energy epoch

A critical review of classical bouncing cosmologies

A Curvature Principle for the interaction between universes

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