Bouncing universes with varying constants

Barrow, John D.; Kimberly, Dagny; Magueijo, João

doi:10.1088/0264-9381/21/18/001

Cited by 109 publications

(127 citation statements)

References 35 publications

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“…Other studies pertaining to the cylic universe, which are not reviewed here, include: 5D dynamics of general braneworld models [174], past-shrinking cycles that spend more time in an entropy conserving Hagedorn phase [175], a cyclic magnetic universe [176,177], phantom accretion onto black holes [178], deformed Hořava-Lifshitz gravity [179], a string-inspired model via a scalartachyon coupling and a contribution from curvature in a closed universe [180], cosmological hysteresis [181], Finsler-like gravity theories constructed on tangent bundles to Lorentz manifolds [182], a combination of cyclic and inflationary phases with quintessence [183] and a cyclic model with a chameleon field [184], among others [185][186][187].…”

Section: B Ekpyrotic and Cyclic Scenariosmentioning

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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Section: B Ekpyrotic and Cyclic Scenariosmentioning

confidence: 99%

A critical review of classical bouncing cosmologies

2015

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“…Two questions have recurred often in theoretical cosmology [1][2][3][4][5][6][7][8][9]: 1) is the Universe eternal or did it have a beginning at some definite time in the past?, and 2) is it possible to make Universes with one or more "bounces" where the scale factor crunches and then bangs? 1 The answers to these two questions are deeply intertwined with the subject matter of the singularity theorems of Penrose and Hawking (discussed comprehensively in [10]).…”

Section: Jhep02(2014)029mentioning

confidence: 99%

A simple harmonic universe

Graham

Horn

Kachru

et al. 2014

J. High Energ. Phys.

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We explore simple but novel bouncing solutions of general relativity that avoid singularities. These solutions require curvature k = +1, and are supported by a negative cosmological term and matter with −1 < w < −1/3. In the case of moderate bounces (where the ratio of the maximal scale factor a + to the minimal scale factor a − is O(1)), the solutions are shown to be classically stable and cycle through an infinite set of bounces. For more extreme cases with large a + /a − , the solutions can still oscillate many times before classical instabilities take them out of the regime of validity of our approximations. In this regime, quantum particle production also leads eventually to a departure from the realm of validity of semiclassical general relativity, likely yielding a singular crunch. We briefly discuss possible applications of these models to realistic cosmology.

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“…From Equations (78) and (79) it follows that ω = −3/2 is exactly the border between a standard scalar field and a ghost (negative kinetic energy) which is required in order to have an expansion minimum in varying constant cosmologies [45] and to mimic the phantom field [46] which violates the weak energy conditionρ +p < 0. This is easily seen after calculating the energy-momentum tensor and expressing the energy density and pressure in terms of scalar field, i.e.,…”

Section: Relation To Brans-dicke and Low-energy-effective Superstringmentioning

confidence: 99%

Conformal Relativity versus Brans–Dicke and Superstring Theories

Blaschke

Dąbrowski

2012

Entropy

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Abstract:We show how conformal relativity is related to Brans-Dicke theory and to low-energy-effective superstring theory. Conformal relativity or the Hoyle-Narlikar theory is invariant with respect to conformal transformations of the metric. We show that the conformal relativity action is equivalent to the transformed Brans-Dicke action for ω = −3/2 (which is the border between standard scalar field and ghost) in contrast to the reduced (graviton-dilaton) low-energy-effective superstring action which corresponds to the Brans-Dicke action with ω = −1. We show that like in ekpyrotic/cyclic models, the transition through the singularity in conformal cosmology in the string frame takes place in the weak coupling regime. We also find interesting self-duality and duality relations for the graviton-dilaton actions.

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Bouncing universes with varying constants

Cited by 109 publications

References 35 publications

A critical review of classical bouncing cosmologies

A critical review of classical bouncing cosmologies

A simple harmonic universe

Conformal Relativity versus Brans–Dicke and Superstring Theories

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