Decoherence problem in an ekpyrotic phase

Tseng, Chien-Yao

doi:10.1103/physrevd.87.023518

Cited by 9 publications

(12 citation statements)

References 32 publications

(55 reference statements)

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“…During the ekpyrotic phase, curvature perturbations at linear order are not amplified [12,35], nor are they sourced by the entropy perturbations. However, at second order curvature perturbations can be sourced by the entropy fluctuations [13].…”

Section: (No) Non-gaussianity During the Ekpyrotic Phasementioning

confidence: 99%

Ekpyrotic perturbations with small non-Gaussian corrections

2014

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The entropic mechanism for producing nearly scale-invariant density perturbations in a contracting ekpyrotic universe relies on having an unstable scalar potential. Here we develop a variant of this mechanism (recently proposed by Qiu, Gao and Saridakis, and by Li), in which there exists a nontrivial coupling between adiabatic and entropic fields, and where an unstable potential is not required. In the model nearly scale-invariant entropy perturbations are generated first. Remarkably, we find that the bispectrum of these perturbations vanishes, with the values of the non-Gaussianity parameters of local, equilateral and orthogonal type all exactly zero. Subsequently, the entropy perturbations can be converted into curvature perturbations by a variety of mechanisms. The bispectrum of the curvature perturbations depends on the nonlinearity of the conversion process and is thus more model-dependent-however, for an efficient conversion process the final bispectrum remains small. The only distinguishing feature compared to single-field slow-roll inflationary models is that the ekpyrotic phase does not generate significant primordial gravitational waves. Thus the present model provides a perfect match to current data from the PLANCK satellite.

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Section: (No) Non-gaussianity During the Ekpyrotic Phasementioning

confidence: 99%

Ekpyrotic perturbations with small non-Gaussian corrections

2014

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“…A distinctive feature of all currently known ekpyrotic models is that, during the ekpyrotic contraction phase, gravitational waves are not amplified. More specifically, the gravitational waves have a blue spectrum, but their quantum state does not become squeezed; hence, they cannot be given a classical interpretation [9,10]. During the expanding phase, the (classical) scalar curvature perturbations provide a source for gravitational waves at second order in perturbation theory, leading to a small-amplitude gravitational wave background [11].…”

Section: Introductionmentioning

confidence: 99%

“…It has been shown [9,10] that, if there is only a single field in the contracting phase, the (adiabatic) perturbations are not amplified and cannot be the seed of structure in the post-bang universe. The currently best-understood way around this problem is the entropic mechanism, where pre-bang isocurvature fluctuations are generated by adding a second ekpyrotic field, φ 2 [14][15][16][17].…”

Section: Introductionmentioning

confidence: 99%

General mechanism for producing scale-invariant perturbations and small non-Gaussianity in ekpyrotic models

2014

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We explore a new type of entropic mechanism for generating density perturbations in a contracting phase in which there are two scalar fields, but only one has a steep negative potential. This first field dominates the energy density and is the source of the ekpyrotic equation of state. The second field has a negligible potential, but its kinetic energy density is coupled to the first field with a non-linear sigma-model type interaction. We show that for any ekpyrotic equation of state it is possible to choose the potential and the kinetic coupling such that exactly scale-invariant (or nearly scale-invariant) entropy perturbations are produced. The corresponding background solutions are stable, and the bispectrum of the entropy perturbations vanishes as no non-Gaussianity is produced during the ekpyrotic phase. Hence, the only contribution to non-Gaussianity comes from the nonlinearity of the conversion process during which entropic perturbations are turned into adiabatic ones, resulting in a local non-Gaussianity parameter fnl ∼ 5.

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“…However, with regard to the generation of primordial curvature perturbations, the presence of a negative potential during the bounce suggests a small modification of existing scenarios. During the ekpyrotic phase, curvature perturbations are not amplified [30][31][32][33]. However, in the presence of a second scalar field, nearly scale-invariant entropy perturbations may be generated [34][35][36][37].…”

Section: Discussionmentioning

confidence: 99%

Nonsingular bouncing cosmology: Consistency of the effective description

2016

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We explicitly confirm that spatially flat non-singular bouncing cosmologies make sense as effective theories. The presence of a non-singular bounce in a spatially flat universe implies a temporary violation of the null energy condition, which can be achieved through a phase of ghost condensation. We calculate the scale of strong coupling and demonstrate that the ghost-condensate bounce remains trustworthy throughout, and that all perturbation modes within the regime of validity of the effective description remain under control. For this purpose we require the perturbed action up to third order in perturbations, which we calculate in both flat and co-moving gauge-since these two gauges allow us to highlight different physical aspects. Our conclusion is that there exist healthy descriptions of non-singular bouncing cosmologies providing a viable resolution of the big-bang singularities in cosmological models. Our results also suggest a variant of ekpyrotic cosmology, in which entropy perturbations are generated during the contracting phase, but are only converted into curvature perturbations after the bounce.

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Decoherence problem in an ekpyrotic phase

Cited by 9 publications

References 32 publications

Ekpyrotic perturbations with small non-Gaussian corrections

Ekpyrotic perturbations with small non-Gaussian corrections

General mechanism for producing scale-invariant perturbations and small non-Gaussianity in ekpyrotic models

Nonsingular bouncing cosmology: Consistency of the effective description

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