Scale-invariant perturbations in ekpyrotic cosmologies without fine-tuning of initial conditions

Levy, Aaron M.; Ijjas, Anna; Steinhardt, Paul J.

doi:10.1103/physrevd.92.063524

Cited by 46 publications

(61 citation statements)

References 34 publications

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“…Refs. [14][15][16][17][18] and references therein). In fact, this is the motivation for having an exotic component in the pre-bounce phase, with an equation-of-state parameter w exotic ≡ P exotic /ρ exotic ≥ 1.…”

Section: Bounce Stabilitymentioning

confidence: 99%

“…Approaching the bounce from the pre-bounce side, the energy density of such an exotic component "grows to dominate all other forms of energy, including inhomogeneities, anisotropy and spatial curvature" (quote from Ref. [16]). For this reason, we have also considered [4,5] asymmetric versions of the degenerate-metric bounce with w pre-bounce = 1 and w post-bounce ∼ 1/3.…”

Section: Bounce Stabilitymentioning

confidence: 99%

“…The actual mechanism which generates the required scale-invariant spectrum of cosmological perturbations may depend on certain details of the pre-bounce phase (cf. Refs [16,18]…”

mentioning

confidence: 99%

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Nonsingular bouncing cosmology from general relativity: Scalar metric perturbations

Klinkhamer

Wang

2020

Phys. Rev. D

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We derive the equations of motion for scalar metric perturbations in a particular nonsingular bouncing cosmology, where the big bang singularity is replaced by a spacetime defect. The adiabatic perturbation solution is obtained for nonrelativistic hydrodynamic matter. We get the same result by working with conformal coordinates. This last method is also valid for vector and tensor metric perturbations, and selected results are presented. PACS numbers: 04.20.Cv, 98.80.Bp, 98.80.Jk Keywords: general relativity, big bang theory, mathematical and relativistic aspects of cosmology *

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Section: Bounce Stabilitymentioning

confidence: 99%

Section: Bounce Stabilitymentioning

confidence: 99%

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Nonsingular bouncing cosmology from general relativity: Scalar metric perturbations

Klinkhamer

Wang

2020

Phys. Rev. D

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“…For example, a new, especially simple version of ekpyrotic (cyclic) cosmology has been identified that fits all current observations, including nearly Gaussian fluctuations and small r [4][5][6][7]. Also, anamorphic bouncing cosmologies have been introduced that use yet different ways to smooth and flatten the universe during a contracting phase and generate a nearly scale-invariant spectrum of perturbations [8].…”

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confidence: 91%

Implications of Planck2015 for inflationary, ekpyrotic and anamorphic bouncing cosmologies

Ijjas

Steinhardt

2016

Class. Quantum Grav.

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The results from Planck2015, when combined with earlier observations from WMAP, ACT, SPT and other experiments, were the first observations to disfavor the "classic" inflationary paradigm. To satisfy the observational constraints, inflationary theorists have been forced to consider plateau-like inflaton potentials that introduce more parameters and more fine-tuning, problematic initial conditions, multiverse-unpredictability issues, and a new 'unlikeliness problem.' Some propose turning instead to a "postmodern" inflationary paradigm in which the cosmological properties in our observable universe are only locally valid and set randomly, with completely different properties (and perhaps even different physical laws) existing in most regions outside our horizon. By contrast, the new results are consistent with the simplest versions of ekpyrotic cyclic models in which the universe is smoothed and flattened during a period of slow contraction followed by a bounce, and another promising bouncing theory, anamorphic cosmology, has been proposed that can produce distinctive predictions.The Planck2015 [1] and Planck2013 [2] observations, combined with the results by the Wilkinson Microwave Anisotropy Probe (WMAP), Atacama Cosmology Telescope (ACT) and South Pole Telescope (SPT) teams have shown that the measured spatial curvature is small; the spectrum of primordial density fluctuations is nearly scale-invariant; there is a small spectral tilt, consistent with a simple dynamical mechanism that caused the smoothing and flattening; and the fluctuations are nearly gaussian. These features are all consistent with the simplest textbook inflationary models. At the same time, Planck2015 also confirmed that r, the ratio of the tensor perturbation amplitude to the scalar perturbation amplitude, is less than 0.1, a result that virtually eliminates all the simplest textbook inflationary models. The development is notable because, as emphasized by Ijjas et al. [3], it is the first time that the classic inflationary picture has been in conflict with observations.The results have led theorists to consider alternative plateau-like inflationary models whose parameters can be adjusted to reduce the expected value of r while retaining all the rest of the classic predictions. However, as we explain in this brief review, the remaining models raise new issues. They require more parameters, more tuning of parameters, more tuning of initial conditions, a worsened multiverse-unpredictability problem, and a new challenge that we call the inflationary 'unlikeliness problem. ' One response to these problems has been that they should be ignored. The classic inflationary picture should be abandoned in favor of an alternative 'postmodern' inflationary picture that allows enough flexibility to fit any combination of observations. The classic and postmodern inflationary pictures are so different that they ought to be viewed as distinct paradigms to be judged separately.A more promising response to Planck2015 has been to develop "bouncing" cosmologie...

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“…The occurrence of blueshift scalar perturbation is nonetheless a minor flaw of ekpyrotic theory which can be easily corrected. Hence, new ekpyrotic theory considers multiple scalar field and successfully generated a scalar spectrum which is scale invariant and slightly redshifted [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. Producing primordial gravitational waves sourced by the gauge field in the ekpyrotic scenario was also studied in [31].…”

Section: Jhep06(2018)041mentioning

confidence: 99%

No-go theorems for ekpyrosis from ten-dimensional supergravity

Uzawa

2018

J. High Energ. Phys.

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In this note we investigate whether the new ekpyrotic scenario can be embedded into ten-dimensional supergravity. We use that the scalar potential obtained from flux compactifications of type II supergravity with sources has a universal scaling with respect to the dilaton and the volume mode. Similar to the investigation of inflationary models, we find very strong constraints ruling out ekpyrosis from analysing the fast-roll conditions. We conclude that flux compactifications tend to provide potentials that are neither too flat and positive (inflation) nor too steep and negative (ekpyrosis).

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Scale-invariant perturbations in ekpyrotic cosmologies without fine-tuning of initial conditions

Cited by 46 publications

References 34 publications

Nonsingular bouncing cosmology from general relativity: Scalar metric perturbations

Nonsingular bouncing cosmology from general relativity: Scalar metric perturbations

Implications of Planck2015 for inflationary, ekpyrotic and anamorphic bouncing cosmologies

No-go theorems for ekpyrosis from ten-dimensional supergravity

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