Evolution of fluctuations during graceful exit in string cosmology

Kawai, Shinsuke; Soda, Jiro

doi:10.1016/s0370-2693(99)00736-4

Cited by 97 publications

(82 citation statements)

References 21 publications

(27 reference statements)

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“…Again the final spectra are found to be blue-tilted. We should also mention that the frequency shift s becomes negative for the Hubble rate which is larger than unity around the graceful exit [61]. In this case the small-scale modes show exponential instability as we pointed out in the dilaton-driven case.…”

Section: B Density Perturbationsmentioning

confidence: 51%

“…(3.16), in which case curvature perturbations are enhanced on super-Hubble scales during super-inflation (R k ∝ (−η E ) −1 ) due to the growth of the second term in eq. (3.15) [61]. We show in Fig.…”

Section: B Density Perturbationsmentioning

confidence: 80%

“…The only difference is an instability of small-scale fluctuation modes during the bounce (see also [61]) which leads to a further steepening of the spectrum. Our result implies that the growing mode of Φ during the contracting phase which is scale-invariant for 0 < p ≪ 1 is effectively uncoupled with the dominant constant mode of Φ in the expanding phase, a result obtained in the context of matching conditions in [13] (in the case of PBB cosmology) and in [15,16] for the Ekpyrotic scenario.…”

Section: Discussion and Open Issuesmentioning

confidence: 99%

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Construction of nonsingular pre-big-bang and ekpyrotic cosmologies and the resulting density perturbations

2002

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We consider the construction of nonsingular Pre-Big-Bang and Ekpyrotic type cosmological models realized by the addition to the action of specific higher-order terms stemming from quantum corrections. We study models involving general relativity coupled to a single scalar field with a potential motivated by the Ekpyrotic scenario. We find that the inclusion of the string loop and quantum correction terms in the string frame makes it possible to obtain solutions of the variational equations which are nonsingular and bouncing in the Einstein frame, even when a negative exponential potential is present, as is the case in the Ekpyrotic scenario. This allows us to discuss the evolution of cosmological perturbations without the need to invoke matching conditions between two Einstein Universes, one representing the contracting branch, the second the expanding branch. We analyze the spectra of perturbations produced during the bouncing phase and find that the spectrum of curvature fluctuations in the model proposed originally to implement the Ekpyrotic scenario has a large blue tilt (nR = 3). Except for instabilities introduced on small scales, the result agrees with what is obtained by imposing continuity of the induced metric and of the extrinsic curvature across a constant scalar field (up to k 2 corrections equal to the constant energy density) matching surface between the contracting and the expanding Einstein Universes. We also discuss nonsingular cosmological solutions obtained when a Gauss-Bonnet term with a coefficient suitably dependent on the scalar matter field is added to the action in the Einstein frame with a potential for the scalar field present. In this scenario, nonsingular solutions are found which start in an asymptotically flat state, undergo a period of super-exponential inflation and end with a graceful exit. The spectrum of fluctuations is also calculated in this case.

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Section: B Density Perturbationsmentioning

confidence: 51%

Section: B Density Perturbationsmentioning

confidence: 80%

Section: Discussion and Open Issuesmentioning

confidence: 99%

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Construction of nonsingular pre-big-bang and ekpyrotic cosmologies and the resulting density perturbations

2002

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“…In this paper, we will consider specific class of models where a scalar field couples with the Gauss-Bonnet term non-minimally. There have been a lot of studies on the implications of scalar coupled Gauss-Bonnet term in the context of inflationary scenario [8][9][10] 1 . In the context of dark energy model, Gauss-Bonnet term has also been considered [11].…”

Section: Introductionmentioning

confidence: 99%

Constraining scalar-Gauss-Bonnet inflation by reheating, unitarity, and Planck data

2017

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We revisit the inflationary dynamics in detail for theories with Gauss-Bonnet gravity coupled to scalar functions, in light of the Planck data. Considering the chaotic inflationary scenario, we constrain the parameters of two models involving inflaton-Gauss-Bonnet coupling by current Planck data. For non zero inflaton-Gauss-Bonnet coupling β, an inflationary analysis provides us a big cosmologically viable region in the space of (m, β), where m is the mass of inflaton. However, we study further on constraining β arising from reheating considerations and unitarity of tree-level amplitude involving we have studied the constraints on β arising from reheating considerations and unitarity of tree level amplitude involving 2 graviton → 2 graviton (hh → hh) scattering. Our analysis, particularly on reheating significantly reduces the parameter space of (m, β) for all models. he quadratic Gauss-Bonnet coupling parameter turns out to be more strongly constrained than that of the linear coupling. For the linear Gauss-Bonnet coupling function, we obtain β 10 3 , with the condition β(m/M P ) 2

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“…The solutions have super inflation phase where the Hubble parameter increases, which indicates the violation of the weak energy condition. Subsequently, cosmological perturbations in this background are investigated [8][9][10][11][12][13][14]. In the process, the so-called gradient instability is found for the first time in [9,10].…”

Section: Introductionmentioning

confidence: 99%

Comment on “Gauss-Bonnet inflation”

Hikmawan¹,

Soda²,

Suroso³

et al. 2016

Phys. Rev. D

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Recently, an interesting inflationary scenario, named Gauss-Bonnet inflation, is proposed by Kanti et al. [1,2]. In the model, there is no inflaton potential but the inflaton couples to the Guass-Bonnet term. In the case of quadratic coupling, they find inflation occurs with graceful exit. The scenario is attractive because of the natural set-up. However, we show there exists the gradient instability in the tensor perturbations in this inflationary model. We further prove the no-go theorem for the Gauss-Bonnet inflation without an inflaton potential.

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Evolution of fluctuations during graceful exit in string cosmology

Cited by 97 publications

References 21 publications

Construction of nonsingular pre-big-bang and ekpyrotic cosmologies and the resulting density perturbations

Construction of nonsingular pre-big-bang and ekpyrotic cosmologies and the resulting density perturbations

Constraining scalar-Gauss-Bonnet inflation by reheating, unitarity, and Planck data

Comment on “Gauss-Bonnet inflation”

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