Viable scalar spectral tilt and tensor-to-scalar ratio in near-matter bounces

Raveendran, Rathul Nath; Sriramkumar, L.

doi:10.1103/physrevd.100.083523

Cited by 17 publications

(12 citation statements)

References 36 publications

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“…To induce the local effects of the singularity, we introduce a regulating Gaussian factor within the expression of a 2 (t) in Eq. (22). Such regulating factor actually controls when the singularity becomes effective.…”

Section: Realization Of a Bounce With A Type-iv Singularity That Loca...mentioning

confidence: 99%

Bounce universe with finite-time singularity

Odintsov¹,

Paul²

2022

Preprint

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This work explains how the presence of a Type-IV singularity (a mild singularity) can influence the dynamics of a bouncing universe. In particular, we examine bounce cosmology that appears with a Type-IV singularity in the context of a ghost free Gauss-Bonnet theory of gravity. Depending on the time of occurrence of the Type-IV singularity, three different cases may arise -when the singularity occurs before the bounce or after the bounce or at the instant of the bounce respectively. However in all of these cases, we find that in the case when the singularity "globally" affects the spacetime, the scalar power spectrum becomes red tilted and the tensor-to-scalar ratio is too large to be consistent with the observational data. Based on these findings, we investigate a different bouncing scenario which also appears with a Type-IV singularity, and the singularity affects the spacetime "locally" around the time when it occurs. As a result, and unlike to the previous scenario, the perturbation modes in the second bouncing scenario are likely to generate far away from the bounce in the deep contracting phase. This finally results to the simultaneous compatibility of the observable quantities with the Planck data, and ensures the viability of the bounce model where the Type-IV singularity has local effects on the spacetime around the time of the singularity. I. INTRODUCTIONAt present, we are living in a cosmological era where, on one hand, we have several cosmological data that includes the scalar spectral index, tensor-to-scalar ratio which describe the early stage of the universe, as well as the late time equation of state parameter, the Om(z) parameter in regard to the dark energy era of the universe. However, on other hand, the modern cosmology is still riddled with the question that whether the universe started its expansion from a Big-Bang singularity or from a non-singular like bouncing scenario. Inflation is one of the cosmological scenarios that successfully describes the early stage of the universe, in particular, it solves the horizon and flatness problems, and most importantly it predicts an almost scale invariant curvature perturbation power spectrum that is well consistent with the recent Planck data [1-5]. However, extrapolating backwards in time, the inflationary scenario results to an initial singularity of the universe, known as the Big-Bang singularity where, due to the geodesic incompleteness, the spacetime curvature diverges at the point of singularity. Bouncing cosmology is one of the alternatives of inflation, that can generate a scale invariant curvature power spectrum, and in addition, the bounce scenario leads to a singular free evolution of the universe . In the present work, we are interested in bounce cosmology that appears with certain features which we will explain.Among various bounce models proposed so far, the matter bounce scenario (MBS) earned a lot of popularity, due to the fact that it produces a scale invariant primordial power spectrum and also leads to a matter dominated universe a...

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Section: Realization Of a Bounce With A Type-iv Singularity That Loca...mentioning

confidence: 99%

Bounce universe with finite-time singularity

Odintsov¹,

Paul²

2022

Preprint

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“…where ρ + n and ρ − m as positive real numbers. The negative energy density terms could either be arising from an exotic matter field (as, for instance, in [23][24][25]) or emerging from the corrections to the Einstein-Hilbert action introduced by a UV-complete quantum gravity theory (as, for instance, in [26]).…”

Section: Effective Matter Content Of Bouncing Models: General Analysismentioning

confidence: 99%

No-boundary Wave Function, Wheeler-DeWitt Equation and Path Integral Analysis of the Bouncing `Quantum' Cosmology

Rajeev,

Mondal,

Chakraborty

2021

Preprint

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Bouncing models are alternatives to inflationary cosmology that replace the initial Big-Bang singularity by a 'bouncing' phase. A deeper understanding of the initial conditions of the universe, in these scenarios, requires knowledge of quantum aspects of bouncing models. In this work, we propose two classes of bouncing models that can be studied with great analytical ease and hence, provide test-bed for investigating more profound problems in quantum cosmology of bouncing universes. Our model's two key ingredients enable us to do straightforward analytical calculations: (i) a convenient parametrization of the minisuperspace of FRLW spacetimes and (ii) two distinct choices of the effective perfect fluids that source the background geometry of the bouncing universe. We study the quantum cosmology of these models using both the Wheeler-de Witt equations and the path integral approach. In particular, we found a bouncing model analogue of the no-boundary wavefunction and presented a Lorentzian path integral representation for the same. We also discuss the introduction of real scalar perturbations.

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“…Despite the above mentioned successes, the MBS is riddled with some problems, like -(1) the tensor to scalar ratio in the scalar-tensor MBS becomes too large to be consistent with the observational data [62], (2) during the contracting phase of the MBS, the anisotropic energy density grows faster compared to that of the bouncing agent and hence the model suffers from the BKL instability [58], (3) as the universe at late time is dominated by pressureless matter, the MBS does not predict the dark energy era of the universe -not consistent with the supernovae observations [59][60][61]. Successful attempts have been made to resolve these issues in modified theories of gravity [20,21,55,56,63,64]. For example, some of our authors proposed a smooth unification from a bounce to the dark energy era with an intermediate deceleration era in higher curvature theories of gravity both in the case of symmetric and asymmetric bounce scenarios [63,64], where the generation era of primordial perturbation modes become different depending on the symmetric / asymmetric character of the bounce.…”

Section: Introductionmentioning

confidence: 99%

Ekpyrotic bounce driven by Kalb-Ramond field

Paul¹,

SenGupta²

2022

Preprint

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We propose an ekpyrotic bounce scenario driven by a second rank antisymmetric Kalb-Ramond field, where the universe initially contracts through an ekpyrotic stage having a non-singular bouncing like behaviour, and consequently, it smoothly transits to an expanding phase. In particular, the KR field has an interaction with a scalaron field (coming from higher curvature d.o.f) by a linear coupling. The interaction energy density between the KR and the scalaron grows faster than a −6 (with a being the scale factor of the universe) during the contraction phase -which has negligible effects at the distant past, however as the universe continues to contract, this interaction energy density gradually grows and plays a significant role in violating the null energy condition or to trigger a non-singular bounce at a minimum value of the scale factor. The existence of the ekpyrotic phase justifies the resolution of the BKL instability, where the anisotropic energy density gets diluted compared to that of the bouncing agent. The bounce being symmetric and ekpyrotic, the energy density of the bouncing agent rapidly decreases after the bounce during the expanding phase of the universe, and consequently the standard Big-Bang cosmology gets recovered. In regard to the perturbation analysis, we find that the comoving Hubble radius diverges at the distant past, which leads to the generation era of the primordial perturbation modes at the deep contracting phase far away from the bounce. In effect, the curvature perturbation gets a blue tilted spectrum over the large scale modes -not consistent with the Planck data. To circumvent this problem, we propose an extended scenario where the ekpyrotic phase is preceded by a quasi-matter dominated pre-ekpyrotic phase, and re-investigate the perturbation power spectrum. As a result, the primordial curvature perturbation, at scales that cross the horizon during the pre-ekpyrotic stage, turns out to be nearly scale invariant that is indeed consistent with the recent Planck data.

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Viable scalar spectral tilt and tensor-to-scalar ratio in near-matter bounces

Cited by 17 publications

References 36 publications

Bounce universe with finite-time singularity

Bounce universe with finite-time singularity

No-boundary Wave Function, Wheeler-DeWitt Equation and Path Integral Analysis of the Bouncing `Quantum' Cosmology

Ekpyrotic bounce driven by Kalb-Ramond field

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