On the initial conditions of scalar and tensor fluctuations in $$f(R,\phi )$$ f ( R , ϕ ) gravity

Cheraghchi, Samira; Shojai, Fatimah

doi:10.1140/epjc/s10052-018-5878-x

Cited by 6 publications

(6 citation statements)

References 26 publications

(37 reference statements)

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“…In f (R) gravity, choosing the gauge condition δF = 0, as a specific choice, effectively removes some degrees of freedom and the Mukhanov-Sasaki equation is easily obtained. Even for a general f (R, f) gravity, one can introduce a new gauge by which the generalized Mukhanovâ-Sasaki equation is derived straightforwardly [24]. 6 Here we do not specify the gauge.…”

Section: Vacuum Ambiguity In F (R) Inflationary Cosmologymentioning

confidence: 99%

Trans-Planckian effect in f(R) cosmology

2023

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Apart from the assumption that the inflation started at an infinite time in the past, the more realistic initial state of the quantum fluctuations is described by a mixed quantum state imposed at a finite value of the initial time.One of the most important non-trivial vacua is the $\alpha$-vacuum, which is specified by a momentum cutoff $\Lambda$ \cite{Danielsson:2002kx}. As a consequence, the initial condition is imposed at different initial times for the different $k$-modes. This modifies the amplitude of the quantum fluctuations, and thus the corresponding power spectra. In this paper, we consider the imprint of the $\alpha$-vacuum state on the power spectrum of scalar perturbations in a generic $f(R)$ gravity by assuming an ultraviolet cutoff $\Lambda$. As a specific model, we consider the Starobinsky model and find the trans-Planckian power spectrum. We find that the leading order corrections to the scalar power spectra in $f(R)$ gravity have an oscillatory behavior as in general relativity \cite{Lim}, and furthermore, the results are in sufficient agreement with the $\Lambda$CDM model.

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Section: Vacuum Ambiguity In F (R) Inflationary Cosmologymentioning

confidence: 99%

Trans-Planckian effect in f(R) cosmology

2023

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“…where h is another constant which can be obtained by the initial condition of metric perturbation in the Jordan frame at the time of horizon crossing [21]. By using equation (29), it is straightforward to see that δF 2F = κ 2 6 δξ (50)…”

Section: Scalar Perturbationsmentioning

confidence: 99%

“…and the Fourier transforms of the metric and scalar perturbations, δφ k , satisfy in the following equations [21] 3…”

Section: Scalar Perturbationsmentioning

confidence: 99%

“…In GR, in the super-horizon regime, the curvature perturbation, (10.3.16) in [18]. On the other hand, according to [21] and [23], in f (R) gravity, the curvature perturbation, in the super-horizon limit, has a dominated constant mode of this order. Hence by comparing this definition of the curvature perturbation with equation (53), we result that the constant h 6…”

Section: Scalar Perturbationsmentioning

confidence: 99%

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Superhorizon effects on the gauge invariant effective energy density inf(R)gravity

Cheraghchi¹,

Shojai²

2019

Phys. Rev. D

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In this article we study the gauge invariant effective energy momentum tensor for cosmological perturbations in f (R) gravity during the inflationary epoch. Considering the super-horizon regime, we derive the effective energy density up to the one-loop corrections. It describes the back-reaction effect of the fluctuations on the background space-time. introductionAccording to the inflationary scenario, the fluctuations of a scalar field, called inflaton, along with the metric perturbations, provide the original seeds for the structure formation of our recent universe, see [1]-[6] and references therein. Due to the non-linearity of Einstein field equations, there is a non-vanishing back-reaction effect through which the fluctuations change the Friedmann-Lematre-Robertson-Walker (FLRW) background metric. This effect could appear in the power spectrum of the cosmic fluctuations of CMB [6]. In general relativity (GR), considering the subhorizon and super-horizon regimes, the back-reaction effects have studied in many articles [7,8,9,10]. Some issues like local observability and causality [11] cause the super-horizon modes have been less studied than the sub-horizon modes. In [12], Unruh has claimed that the effect of long wavelength perturbation is invisible for any local observer but in the recent articles of Brandenberger and his collaborators [13], they mention how the super-horizon modes could affect the local physics. In GR, during the slow roll inflation, it is shown that the light scalar field fluctuations along with the scalar metric perturbations lead to a strong infrared enhancement in the gauge invariant energy momentum tensor (EMT) due to the super-horizon modes, but a combination of slow roll parameters can regulate this infrared enhancement [14,15,16]. In this article we would like to consider the effective EMT in a gauge invariant manner up to the one loop quantum correction of the scalar and tensor perturbations in f (R) gravity with an extra scalar field. Although f (R) gravity can produce an inflationary era in the early universe, inclusion of some additional scalar fields to it, can lead to non-adiabatic perturbations. These fields are responsible for primordial non-Gaussianity in the density perturbation [17]. Moreover multifield inflationary models have rich dynamics and according to [18] there is no particular reason to believe that a single scalar field drives the inflationary expansion of the universe. These models may also be motivated by high-energy physics, such as supersymmetric theories in which there are many superpartners scalar

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“…Their investigation showed that this theory can support a viable inflationary model that includes a well-functioning exit strategy. Numerous cosmological issues are examined by researchers in this theory [18]. This theory has further been modified by including the kinetic term in the action, called f (R, j, χ) theory [19].…”

Section: Introductionmentioning

confidence: 99%

Anisotropic stellar structures admitting Karmakar condition in f(R, φ, χ) theory

Sharif,

Gul

2024

Phys. Scr.

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In this paper, we employ the Karmarkar condition to study thegeometry of compact stars experiencing anisotropy in the context of$f(\textsf{R},\varphi,\chi)$ gravity, where $\textsf{R}$, $\varphi$and $\chi$ denote the Ricci scalar, scalar field and kinetic term,respectively. The field equations corresponding to the well-knownmodel $f(\textsf{R},\varphi,\chi)=\textsf{R}^{\lambda}+\chi+\varphi$are formulated, $\lambda$ being the arbitrary constant. The unknownconstants involved in Karmarkar condition are calculated by matchingthe internal and external regimes at the hypersurface. Weinvestigate the acceptable behavior of matter variables andanisotropy. The viability of all the resulting models is checkedusing energy bounds. We also discuss some important factors such asmass, compactness and redshift parameters. Finally, we investigatethe stable aspect of considered stars through causality conditionand Herrera cracking technique. We conclude that all the discussedstellar structures in this theory satisfy the required constraints.

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On the initial conditions of scalar and tensor fluctuations in $$f(R,\phi )$$ f ( R , ϕ ) gravity

Cited by 6 publications

References 26 publications

Trans-Planckian effect in f(R) cosmology

Trans-Planckian effect in f(R) cosmology

Superhorizon effects on the gauge invariant effective energy density inf(R)gravity

Anisotropic stellar structures admitting Karmakar condition in f(R, φ, χ) theory

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