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18 pages, 872 KiB

Open AccessArticle

Hyperbolic Scenario of Accelerating Universe in Modified Gravity

by Raja Azhar Ashraaf Khan, Rishi Kumar Tiwari, Jumi Bharali, Amine Bouali, G. Dilara Açan Yildiz and Ertan Güdekli

Symmetry 2023, 15(6), 1238; https://doi.org/10.3390/sym15061238 - 9 Jun 2023

Cited by 2 | Viewed by 1632

Abstract

Throughout this study, locally rotationally symmetric (LRS) Bianchi type-V space-time is pondered with Tsallis holographic dark energy (THDE) with the Granda–Oliveros (GO) cut-off in the Sáez–Ballester (SB) theory of gravity. A parameterization of the deceleration parameter

(q)

has been suggested: [...] Read more.

Throughout this study, locally rotationally symmetric (LRS) Bianchi type-V space-time is pondered with Tsallis holographic dark energy (THDE) with the Granda–Oliveros (GO) cut-off in the Sáez–Ballester (SB) theory of gravity. A parameterization of the deceleration parameter

(q)

has been suggested:

q = α - \frac{β}{H^{2}}

. The proposed deceleration parameterization demonstrates the Universe’s phase transition from early deceleration to current acceleration. Markov chain Monte Carlo (MCMC) was utilized to have the best-fit value for our model parameter and confirm that the model satisfies the recent observational data. Additional parameters such as deceleration parameter q with cosmographic parameters jerk, snap, and lerk have also been observed physically and graphically. The constructed model is differentiated from other dark energy models using statefinder pair analysis. Some important features of the model are discussed physically and geometrically. Full article

(This article belongs to the Special Issue Application of Symmetry in Gravity Researches)

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13 pages, 502 KiB

Open AccessArticle

A Transition Model in f(R,T) Theory via Observational Constraints

by Rishi Kumar Tiwari, Bhupendra Kumar Shukla, Değer Sofuoğlu and Dilay Kösem

Symmetry 2023, 15(4), 788; https://doi.org/10.3390/sym15040788 - 24 Mar 2023

Cited by 7 | Viewed by 1933

Abstract

A particular form of the time-dependent deceleration parameter is used to examine the accelerated expansion of the universe and the phase transition in this expansion in the context of

f (R, T)

gravity theory for the flat FRW model. The [...] Read more.

A particular form of the time-dependent deceleration parameter is used to examine the accelerated expansion of the universe and the phase transition in this expansion in the context of

f (R, T)

gravity theory for the flat FRW model. The modified field equations are solved under the choice of

f (R, T) = R + 2 f (T)

. The best fit values of the model parameters that would be consistent with the recent observational datasets that are estimated. For this estimation, 57 points from Cosmic Chronometers (CC) datasets and 1048 points from Pantheon supernovae datasets are used. Bayesian analysis and likelihood function are applied together with Markov Chain Monte Carlo (MCMC) method at

1 σ

and

2 σ

confidence levels. Then, the physical behavior of parameters such as density, pressure and cosmographic parameters corresponding to these constrained values of the model parameters are analyzed. Looking at the deceleration parameter, it is seen that the universe has passed from a decelerating expansion phase to an accelerating phase. As a result, it has been shown that the cosmological model

f (R, T)

that we discussed can explain the accelerating expansion of the late universe well without resorting to any dark energy component in the energy-momentum tensor. Full article

(This article belongs to the Special Issue Symmetry and Asymmetry in Gravity Research)

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7 pages, 278 KiB

Open AccessProceeding Paper

Bianchi Type-I Universe in Modified Theory of Gravity

by Bhupendra Kumar Shukla, Rishi Kumar Tiwari and Aroonkumar Beesham

Phys. Sci. Forum 2023, 7(1), 28; https://doi.org/10.3390/ECU2023-14049 - 17 Feb 2023

Viewed by 1126

Abstract

In this paper, we have studied an anisotropic Bianchi-I cosmological model in

f (R, T)

gravity. To obtain the exact solutions of the field equations, we have used the condition

σ / θ

to be a function of the scale [...] Read more.

In this paper, we have studied an anisotropic Bianchi-I cosmological model in

f (R, T)

gravity. To obtain the exact solutions of the field equations, we have used the condition

σ / θ

to be a function of the scale factor (IJTP, 54, 2740-2757, 2015). Our model possesses an initial singularity. It initially exhibits decelerating expansion and transits to accelerating expansion at late times. We have also discussed the physical and geometrical properties of the model. Full article

(This article belongs to the Proceedings of The 2nd Electronic Conference on Universe)

19 pages, 1885 KiB

Open AccessArticle

Perturbations in Bianchi-V Spacetimes with Varying Λ, G and Viscous Fluids

by Amare Abebe, Alnadhief H. A. Alfedeel, Değer Sofuoğlu, Eltegani I. Hassan and Rishi Kumar Tiwari

Universe 2023, 9(2), 61; https://doi.org/10.3390/universe9020061 - 19 Jan 2023

Cited by 6 | Viewed by 1561

Abstract

As a follow-up of a recent article in which we investigated the cosmological background expansion history of the universe in Bianchi type-V cosmological models with bulk viscous fluid and evolving cosmological

Λ

and Newtonian G parameters, we study the evolution of the cosmological [...] Read more.

As a follow-up of a recent article in which we investigated the cosmological background expansion history of the universe in Bianchi type-V cosmological models with bulk viscous fluid and evolving cosmological

Λ

and Newtonian G parameters, we study the evolution of the cosmological perturbations in the current work. In particular, we analyse the evolution of the viscous matter over-density that leads to formation of large-scale structures in the Bianchi-V model, and compare the results with standard

Λ

CDM solutions. Our results suggest that introducing viscous fluid in the background described by Bianchi-V geometry with evolving

Λ

and G amplifies the structure-growth rate. Full article

(This article belongs to the Section Cosmology)

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11 pages, 359 KiB

Open AccessArticle

The Cosmology of a Non-Minimally Coupled f(R,T) Gravitation

by Değer Sofuoğlu, Rishi Kumar Tiwari, Amare Abebe, Alnadhief H. A. Alfedeel and Eltegani I. Hassan

Physics 2022, 4(4), 1348-1358; https://doi.org/10.3390/physics4040086 - 7 Nov 2022

Cited by 13 | Viewed by 1995

Abstract

A non-minimally coupled cosmological scenario is considered in the context of

f (R, T) = f_{1} (R) + f_{2} (R) f_{3} (T)

gravity (with R being the Ricci scalar and T [...] Read more.

A non-minimally coupled cosmological scenario is considered in the context of

f (R, T) = f_{1} (R) + f_{2} (R) f_{3} (T)

gravity (with R being the Ricci scalar and T the trace of the energy-momentum tensor) in the background of the flat Friedmann–Robertson–Walker (FRW) model. The field equations of this modified theory are solved using a time-dependent deceleration parameter for a dust. The behavior of the model is analyzed taking into account constraints from recent observed values the deceleration parameter. It is shown that the analyzed models can explain the transition from the decelerating phase to the accelerating one in the expansion of the universe, by staying true to the results of the observable universe. It is shown that the models are dominated by a quintessence-like cosmological dark fluid at the late universe. Full article

(This article belongs to the Special Issue New Advances in Quantum Geometry)

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16 pages, 408 KiB

Open AccessArticle

FLRW Cosmological Models with Dynamic Cosmological Term in Modified Gravity

by Rishi Kumar Tiwari, Aroonkumar Beesham and Bhupendra Kumar Shukla

Universe 2021, 7(9), 319; https://doi.org/10.3390/universe7090319 - 28 Aug 2021

Cited by 2 | Viewed by 2531

Abstract

Although the standard lambda cold dark matter cosmological model is quite successful in describing the universe, there are still several issues that are still not resolved. Some of these are the cosmological constant problem, certain anomalies in the cosmic microwave background radiation and [...] Read more.

Although the standard lambda cold dark matter cosmological model is quite successful in describing the universe, there are still several issues that are still not resolved. Some of these are the cosmological constant problem, certain anomalies in the cosmic microwave background radiation and whether general relativity is valid on large scales. Therefore, it is interesting to examine modified theories in an attempt to solve these problems, and to examine the entire range of possibilities that are allowed. In this work, we examine one of these modified theories, viz., f(R,T) gravity. We study the homogeneous and isotropic models in this theory, which have some pleasing features, such as no initial singularity, a dynamic cosmological term, and a transition from early deceleration to late-time acceleration as intimated by observations. The physical parameters of the model, as well as the energy conditions, are discussed and a viable cosmological model can be constructed. Full article

(This article belongs to the Special Issue Selected Papers from the 1st International Electronic Conference on Universe (ECU 2021))

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12 pages, 317 KiB

Open AccessArticle

Anisotropic Cosmological Model in a Modified Theory of Gravitation

by Rishi Kumar Tiwari, Aroonkumar Beesham, Soma Mishra and Vipin Dubey

Universe 2021, 7(7), 226; https://doi.org/10.3390/universe7070226 - 4 Jul 2021

Cited by 12 | Viewed by 2990

Abstract

Current observations indicate that, on a large enough scale, the universe is homogeneous and isotropic. However, this does not preclude the possibility of some anisotropy having occurred during the early stages of the evolution of the universe, which could then have been damped [...] Read more.

Current observations indicate that, on a large enough scale, the universe is homogeneous and isotropic. However, this does not preclude the possibility of some anisotropy having occurred during the early stages of the evolution of the universe, which could then have been damped out later. This idea has aroused interest in the Bianchi models, which are homogeneous but anisotropic. Secondly, there is much interest in modified gravity these days due to the problems that the usual ΛCDM model faces in general relativity. Hence, in this paper, a study was conducted on the Bianchi type-I cosmological model in f(R,T)-modified gravity. Following some ideas from cosmography, a specific form of the deceleration parameter was assumed, leading to a model that exhibited a transition from early deceleration to late-time acceleration. The derived model approached isotropy at late times. The physical properties of the model were discussed, and expressions for the various parameters of the model were derived. It is also possible to make progress towards solving the cosmological constant problem, since in this model in f(R,T) gravity, a variable cosmological-type parameter arose, which was large early on but decreased to a constant value in later times. Full article

(This article belongs to the Special Issue Selected Papers from the 1st International Electronic Conference on Universe (ECU 2021))

7 pages, 352 KiB

Open AccessProceeding Paper

Reconstruction of Models with Variable Cosmological Parameter in f(R,T) Theory

by Rishi Kumar Tiwari, Aroonkumar Beesham and Bhupendra Kumar Shukla

Phys. Sci. Forum 2021, 2(1), 59; https://doi.org/10.3390/ECU2021-09372 - 1 Mar 2021

Cited by 1 | Viewed by 1687

Abstract

The standard ΛCDM model is reasonably successful in describing the universe, and is the most widely acceptable model in cosmology. However, there are several theoretical issues, such as the initial singularity, the cosmological constant problem, the particle nature of dark matter, the existence [...] Read more.

The standard ΛCDM model is reasonably successful in describing the universe, and is the most widely acceptable model in cosmology. However, there are several theoretical issues, such as the initial singularity, the cosmological constant problem, the particle nature of dark matter, the existence of anomalies in the cosmic microwave background radiation and on small scales, the predictions and tests of the inflationary scenario and whether general relativity is valid on the largest possible scales. Hence, there is growing interest in looking at modified theories. In this presentation, a reconstruction is made of the Friedmann–Lemaitre–Robertson–Walker models with a dynamic cosmological parameter in f(R,T) modified gravity. This theory has a number of pleasing features, such as the avoidance of the initial Big Bang singularity and a variable cosmological parameter. A dynamic cosmological parameter, which arises naturally in this theory, can solve the cosmological constant problem, and it is also a candidate for dark energy. In addition, a variable cosmological parameter fits observations better than the standard ΛCDM model. The model exhibits a transition from deceleration to acceleration. The time evolution of the physical parameters such as energy density, pressure and equation of state are analyzed. Full article

(This article belongs to the Proceedings of The 1st Electronic Conference on Universe)

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6 pages, 256 KiB

Open AccessProceeding Paper

Bianchi Type I Cosmological Model in f(R,T) Gravity

by Rishi Kumar Tiwari, Aroonkumar Beesham, Soma Mishra and Vipin Dubey

Phys. Sci. Forum 2021, 2(1), 38; https://doi.org/10.3390/ECU2021-09290 - 22 Feb 2021

Cited by 3 | Viewed by 2073

Abstract

Although the present universe is believed to be homogeneous and isotropic on large scales, there is some evidence of some anisotropy at early times. Hence, there is interest in the Bianchi models, which are homogeneous but anisotropic. In this presentation, the Bianchi type–I [...] Read more.

Although the present universe is believed to be homogeneous and isotropic on large scales, there is some evidence of some anisotropy at early times. Hence, there is interest in the Bianchi models, which are homogeneous but anisotropic. In this presentation, the Bianchi type–I space-time in the framework of the f(R,T) modified theory of gravity has been investigated for the specific choice of f(R,T) = R + 2f(T), where f(T) = −mT, m = constant. The solution of the modified gravity field equations has been generated by assuming that the deceleration parameter q is a function of the Hubble parameter H, i.e., q = b − n/H (where b and n are constants, and n > 0), which yields the scale factor a = k[exp(nt) − 1]^1/(1+b) (where k is a constant). The model exhibits deceleration at early times, and is currently accelerating. It is also seen that the model approaches isotropy at late times. Expressions for the Hubble parameter in terms of red-shift, luminosity distance, and state-finder parameter are derived and their significance is described in detail. The physical properties of the cosmological model are also discussed. An interesting feature of the model is that it has a dynamic cosmological parameter, which is large during the early universe, decreases with time, and approaches a constant at late times. This may help in solving the cosmological constant problem. Full article

(This article belongs to the Proceedings of The 1st Electronic Conference on Universe)

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