Bulk viscous matter and recent acceleration of the universe based on causal viscous theory

In order to test if there is energy transfer between dark energy (DE) and dark matter (DM), we investigate cosmological constraints on two forms of nontrivial interaction between the DM sector and the sector responsible for the acceleration of the universe, in light of the newly revised observations including OHD, CMB, BAO and SNe Ia. More precisely, we find the same tendencies for both phenomenological forms of the interaction term Q = 3γHρ, i.e. the parameter γ to be a small number, |γ| ≈ 10-2. However, concerning the sign of the interaction parameter, we observe that γ > 0 when the interaction between dark sectors is proportional to the energy density of dust matter, whereas the negative coupling (γ < 0) is preferred by observations when the interaction term is proportional to DE density. We further discuss two possible explanations to this incompatibility and apply a quantitative criteria to judge the severity of the coincidence problem. Results suggest that the γm IDE model with a positive coupling may alleviate the coincidence problem, since its coincidence index C is smaller than that for the γd IDE model, the interacting quintessence and phantom models by four orders of magnitude.

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Hamiltonian formalism for nonlocal gravity models

International Journal of Geometric Methods in Modern Physics ◽

10.1142/s0219887822500360 ◽

2022 ◽

Author(s):

Pawan Joshi ◽

Utkarsh Kumar ◽

Sukanta Panda

Keyword(s):

Ricci Tensor ◽

Degrees Of Freedom ◽

Hamiltonian Formalism ◽

Riemann Tensor ◽

Ricci Scalar ◽

Gravity Models ◽

Lagrangian Formalism ◽

Acceleration Of The Universe ◽

Nonlocal Action ◽

The Universe

Nonlocal gravity models are constructed to explain the current acceleration of the universe. These models are inspired by the infrared correction appearing in Einstein–Hilbert action. Here, we develop the Hamiltonian formalism of a nonlocal model by considering only terms to quadratic order in Riemann tensor, Ricci tensor and Ricci scalar. We show how to count degrees of freedom using Hamiltonian formalism including Ricci tensor and Ricci scalar terms. In this model, we have also worked out with a choice of a nonlocal action which has only two degrees of freedom equivalent to GR. Finally, we find the existence of additional constraints in Hamiltonian required to remove the ghosts in our full action. We also compare our results with that of obtained using Lagrangian formalism.

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On generalized Lemaitre–Tolman–Bondi metric: Fractal matter at the end of matter–antimatter recombination

International Journal of Modern Physics D ◽

10.1142/s0218271821500863 ◽

2021 ◽

pp. 2150086

Author(s):

Sergio L. Cacciatori ◽

Alessio Marrani ◽

Federico Re

Keyword(s):

Dark Matter ◽

Relativistic Effects ◽

Partial Explanation ◽

First Order ◽

Acceleration Of The Universe ◽

Particular Solution ◽

Ancient Times ◽

The Universe ◽

Recombination Epoch ◽

Ltb Metric

Many recent researches have investigated the deviations from the Friedmannian cosmological model, as well as their consequences on unexplained cosmological phenomena, such as dark matter and the acceleration of the Universe. On one hand, a first-order perturbative study of matter inhomogeneity returned a partial explanation of dark matter and dark energy, as relativistic effects due to the retarded potentials of far objects. On the other hand, the fractal cosmology, now approximated by a Lemaitre–Tolman–Bondi (LTB) metric, results in distortions of the luminosity distances of SNe Ia, explaining the acceleration as apparent. In this work, we extend the LTB metric to ancient times. The origin of the fractal distribution of matter is explained as the matter remnant after the matter–antimatter recombination epoch. We show that the evolution of such a inhomogeneity necessarily requires a dynamical generalization of LTB, and we propose a particular solution.

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Accelerating Universe with Viscous Cosmic String in Quadratic Form of Teleparallel Gravity

Journal of Scientific Research ◽

10.3329/jsr.v11i3.39220 ◽

2019 ◽

Vol 11 (3) ◽

pp. 249-262

Author(s):

S. R. Bhoyar ◽

V. R. Chirde ◽

S. H. Shekh

Keyword(s):

Cosmic String ◽

Teleparallel Gravity ◽

Negative Slope ◽

Accelerating Universe ◽

Field Equations ◽

Physical Behavior ◽

Torsion Scalar ◽

Bulk Viscous ◽

The Universe ◽

Physical Quantities

In this paper, we have investigated Kantowaski-Sachs cosmological model with bulk viscous and cosmic string in the framework of Teleparallel Gravity so called f(T) gravity, where T denotes the torsion scalar. The behavior of accelerating universe is discussed towards the particular choice of f(T) = Α(T) + β(T)m. The exact solutions of the field equations are obtained by applying variable deceleration parameter which is linear in time with a negative slope. The physical behavior of these models has been discussed using some physical quantities. Also, the function of the torsion scalar for the universe is evaluated.

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Cosmological dynamics of f(R) models in dynamical system analysis

International Journal of Modern Physics A ◽

10.1142/s0217751x20501249 ◽

2020 ◽

Vol 35 (22) ◽

pp. 2050124

Author(s):

Parth Shah ◽

Gauranga C. Samanta

Keyword(s):

Dynamical System ◽

Asymptotic Behavior ◽

Critical Points ◽

System Analysis ◽

Late Time ◽

Field Equations ◽

Acceleration Phase ◽

First Case ◽

Acceleration Of The Universe ◽

The Universe

In this work we try to understand the late-time acceleration of the universe by assuming some modification in the geometry of the space and using dynamical system analysis. This technique allows to understand the behavior of the universe without analytically solving the field equations. We study the acceleration phase of the universe and stability properties of the critical points which could be compared with observational results. We consider an asymptotic behavior of two particular models [Formula: see text] and [Formula: see text] with [Formula: see text], [Formula: see text], [Formula: see text] for the study. As a first case we fix the value of [Formula: see text] and analyze for all [Formula: see text]. Later as second case, we fix the value of [Formula: see text] and calculation are done for all [Formula: see text]. At the end all the calculations for the generalized case have been shown and results have been discussed in detail.

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Cosmic acceleration with tachyon field

Modern Physics Letters A ◽

10.1142/s0217732318501997 ◽

2018 ◽

Vol 33 (34) ◽

pp. 1850199 ◽

Cited By ~ 1

Author(s):

A. I. Keskin

Keyword(s):

Scalar Field ◽

Energy Momentum Tensor ◽

Momentum Tensor ◽

Cosmic Acceleration ◽

Late Time ◽

De Sitter ◽

Tachyon Field ◽

Minimal Coupling ◽

Acceleration Of The Universe ◽

The Universe

In this study, we examine two models of the scalar field, that is, a normal scalar field and a tachyon scalar field in [Formula: see text] gravity to describe cosmic acceleration of the universe, where [Formula: see text], [Formula: see text] and [Formula: see text] are Ricci curvature scalar, trace of energy–momentum tensor and kinetic energy of scalar field [Formula: see text], respectively. Using the minimal-coupling Lagrangian [Formula: see text], for both the scalar models we obtain a viable cosmological system, where [Formula: see text] and [Formula: see text] are real constants. While a normal scalar field gives a system describing expansion from the deceleration to the late-time acceleration, tachyon field together with [Formula: see text] in the system produces a quintessential expansion which is very close to de Sitter point, where we find a new condition [Formula: see text] for inflation.

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