Time Varying Deceleration Parameter Research Articles

Dynamics of Bianchi Type-III Universe with Time-Varying Deceleration Parameter in f (G) Gravity

Dynamics of Bianchi Type-III Universe with Time-Varying Deceleration Parameter in f (G) Gravity

Cosmological models with variable parameters

The present study deals with the investigation of the Friedmann-Lemaitre-Robertson-Walker models (often FLRW-models) with time varying G and ∧ in the framework of General theory of Relativity. The Einstein field equations have been solved by considering the time-varying deceleration parameter q(t) and Scale factor , where m and β are arbitrary constants. We have analysed the value of m, which will generate a transition for universe from early decelerating phase to recent acceleration phase. The physical and graphic behaviour have also been planned to study in this communication.

Phase transition of Bianchi-type I cosmological model in [formula omitted] gravity

In the context of the f(T) theory of gravity, where T represents as a torsion scalar, we explore the Locally Rotational Symmetric Bianchi type-I dark energy model. An effective cosmological model is used, and its isotropization is examined, producing a conclusion that accurately depicts the evolution of our Universe. We looked at dark energy (DE) and dark matter (DM) in both interacting and non-interacting forms. The exact solution of the field equations was achieved by using special form of the time-varying deceleration parameter in terms of the Hubble parameter. The dark matter energy density (Ω(dm)) decreases with redshift while dark energy density (Ω(de)) grow with redshift. The physical and geometrical behaviour of the models are examined in both forms. A combination of dark matter and dark energy can eliminate the coincidence issue in the ΛCDM model. Using the most recent observations, we also discussed the physical characteristics of the generated models about the early Universe and late-time acceleration. It has been noted that this model has an initial singularity, initial decelerating growth and late-time transitions to an accelerating expansion phase.

String cosmological scenario in periodic time-varying deceleration parameter in Lyra geometry

In this paper, we have studied the bulk viscous fluid connected with string cosmological model in a higher dimensional (five dimensions) Bianchi type-III space–time in the framework of Lyra’s manifold. To get the deterministic solution of the field equations, we assume two physically appropriate conditions: ( i) θ in the model parameter that is proportional to the eigen value of [Formula: see text] of the shear tensor [Formula: see text] and ( ii) the periodic time-varying deceleration parameter q specified as q = mcos ( kt) − 1, where m, k > 0. In this paper, we have studied the periodic time-varying behaviour of a few quantities, such as the deceleration parameter q, the energy density ρ, the proper pressure p, the string tension density λ, the total pressure [Formula: see text], the energy density for particle ρp, and the displacement vector β( t), and we have discussed the physical significance of these quantities. It has been found that in the early stages of the universe’s evolution, string prevails over particles, whereas the universe is dominated by massive string at late time. It is also noted that the string phase of the cosmos does not vanish in the model since particle density is constantly positive throughout the universe’s history. In normal gauge treatment, β( t) acts like cosmological constant (Λ), and the solutions are compatible with observations. For the stability analysis, we have investigated the nature of various energy conditions (ECs). The positive behaviour of DEC indicates the model’s validation; on the other hand, SEC and WEC are violating, indicating the universe’s accelerated expansion.

Cosmic expansion of the universe characterized by anisotropic viscous fluid cosmological models

In this paper, the cosmic expansion of the universe filled with viscous fluid has been discussed in Bianchi type-I space-time. To solve Einstein’s field equations, a time-varying deceleration parameter, ( q( t)) yields a time variable scale factor [Formula: see text], with " α" and " n" are positive constants. We also consider a time-varying gravitational constant ( G) via the formulation 8π G = G0 a m , where " G0" and " m" are positive constants. Further, based on the third constraint, two different viscous cosmological models, one by considering the perfect gas equation of state p = ωρ and the other by considering time-varying bulk viscosity coefficient ( ξ( t)), have been constructed. It is mentioned here that this particular choice of scale factor generates a class of accelerating models for n < 1, while for n > 1, the transition of the universe from early deceleration to current acceleration takes place. An important feature of the models is decreasing dark energy candidate, i.e., cosmological constant (Λ) with time, fine-tuning with the results from recent supernovae Ia observations. The various physical parameters involved in the models have been discussed analytically and graphically and are found in good agreement with recent observations. For the physical acceptability of both the models, energy conditions are shown graphically. The models are found to be physically acceptable, particularly in identifying the universe’s transition. We have discussed the statefinder and observed that all the trajectories indicate dark energy candidates as Chaplygin gas and quintessence for different constraints ( α, n). We have also calculated the cosmographic parameters such as jerk, lerk, and maxout to analyze the physical behavior of the DE models.

The study of hypersurface-homogeneous space-time in Renyi holographic dark energy

The investigation of this work is carried on the hypersurface-homogeneous space-time in the presence of two fluids, one being the pressureless matter and the other being the Renyi holographic dark energy (RHDE). Eventually, this work solves the cosmological model with Renyi HDE by taking the Hubble horizon as an infrared (IR) cutoff ([Formula: see text]). The geometrical and matter parts of space-time are solved within the Saez–Ballester scalar-tensor theory of gravitation. Interestingly, this study obtains a time-varying deceleration parameter ([Formula: see text]) that exhibits a transition from deceleration to acceleration phase. For the configurations of parameter [Formula: see text] emerging in space-time, three physically plausible cosmological hypotheses of the cosmos are outlined in this work. The kinematical properties were obtained and discussed for the three values of [Formula: see text]. The study of cosmic expansion in the accelerated phase of this work is done through various cosmological parameters like EoS, deceleration parameter, statefinder parameter, etc. The Hubble parameter is derived in terms of cosmic time and redshift both. The present value of this parameter is estimated by taking [Formula: see text] point data sets of observational Hubble data (OHD). The stability of the model is verified through the analysis of squared speed of sound ([Formula: see text]) parameter. In this work, the obtained results match with recent observational data.

Bulk viscous fluid in extended symmetric teleparallel gravity

In this paper, we investigate the existence of bulk viscous FLRW cosmological models in a recently proposed extended symmetric teleparallel gravity or fQ,T gravity in which Q is the non-metricity and T is the trace of the energy–momentum tensor. We consider a simple coupling between matter and non-metricity, specifically, fQ,T=αQm+1+λT and fQ,T=αQ+λT where α, λ and m are free model parameters. The exact cosmological solutions are found by assuming the scale factor in the form of the hybrid expansion law. This type of relation generates a time-varying deceleration parameter with the transition of the Universe from the early decelerating phase to the present accelerating phase. In the presence of viscous fluid, we analyze some cosmological parameters of our cosmological model such as the energy density, bulk viscous pressure, bulk viscous coefficient, equation of state parameter, and energy conditions. Finally, we conclude our fQ,T cosmological models agree with the recent astronomical observations.

Roles of a periodic time varying deceleration parameter in particle creation with (d+2) dimensional FLRW Universe

In the present paper, we investigate the cosmic behavior of particle creation (PC) in (d+2) dimensional FLRW (Friedmann–Lemaitre–Robertson–Walker) models with time dependent gravitational constant and cosmological constant. The gravitational field equations are solved by assuming a periodically time-varying deceleration parameter (PTVDP) i.e. q=mcoskt−1 (Shen and Zhao, 2014) which produces a scale factor a(t)=a0tankt21m, where a0 shows current scale factor and k displays the PTVDP periodicity and can be regarded as a parameter of cosmic frequency, m is an enhancement element that increases the PTVDP peak. Here, we studied the periodic time-varying behavior of a few quantities, such as the deceleration parameter q, the energy density ρ, the entropy S, PC rate ψ, the cosmological constant Λ, Newton’s gravitational constant G and discuss their physical significance. We have also discussed the density parameter, proper distance, angular distance, luminosity distance, apparent magnitude, age of the Universe, and the look-back time with redshift z and have observed the role of particle formation in universe evolution in early and late times. The periodic nature of various physical parameters is also discussed which are supporting the recent observations.

Cosmological parameters in Barrow holographic dark energy

This work is dedicated to the cosmological scenario of the Barrow holographic dark energy (BHDE) in the light of time varying deceleration parameter. We investigate the evolution of a spatially flat Friedmann–Robertson–Walker (FRW) universe filled with pressure-less dark matter and BHDE density. We analyze the evolutionary history of essential cosmological parameters, including the deceleration parameter, the equation of state parameter, and the density parameter, using the Hubble horizon as the IR cut-off, and find appropriate characteristics in the BHDE model. Also, we examine the various structures of the model by reconstructing the scalar field potential to describe the universe accelerated expansion phase. For more accuracy of the dark energy model characteristics, we include the statefinder pair (r,s) which mimics the [Formula: see text]CDM model.

On a Bianchi type-I space-time with bulk viscosity in f(R,T) gravity

In this paper, we present a spatially homogeneous and anisotropic Bianchi type-I cosmological model with a viscous bulk fluid in [Formula: see text] gravity where [Formula: see text] and [Formula: see text] are the Ricci scalar and trace of the energy-momentum tensor, respectively. The field equations are solved explicitly using the hybrid law of the scale factor, which is related to the average Hubble parameter and gives a time-varying deceleration parameter (DP). We found the deceleration parameter describing two phases in the universe, the early deceleration phase [Formula: see text] and the current acceleration phase [Formula: see text]. We have calculated some physical and geometric properties and their graphs, whether in terms of time or redshift. Note that for our model, the bulk viscous pressure [Formula: see text] is negative and the energy density [Formula: see text] is positive. The energy conditions and the [Formula: see text] analysis for our spatially homogeneous and anisotropic Bianchi type-I model are also discussed.

Time Varying Deceleration Parameter in <i>f</i>(<i>R</i>, <i>T</i>) Gravity

In the present study, a homogeneous and anisotropic LRS Bianchi type I universe model is considered in f(R, T) theory of gravity. In order to find an exact solution of the field equations of the model, the model presented is based on a unique condition of periodically time varying deceleration parameter. The physical and geometrical characteristics of the universe model have been studied. It has been shown that the model has point-type singularity and all the cosmological parameters possess periodic time behavior. The model has a cyclic expansion history, for example, the model starts with the decelerating expansion, and later it transits to an accelerating phase of expansion and then goes to super-exponential phase of expansion in a period.

Phase transition of LRS Bianchi type-I cosmological model in f(R,T) gravity

In this work, LRS Bianchi type-I cosmological model with perfect fluid source in [Formula: see text] gravity theory, where [Formula: see text] is the Ricci scalar and [Formula: see text] is the trace of the stress energy-momentum tensor, has been studied in order to investigate early time deceleration and late time acceleration of the universe. By proposing a new special form of time-varying deceleration parameter in terms of Hubble parameter, the exact solution of the field equations has been obtained. The physical and geometric quantities of the model have been derived and their evolution has been discussed. Our model has an initial singularity and initially exhibits decelerating expansion and transits to accelerating expansion phase at last eras. The nature of the matter source of the model is consistent with the standard model in frame of the structure formation.

Quadratically varying deceleration parameter in f(R,T) gravity

In this paper, we have proposed a new form for the varying deceleration parameter that is a generalization of the form of Ö. Akarsu and T. Dereli [Cosmological models with linearly varying deceleration parameter, Int. J. Theor. Phys. 51(2012) 612]. LRS Bianchi type-I cosmological model filled with a perfect fluid source in [Formula: see text] gravity theory, where [Formula: see text] is the Ricci scalar and [Formula: see text] is the trace of the stress energy–momentum tensor, has been studied in order to investigate early time deceleration and late time acceleration of the universe by using this new form of time-varying deceleration parameter. The time evolution of physical and dynamical parameters have been analyzed and shown by graphs. Moreover, the deceleration parameter has been considered in terms of redshift. It has been shown that the model starts with a big bang and ends with a big rip. It is filled by a quintessence like fluid at the early time and by a phantom like fluid at the late time.

Mixed fluid cosmological model in f(R, T) gravity

We construct a locally rotationally symmetric (LRS) Bianchi type-I cosmological model in f(R, T) theory of gravity when the source of gravitation is a mixture of barotropic fluid and dark energy (DE) by employing a time-varying deceleration parameter. We observe through the behavior of the state finder parameters (r, s) that our model begins from the Einstein static era and goes to ΛCDM era. The equation of state (EOS) parameter (ωd) for DE varies from the phantom (ω < –1) phase to quintessence (ω > –1) phase, which is consistent with observational results. It is found that the discussed model can reproduce the current accelerating phase of the expansion of the universe.

Anisotropic bulk viscous string cosmological models of the Universe under a time-dependent deceleration parameter

We investigate a new class of LRS Bianchi type-II cosmological models by revisiting in the paper of Mishra {\it et al} (2013) by considering a new deceleration parameter (DP) depending on the time in string cosmology for the modified gravity theory suggested by S$\acute{a}$ez \& Ballester (1986). We have considered the energy-momentum tensor proposed by Leteliar (1983) for bulk viscous and perfect fluid under some assumptions. To make our models consistent with recent astronomical observations, we have used scale factor (Sharma {\it et al} 2018; Garg {\it et al} 2019) $ a(t)=\exp{[\frac{1}{\beta}\sqrt{2 \beta t + k}]}$, where $\beta $ and $k$ are positive constants and it provides a time-varying DP. By using the recent constraints ($H_{0}=73.8$, and $q_{0} = -0.54$) from SN Ia data in combination with BAO and CMB observations (Giostri {\it et al}, arXiv:1203.3213v2[astro-ph.CO]), we affirm $\beta = 0.0062$ and $k = 0.000016$. For these constraints, we have substantiated a new class of cosmological transit models for which the expansion takes place from early decelerated phase to the current accelerated phase. Also, we have studied some physical, kinematic and geometric behavior of the models, and have found them consistent with observations and well established theoretical results . We have also compared our present results with those of Mishra {\it et al} (2013) and observed that the results in this paper are much better, stable under perturbation and in good agreement with cosmological reflections.

Bianchi–III Transitioning Space-Time in a Nonsingular Hybrid Universe within Lyra’s Cosmology

The existence of Lyra’s cosmology is studied with the least interaction between normal matter and dark energy using Bianchi-III space-time in a nonsingular hybrid universe. The model explains the phase transition of the universe from deceleration to acceleration in the presence of a perfect fluid and naturewise anisotropic dark energy interacting very minimally to keep the energy-momentum tensors conserved independently. We take the hybrid average scale factor as $$a=(t^{m}e^{\lambda t})^{1/n}$$, where $$m,\lambda,n$$ are positive constants, which, with a time-varying deceleration parameter, describes both the early and late time features of universe. The time-varying displacement $$\beta(t)$$ of Lyra’s manifold used in the present model correlates with the nature of the cosmological constant $$\Lambda(t)$$. The physical and geometric properties of the model for a nonuniformly accelerated universe are also discussed with a comparison with other models.

Axially magnetized dark energy cosmological model

We investigate the behavior of the skewness parameters for an anisotropic universe in the framework of General Relativity. Non-interacting dark energy is considered in presence of electromagnetic field. A time-varying deceleration parameter simulated by a hybrid scale factor is considered. The dynamics of the universe is investigated in presence and absence of magnetic field. The equation of state parameter of dark energy evolves within the range predicted by the observations. Magnetic field is observed to have a substantial effect on the cosmic dynamics and the skewness parameters. The models discussed here end in a big rip and become isotropic at finite time.

Tsallis holographic dark energy in FRW universe with time varying deceleration parameter

By considering the time-varying deceleration parameter (DP), we have investigated Tsallis holographic dark energy (THDE), infrared cut-off with the Hubble horizon proposed by Tavayef, et al. (2018) in the framework of Friedmann Robertson Walker universe. The proposed THDE model describes that the universe is in an accelerating expansion mode. Interestingly enough, the EoS parameter in our model is representing the outstanding evolution of the universe as, the quintessence era (ωT≥−1), phantom era (ωT<−1), according to the value of dimensionless parameter δ. To analyze the different structure of the model, we try to accommodate the two perspectives of (DE) by reconstructing the scalar field potential, which depicts the accelerated expansion at present. We examine the validity of the model by drawing the statefinder parameters and comparing them with the recent observational outcomes of cosmography.

Stability of LRS Bianchi type-I cosmological models in f(R, T)-gravity

This paper examines the stability of the transition from the early decelerating stage of the Universe to the recent accelerating stage for the perfect fluid cosmological {locally rotationally symmetric (LRS) Bianchi-I model in f(R, T) theory. To determine the solution of field equations, the idea of a time-varying deceleration parameter (DP) which yields a scale factor, for which the Universe attains a phase transition scenario and is consistent with recent cosmological observations, is used. The time-dependent DP yields a scale factor , where β and k are respectively arbitrary and integration constants. By using the recent constraints (H0 = 73.8, and q0 = −0.54) from Type Ia Supernova (SN Ia) data in combination with Baryonic Acoustic Oscillations (BAO) and Cosmic Microwave Background (CMB) observations (Giostri et al.), we obtain the values of β = 0.0062 and k = 0.000016 for which we have derived a cosmological model from the early decelerated phase to the present accelerating phase. By applying other recent constraints (H0 = 73.8, q0 = −0.73) from SNe Ia Union data (Cunha), we obtain the values of β = 0.0036 and k = 0.000084 for which we have derived a cosmological model in the accelerating phase only. We have compared both models with experimental data. The stability of the background solution has been examined also for the metric perturbations alongside the properties of future singularities in a Universe ruled by dark energy with phantom type fluid. We demonstrate the presence of a stable fixed point with a condition of state ω < − 1 and numerically affirm this is really a late-time attractor in the ghost overwhelmed Universe. Some physical and geometric properties of the model are found and examined.

Bulk viscous embedded hybrid dark energy models

In this paper, we have constructed the cosmological model of the universe in a two-fluid environment with a newly developed mathematical formalism. In order to construct the model, Binachi type V (BV) space time is considered with a time varying deceleration parameter. Both the fluids, the viscous fluid and the dark energy (DE) fluid have shown their dominance respectively in early time and late time of the cosmic evolution. The scale factor that simulates the cosmic transition based on the value of the bulk viscous coefficient. Within the developed formalism, a general form of the skewness parameters is also obtained as a functional form of the scale factor. The physical parameter of the model such as equation of state (EoS) parameter is also derived and analysed. The state finder diagnostic pair is also obtained to understand the geometrical nature of the model.