Reconstructing dark energy potentials from parameterized deceleration parameters

In this paper, the properties of dark energy are investigated according to the parameterized deceleration parameter q（z）, which is used to describe the extent of the accelerating expansion of the universe. The potential of dark energy V（φ） and the cosmological parameters, such as the dimensionless energy density Ωφ, Ωm, and the state parameter wφ, are connected to it. Concretely, by giving two kinds of parameterized deceleration parameters q（z） = a ＋ bz/（1 ＋ z） and q（z） = 1/2 ＋ （az ＋ b）/（1 ＋ z）^2, the evolution of these parameters and the reconstructed potentials V（φ） are plotted and analysed. It is found that the potentials run away with the evolution of universe.

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.

Observational insights into the accelerating universe through reconstruction of the deceleration parameter

Recent developments in the exploration of the universe suggest that it is in an accelerated phase of ex-pansion.Accordingly,our study aims to probe the current scenario of the universe with the aid of the reconstruction technique.The primary factor that describes cosmic evolution is the deceleration parameter.Here,we provide a physically plausible,newly defined model-independent parametric form of the deceleration parameter.Further,we constrain the free parameters through statistical MCMC analysis for different datasets,including the most recent Pantheon+.With the statistically obtained results,we analyze the dynamics of the model through the phase trans-ition,EoS parameter,and energy conditions.Also,we make use of the tool Om diagnostic to test our model.

Observational constraints on the accelerating universe in the framework of a 5D bounce cosmological model

In the framework of a five-dimensional （5D） bounce cosmological model, a useful function f（z） is obtained by giving a concrete expression of deceleration parameter q（z） = q1 ＋q2/1＋1n（1＋z） Then using the obtained Hubble parameter H（z） according to the function f（z）, we constrain the accelerating universe from recent cosmic observations： the 192 ESSENCE SNe Ia and the 9 observational H（z） data. The best fitting values of transition redshift zT and current deceleration parameter q0 are given as zT =0.65-0.12^＋0.25 and q0=-0.76-0.15^＋0.15（1σ）. Furthermore, in the 5D bounce model it can be seen that the evolution of equation of state （EOS） for dark energy Wde can cross over -1 at about z = 0.23 and the current value W0de ： =-1.15 〈 -1. On the other hand, by giving a concrete expression of model-independent EOS of dark energy Wde, in the 5D bounce model we obtain the best fitting values zT = 0 .66-0.08^＋0.11 and q0=-0.69-0.10^＋0.10（1σ） from the recently observed data： the 192 ESSENCE SNe Ia, the observational H（z） data, the 3-year Wilkinson Microwave Anisotropy Probe （WMAP）, the Sloan Digital Sky Survey （SDSS） baryon acoustic peak and the x-ray gas mass fraction in clusters.

A stable flat universe with variable cosmological constant in f(R,T)gravity

In this paper, a general FRW cosmological model has been constructed in f（R, T） gravity reconstruction with variable cosmological constant. A number of solutions to the field equations has been generated by utilizing a form for the Hubble parameter that leads to Berman＇s law of constant deceleration parameter q =m - 1. The possible decelerating and accelerating solutions have been investigated. For （q 〉 0） we get a stable fiat decelerating radiation-dominated universe at q = 1. For （q 〈 0） we get a stable accelerating solution describing a flat universe with positive energy density and negative cosmological constant. Nonconventional mechanisms that are expected to address the late-time acceleration with negative cosmological constant have been discussed.

Bianchi Type-I Anisotropic Universe with Metric Potential in Saez-Ballester Theory of Gravitation

Bianchi Type-I cosmological model in the presence of Saez-Ballester theory gravitation is studied. An exact solution of the field equation is given by considering the cosmological model yield a metric potential included with a real number. The relation between the deceleration parameter and Hubble parameter and average scale factor is used in that cosmological model. The effect of the viscosity on the entropy of the universe is utilized by energy momentum tensor with bulk viscous terms in a conservative manner. We obtained a formula for calculating the entropy of the universe in terms of viscosity and used it to compare to the study. Also, various physical and kinematical properties have been discussed.

Investigation of the Cosmic evolution in the presence of non-relativistic neutrinos

The neutrinos of the early universe evolved from a relativistic phase at very early times to a massive particle behavior at later times.First,the kinetic energy of neutrinos is relativistic,and as a result,neutrinos can be described as massless particles.As the Universe expands,the temperature drops and the kinetic energy decreases,and the neutrinos turn into a non-relativistic phase with a non-negligible mass.In this paper,we first put constraints on the total mass of neutrinos.Then we investigate the effect of neutrinos on the CMB power spectrum,P(k),in the case of massless and massive neutrinos using the publicly available Boltzmann code CAMB and we prove that when neutrino coupled to scalar field the CMB power spectrum has a little shift,which means that the power spectrum of CMB is greatly affected by the background energy density and the accelerated expansion of the Universe.Furthermore,we investigate the effect of perturbed quintessence on this spectrum and find that the highest peaks of this spectrum are shifted to smaller scales.Also,we estimate the Deceleration-Acceleration(DA)redshift transition(z_(da))using the coupling canonical scalar field with neutrinos.For Pantheon data we obtain z_(da)=0.7±0.05 and for CC data z_(da)=0.68±0.03.In the presence of neutrinos the DA redshift transition is z_(da)=0.42±0.03 for Pantheon data and z_(da)=0.49±0.05 for CC data.These results indicate that neutrinos can affect this phase transition.The results obtained in this article show that when the mass of neutrinos increases,the value of the background energy density increases,resulting in a higher power spectrum peak.Also,by examining the effect of coupling neutrinos to dark energy,we find that the transition occurs at lower redshift.

Periodic cosmic evolution in f(Q)gravity formalism

We study the periodic cosmic transit behavior of the accelerated universe in the framework of symmetric teleparallelism. The exact solution of field equations is obtained by employing a wellknown deceleration parameter(DP) called periodic varying DP, q =m coskt-1. The viability and physical reliability of the DP are studied by using observational constraints. The dynamics of periodicity and singularity are addressed in detail with respect to time and redshift parameter.Several energy conditions are discussed in this setting.