Narrow Results

In this paper, two simple parametrized deceleration parameters and areconstrained by using the latest SNe Ia Gold dataset, observational Hubble data and their combination. It is found that the transition redshift from decelerated expansion to accelerated expansion z_T and current decelerated parameter values q₀ are consistent with each other in 1σ region by only using SNe Ia Gold dataset and observational Hubble data in three parametrizations respectively. By combining the SNe Ia Gold dataset and observational Hubble data together, a tight constraint is obtained. With this combined constraint, z_T is and with 1σ error in the parametrizations respectively.

In this paper, the 1σ confidence level of transition redshift and present deceleration parameter are studied using modified Chaplygin gas (MCG) model from recent observations: the 182 Gold SNe Ia, three-year WMAP, SDSS baryon acoustic peak and X-ray gas mass fraction in clusters. Concretely, the best fit values of the transition redshift and present deceleration parameter with the 1σ confidence level are and , which indicate the best fit values of z_T and q₀ for MCG case are surprisingly close to the central values given by Refs. 20 and 21. Furthermore, we plot the evolution of cosmological quantities from MCG model by using the best-fit parameters given by combining constraint, and compare them with ΛCDM model.

In this paper, a parametrized deceleration parameter q(a) = q₀+q₁(1-a) is constrained by using the current cosmic observational data from type Ia Supernova (Sne Ia) and Cosmic Microwave Background Radiation (CMB). When the CMB dataset is added as a strong constraint, it is found that the 1σ error is largely reduced. The values of transition redshift z_T from decelerated expansion to accelerated expansion and current deceleration parameter q₀ are larger than that obtained from the case where Sne Ia dataset is used alone. With comparison to the case of Sne Ia 182 dataset used,¹⁵ it is found that the value of transition redshift is smaller than that in Sne 192 dataset case. This is the so-called dataset dependence.

The present-day accelerated expansion of the universe is naturally addressed within the Brans–Dicke theory just by using holographic dark energy model with inverse of Hubble scale as IR cutoff and power law temporal behavior of scale factor. It is also concluded that if the universe continues to expand, then one day it might be completely filled with dark energy.

In this paper, we reconstruct the late-time cosmological dynamics using a purely kinematic approach. In particular, considering a divergence-free parametrization for deceleration parameter $q$, we first derive the jerk parameter $j$ and then confront it with combination of various cosmological datasets. We use the most recent observational datasets consisting of the 1048 Pantheon Supernovae Ia data points in the redshift range $0.01<z<2.3$, the 51 data points of observational Hubble parameter (OHD) measurements in the redshift range $0.07\le z\le 2.36$, the Hubble constant $H_0$ (R19) and the CMB shift parameter measurements. We study the evolution of different cosmological quantities for the present model and compare it with the concordance $\mathrm{\Lambda }$CDM model. We find that only the combined Pantheon+OHD+R19 data shows good agreement with the $\mathrm{\Lambda }$CDM $(j=1)$ model within $1\sigma$ confidence region. We also find that our model successfully generates late time cosmic acceleration along with a decelerated expansion in the past.

Constraints on a parametrized deceleration parameter, q(a) = q₀ + q₁(1 - a), are investigated using cosmological observations of type Ia supernovae (SN Ia), baryon acoustic oscillations (BAOs) and cosmic microwave background (CMB) and observational Hubble data (OHD) which correspond to the cosmological distance measure. When a high redshift dataset of CMB is added in the SN + BAO + OHD case, a stronger constraint is obtained than for the lower redshift case, SN + BAO + OHD. The evolutions of the deceleration and Hubble parameters with respect to redshift z are reconstructed from cosmic observations. With cosmic observations as constraints, it is found that a smaller current deceleration parameter and a larger transition redshift are obtained in the SN + BAO + OHD case than in the SN + BAO + OHD + CMB case. In the SN + BAO + OHD + CMB case, slightly larger Hubble parameter values will be obtained than in the SN + BAO + OHD case.