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Shafieloo ea al. firstly proposed the possibility that the current cosmic acceleration (CA) is slowing down. However, this is rather counterintuitive because a slowing down CA cannot be accommodated in most mainstream cosmological models. In this work, by exploring the evolutionary trajectories of dark energy equation of state $w(z)$ and deceleration parameter $q(z)$, we present a comprehensive investigation on the slowing down of CA from both the theoretical and the observational sides. For the theoretical side, we study the impact of different $w(z)$ by using six parametrization models, and then discuss the effects of spatial curvature. For the observational side, we investigate the effects of different type Ia supernovae (SNe Ia), different baryon acoustic oscillation (BAO), and different cosmic microwave background (CMB) data, respectively. We find that (1) The evolution of CA are insensitive to the specific form of $w(z)$; in contrast, a non-flat Universe more favors a slowing down CA than a flat Universe. (2) SNLS3 SNe Ia datasets favor a slowing down CA at 1$\sigma$ confidence level, while JLA SNe Ia samples prefer an eternal CA; in contrast, the effects of different BAO data are negligible. (3) Compared with CMB distance prior data, full CMB data more favor a slowing down CA. (4) Due to the low significance, the slowing down of CA is still a theoretical possibility that cannot be confirmed by the current observations., Comment: 15 pages, 10 figures, 6 tables, accepted for publication in ApJ

In this work, we explore the cosmological implications of different baryon acoustic oscillation (BAO) data, including the BAO data extracted by using the spherically averaged one-dimensional galaxy clustering (GC) statistics (hereafter BAO1) and the BAO data obtained by using the anisotropic two-dimensional GC statistics (hereafter BAO2). To make a comparison, we also take into account the case without BAO data (hereafter NO BAO). Firstly, making use of these BAO data, as well as the SNLS3 type Ia supernovae sample and the Planck distance priors data, we give the cosmological constraints of the $\Lambda$CDM, the $w$CDM, and the Chevallier-Polarski-Linder (CPL) model. Then, we discuss the impacts of different BAO data on cosmological consquences, including its effects on parameter space, equation of state (EoS), figure of merit (FoM), deceleration-acceleration transition redshift, Hubble parameter $H(z)$, deceleration parameter $q(z)$, statefinder hierarchy $S^{(1)}_3(z)$, $S^{(1)}_4(z)$ and cosmic age $t(z)$. We find that: (1) NO BAO data always give a smallest fractional matter density $\Omega_{m0}$, a largest fractional curvature density $\Omega_{k0}$ and a largest Hubble constant $h$; in contrast, BAO1 data always give a largest $\Omega_{m0}$, a smallest $\Omega_{k0}$ and a smallest $h$. (2) For the $w$CDM and the CPL model, NO BAO data always give a largest EoS $w$; in contrast, BAO2 data always give a smallest $w$. (3) Compared with the case of BAO1, BAO2 data always give a slightly larger FoM, and thus can give a cosmological constraint with a slightly better accuracy. (4) The impacts of different BAO data on the cosmic evolution and the comic age are very small, and can not be distinguished by using various dark energy diagnosis and the cosmic age data., Comment: 12 pages, 7 figures, 3 tables, accepted by Sci. China Phys. Mech. Astron

Inspired by the multiverse scenario, we study a heterotic dark energy model in which there are two parts, the first being the cosmological constant and the second being the holographic dark energy, thus this model is named the $\Lambda$HDE model. By studying the $\Lambda$HDE model theoretically, we find that the parameters $d$ and $\Omega_{hde}$ are divided into a few domains in which the fate of the universe is quite different. We investigate dynamical behaviors of this model, and especially the future evolution of the universe. We perform fitting analysis on the cosmological parameters in the $\Lambda$HDE model by using the recent observational data. We find the model yields $\chi^2_{\rm min}=426.27$ when constrained by $\rm Planck+SNLS3+BAO+HST$, comparable to the results of the HDE model (428.20) and the concordant $\Lambda$CDM model (431.35). At 68.3\% CL, we obtain $-0.07<\Omega_{\Lambda0}<0.68$ and correspondingly $0.04<\Omega_{hde0}<0.79$, implying at present there is considerable degeneracy between the holographic dark energy and cosmological constant components in the $\Lambda$HDE model., Comment: 17 pages, 6 figures

We explore the cosmological consequences of interacting dark energy (IDE) models using the SNLS3 supernova samples. In particular, we focus on the impacts of different SNLS3 light-curve fitters (LCF) (corresponding to "SALT2", "SiFTO", and "Combined" sample). Firstly, making use of the three SNLS3 data sets, as well as the Planck distance priors data and the galaxy clustering data, we constrain the parameter spaces of three IDE models. Then, we study the cosmic evolutions of Hubble parameter $H(z)$, deceleration diagram $q(z)$, statefinder hierarchy $S^{(1)}_3(z)$ and $S^{(1)}_4(z)$, and check whether or not these dark energy diagnosis can distinguish the differences among the results of different SNLS3 LCF. At last, we perform high redshift cosmic age test using three old high redshift objects (OHRO), and explore the fate of the Universe. We find that, the impacts of different SNLS3 LCF are rather small, and can not be distinguished by using $H(z)$, $q(z)$, $S^{(1)}_3(z)$, $S^{(1)}_4(z)$, and the age data of OHRO. In addition, we infer, from the current observations, how far we are from a cosmic doomsday in the worst case, and find that the "Combined" sample always gives the largest 2$\sigma$ lower limit of the time interval between "big rip" and today, while the results given by the "SALT2" and the "SiFTO" sample are close to each other. These conclusions are insensitive to a specific form of dark sector interaction. Our method can be used to distinguish the differences among various cosmological observations., Comment: 12 pages, 7 figures, 2 tables, accepted for publication in Astronomy and Astrophysics

The Baryon Acoustic Oscillations (BAO) provide a standard ruler for studying cosmic expansion. The recent observations of BAO in SDSS DR9 and DR11 take measurements of $H(z)$ at several different redshifts. It is argued that the behavior of dark energy could be constrained more effectively by adding high-redshift Hubble parameter data, such as the SDSS DR11 measurement of $H(z) = 222\pm7$ km/sec/Mpc at z = 2.34. In this paper, we investigate the significance of these BAO data in the flat $\Lambda$CDM model, by combining them with the recent observational data of the Hubble constant from local distance ladder and the Cosmic Microwave Background (CMB) measurements from Planck+WP. We perform a detailed data analysis on these datasets and find that the recent observations of BAO in SDSS DR9 and DR11 have considerable tension with the Planck + WP measurements in the framework of the standard $\Lambda$CDM model. The fitting results show that the main contribution to the tension comes from the Hubble parameter measurement at redshift of $z=2.34$. But there is no visible tension once the joint data analysis by combining the datasets of SDSS and Planck+WP is performed. Thus in order to see whether dark energy does evolve, we need more independent measurements of the Hubble parameter at high redshifts., Comment: 3 pages, 2 figure, 2 tables

We investigate a second order $parabolic$ parametrization, $w(a)=w_t+w_a(a_t-a)^2$, which is a direct characterization of a possible $turning$ in $w$. The cosmological consequence of this parametrization is explored by using the observational data of the SNLS3 type Ia supernovae sample, the CMB measurements from WMAP9 and Planck, the Hubble parameter measurement from HST, and the baryon acoustic oscillation (BAO) measurements from 6dFGS, BOSS DR11 and improved WiggleZ. We found the existence of a turning point in $w$ at $a\sim0.7$ is favored at 1$\sigma$ CL. In the epoch $0.55< a< 0.9$, $w<-1$ is favored at 1$\sigma$ CL, and this significance increases near $a=0.8$, reaching a 2$\sigma$ CL. The parabolic parametrization achieve equivalent performance to the $\Lambda$CDM and Chevallier-Polarski-Linder (CPL) models when the Akaike information criterion was used to assess them. Our analysis shows the value of considering high order parametrizations when studying the cosmological constraints on $w$., Comment: 8 pages, 2 figures, 3 tables