arXiv subjects

Qing-Jun Zhang

Publications and source records attributed to Qing-Jun Zhang.

4 recordsLinked to original sources

arXiv2011

Modelling Time-varying Dark Energy with Constraints from Latest Observations

We introduce a set of two-parameter models for the dark energy equation of state (EOS) $w(z)$ to investigate time-varying dark energy. The models are classified into two types according to their boundary behaviors at the redshift $z=(0,\infty)$ and their local extremum properties. A joint analysis based on four observations (SNe + BAO + CMB + $H_0$) is carried out to constrain all the models. It is shown that all models get almost the same $χ^2_{min}\simeq 469$ and the cosmological parameters $(Ω_M, h, Ω_bh^2)$ with the best-fit results $(0.28, 0.70, 2.24)$, although the constraint results on two parameters $(w_0, w_1)$ and the allowed regions for the EOS $w(z)$ are sensitive to different models and a given extra model parameter. For three of Type I models which have similar functional behaviors with the so-called CPL model, the constrained two parameters $w_0$ and $w_1$ have negative correlation and are compatible with the ones in CPL model, and the allowed regions of $w(z)$ get a narrow node at $z\sim 0.2$. The best-fit results from the most stringent constraints in Model Ia give $(w_0,w_1) = (-0.96^{+0.26}_{-0.21}, -0.12^{+0.61}_{-0.89})$ which may compare with the best-fit results $(w_0,w_1) = (-0.97^{+0.22}_{-0.18}, -0.15^{+0.85}_{-1.33})$ in the CPL model. For four of Type II models which have logarithmic function forms and an extremum point, the allowed regions of $w(z)$ are found to be sensitive to different models and a given extra parameter. It is interesting to obtain two models in which two parameters $w_0$ and $w_1$ are strongly correlative and appropriately reduced to one parameter by a linear relation $w_1 \propto (1+w_0)$.

astro-ph.CO↗

arXiv2010

Time-Varying Dark Energy Constraints From the Latest SN Ia, BAO and SGL

Based on the latest SNe Ia data provided by Hicken et al. (2009) with using MLCS17 light curve fitter, together with the Baryon Acoustic Oscillation(BAO) and strong gravitational lenses(SGL), we investigate the constraints on the dark energy equation-of-state parameter $w$ in the flat universe, especially for the time-varying case $w(z)=w_0+w_zz/(1+z)$. The constraints from SNe data alone are found to be: (a) $(Ω_M, w)=(0.358, -1.09)$ as the best-fit results; (b) $(w_0, w_z)=(-0.73^{+0.23}_{-0.97}, 0.84^{+1.66}_{-10.34})$ for the two parameters in the time-varying case after marginalizing the parameter $Ω_M$; (c) the likelihood of parameter $w_z$ has a high non-Gaussian distribution; (d) an extra restriction on $Ω_M$ is necessary to improve the constraint of the SNe Ia data on the parameters ($w_0$, $w_z$). A joint analysis of SNe Ia data and BAO is made to break the degeneracy between $w$ and $Ω_M$, and leads to the interesting maximum likelihoods $w_0 = -0.94$ and $w_z = 0$. When marginalizing the parameter $Ω_M$, the fitting results are found to be $(w_0, w_z)=(-0.95^{+0.45}_{-0.18}, 0.41^{+0.79}_{-0.96})$. After adding the splitting angle statistic of SGL data, a consistent constraint is obtained $(Ω_M, w)=(0.298, -0.907)$ and the constraints on time-varying dark energy are further improved to be $(w_0, w_z) = (-0.92^{+0.14}_{-0.10}, 0.35^{+0.47}_{-0.54})$, which indicates that the phantom type models are disfavored.

astro-ph.CO↗

arXiv2009

Dark Energy and Hubble Constant From the Latest SNe Ia, BAO and SGL

Based on the latest MLCS17 SNe Ia data provided by Hicken et al. (2009), together with the Baryon Acoustic Oscillation (BAO) and strong gravitational lenses (SGL), we investigate the dark energy equation-of-state parameter for both constant $w$ and time-varying $w(z)=w_0+w_az/(1+z)$ in the flat universe, and its correlation with the matter density $Ω_M$ and Hubble constant $h$. The constraints from SNe data alone arrive at: (a) the best-fit results are $(Ω_M, w, h)=(0.358, -1.09, 0.647)$, while both $Ω_M$ and $w$ are very sensitive to the difference $Δh =h-\tilde{h}$ of the Hubble constant deviating to the prior input $\tilde{h}=0.65$; (b) the likelihoods of parameters are found to be: $w = -0.88^{+0.31}_{-0.39}$ and $Ω_M=0.36^{+0.09}_{-0.15}$, which is consistent with the $Λ\rm CDM$ at 95% C.L.; (c) the two parameters in the time-varying case are found to be $(w_0, w_a)=(-0.73^{+0.23}_{-0.97}, 0.84^{+1.66}_{-10.34})$ after marginalizing other parameters; (d) there is a clear degeneracy between constant $w$ and $Ω_M$, which depresses the power of SNe Ia to constrain both of them; (e) the likelihood of parameter $w_a$ has a high non-Gaussian distribution; (f) an extra restriction on $Ω_M$ is necessary to improve the constraint of the SNe Ia data on ($w_0$, $w_a$). A joint analysis of SNe Ia data and BAO is made to break the degeneracy between $w$ and $Ω_M$, and it provides a stringent constrain with the likelihoods: $w = -0.88^{+0.07}_{-0.09}$ and $Ω_M=0.29^{+0.02}_{-0.03}$. For the time-varying $w(z)$, it leads to the interesting maximum likelihoods $w_0 = -0.94$ and $w_a = 0$. When marginalizing the parameters $Ω_M$ and $h$, the fitting results are found to be $(w_0, w_a)=(-0.95^{+0.45}_{-0.18}, 0.41^{+0.79}_{-0.96})$.

astro-ph.CO↗

arXiv2008

Constraining Dark Energy From Splitting Angle Statistic of Strong Gravitational Lenses

Utilizing the CLASS statistical sample, we investigate the constraint of the splitting angle statistic of strong gravitational lenses(SGL) on the equation-of-state parameter $w=p/ρ$ of the dark energy in the flat cold dark matter cosmology. Through the comoving number density of dark halos described by Press-Schechter theory, dark energy affects the efficiency with which dark-matter concentrations produce strong lensing signals. The constraints on both constant $w$ and time-varying $w(z)=w_0+w_az/(1+z)$ from the SGL splitting angle statistic are consistently obtained by adopting a two model combined mechanism of dark halo density profile matched at the mass scale $M_c$. Our main observations are: (a) the resulting model parameter $M_c$ is found to be $M_c \sim 1.4$ for both constant $w$ and time-varying $w(z)$, which is larger than $M_c \sim 1$ obtained in literatures; (b) the fitting results for the constant $w$ are found to be $w =-0.89^{+0.49}_{-0.26}$ and $w =-0.94^{+0.57}_{-0.16}$ for the source redshift distributions of the Gaussian models $g(z_s)$ and $g^c(z_s)$ respectively, which are consistent with the $Λ\rm CDM$ at 95% C.L; (c) the time-varying $w(z)$ is found to be for $σ_8 = 0.74$: $(M_c; w_0, w_a)=(1.36; -0.92, -1.31)$ and $(M_c; w_0, w_a)=(1.38; -0.89, -1.21)$ for $g(z_s)$ and $g^c(z_s)$ respectively.

astro-ph↗