Observational constraints on unified dark matter including Hubble parameter data

We constrain a unified dark matter (UDM) model from the latest observational data. This model assumes that the dark sector is degenerate. Dark energy and dark matter are the same component. It can be described by an affine equation of state $P_X= p_0 +αρ_X$. Our data set contains the newly revised $H(z)$ data, type Ia supernovae (SNe Ia) from Union2 set, baryonic acoustic oscillation (BAO) observation from the spectroscopic Sloan Digital Sky Survey (SDSS) data release 7 (DR7) galaxy sample, as well as the cosmic microwave background (CMB) observation from the 7-year Wilkinson Microwave Anisotropy Probe (WMAP7) results. By using the Markov Chain Monte Carlo (MCMC) method, we obtain the results in a flat universe: $Ω_Λ$=$0.719_{-0.0305}^{+0.0264}(1σ)_{-0.0458}^{+0.0380}(2σ)$, $α$=$1.72_{-4.79}^{+3.92}(1σ)_{-7.30}^{+5.47}(2σ)(\times10^{-3})$, $Ω_bh^2$=$0.0226_{-0.0011}^{+0.0011}(1σ)_{-0.0015}^{+0.0016}(2σ)$. Moreover, when considering a non-flat universe, $Ω_Λ$=$0.722_{-0.0447}^{+0.0362}(1σ)_{-0.0634}^{+0.0479}(2σ)$, $α$=$0.242_{-0.775}^{+0.787}(1σ)_{-1.03}^{+1.10}(2σ)(\times10^{-2})$, $Ω_bh^2$=$0.0227_{-0.0014}^{+0.0015}(1σ)_{-0.0018}^{+0.0021}(2σ)$, $Ω_k$=$-0.194_{-1.85}^{+2.02}(1σ)_{-2.57}^{+2.75}(2σ)(\times10^{-2})$. These give a more stringent results than before. We also give the results from other combinations of these data for comparison. The observational Hubble parameter data can give a more stringent constraint than SNe Ia. From the constraint results, we can see the parameters $α$ and $Ω_k$ are very close to zero, which means a flat universe is strongly supported and the speed of sound of the dark sector seems to be zero.

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