Equation of State of Dark Matter after Planck Data
In this paper, we loosen the zero equation of state of dark matter to a constant $w_{dm}$. By using the currently available cosmic observations which include the type-Ia supernovae, the baryon acoustic oscillation, the WiggleZ measurements of matter power spectrum and the cosmic microwave background radiation from the firs release of {\it Planck} dat through the Markov chain Monte Carlo method, we found the equation of state of dark matter in $3σ$ regions: $w_{dm}=0.000707_{-0.000747-0.00149-0.00196}^{+0.000746+0.00146+0.00193}$. The difference of the minimum $χ^2$ between the $Λ$CDM and $Λ$wDM models is $Δχ^2_{min}=0.446$ for one extra model parameter $w_{dm}$. Although the currently available cosmic observations favor the $Λ$wDM mildly, no significant deviation from the $Λ$CDM model is found in $1σ$ regions.