# A Little Warm Dark Matter: Constraint to its Equation of State after Planck

Is the dark matter warm or cold? That would be determined by the cosmic observations. In this paper, we loose the constraint to zero $w_{dm}$ and assume a constant equation of state (EoS) of dark matter. By using the currently available cosmic observations which include type-Ia supernovae (SN), baryon acoustic oscillation (BAO) and cosmic microwave background radiation (CMB) from the firs release of {\it Planck} through the Markov Chain Monte Carlo method, we found the EoS of dark matter in $3\sigma$ regions: $w_{dm}=0.00118_{-0.000817-0.00162-0.00212}^{+0.000823+0.00161+0.00211}$. It implies a little warm dark matter is favored up to $1\sigma$ region.

Date added: Tue, 8 Oct 13