Hydrodynamic approaches to RHIC physics

One of the most intriguing findings in the experiments at Relativistic Heavy Ion Collider (RHIC) in Brookhaven National Laboratory (BNL) is a large magnitude of momentum anisotropy in comparison with the ones observed at lower collision energies. The momentum anisotropy in noncentral collisions is characterized by the second Fourier coefficient $v_2$ of the azimuthal distribution for observed particles. The magnitude of $v_2$ and its transverse momentum $p_T$ dependence for identified hadrons are comparable with results from \textit{ideal} hydrodynamic simulations around midrapidity ($\mid \eta \mid <\sim 1$), in low transverse momentum region ($p_T <\sim 1$ GeV/$c$), and up to semicentral collisions ($b < \sim 5$ fm). This is evidence for a recent announcement of the discovery of the perfect fluidity in the strongly coupled/interacting quark gluon plasma (sQGP) as distinct from the weakly coupled/interacting QGP (wQGP) which had been assumed to be created for a long time. Though the ideal hydrodynamic description at RHIC is apparently successful, this in turn raises a couple of new questions about hydrodynamic behavior of bulk matter produced in relativistic heavy ion collisions. In this talk, I will present current understanding of matter produced at RHIC and discuss open issues in modeling dynamics of relativistic heavy ion collisions based on hydrodynamics.