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[116]. Unfortunately even these highly computationally intensive calculations that follow evolution all the way until free streaming still do not describe the observed HBT radii. 

The second open issue is the role of viscous corrections. Past 3 + 1 D rel- ativistic hydrodynamic treatments of Rhic were all ideal; the viscous terms were explicitly set to zero. There have been some attempts to quantify the magnitude of these corrections [121]. However the rigorous treatment of vis- cosity in relativistic hydrodynamics is still an open problem. The inclusion of only the first order correction terms leads to unstable solutions with acausal 

