Hydrodynamics and fluctuations in relativistic heavy-ion collisions

Ultrarelativistic heavy-ion collisions at the RHIC collider in
Brookhaven create a small lump of strongly-interacting matter which freely expands into the vacuum and eventually reaches detectors as particles. Detailed analyses of data suggest that the matter formed at RHIC is so strongly interacting that it behaves like a fluid, whose expansion follows the rule of relativistic hydrodynamics. The viscosity to entropy ratio of this fluid is lower than for any other known substance. In this talk, I discuss recent developments in this field. In particular, I argue that that the fluid is not smooth, but granular, due to the nucleonic structure of atomic nuclei, and that the resulting fluctuations have large effects on several observables.