Flow and flow fluctuations in relativistic heavy-ion collisions

The results from experiments at the BNL Relativistic Heavy Ion
Collider (RHIC)  and the recently-begun heavy-ion program at the CERN
Large Hadron Collider (LHC) indicate strong collective behavior of the
collision medium.  This is especially apparent from the large value of
a much-studied observable called "elliptic flow", which measures an
anisotropy in the distribution of detected particles via the second
Fourier harmonic in azimuthal angle, indicating that the medium
behaves like a very low-viscosity fluid.  Recently it was noticed that
event-by-event fluctuations can generate significant anisotropy in
other Fourier harmonics, with implications that were not previously
appreciated.  In particular, all long-range two-particle correlations
can be explained by collective flow alone.  In addition, this new
understanding suggests a large number of additional independent flow
observables that can be measured and used to tightly constrain
theoretical models.  Some of these data have recently been released.
After an introduction on flow in heavy-ion collisions I summarize
recent progress in understanding the effects of flow fluctuations and
discuss what can be learned from these new flow measurements.