Universal hydrodynamics and charged hadron multiplicity at energies available at the CERN Large Hadron Collider

Time evolution of a "little bang" created in heavy-ion collisions can be divided into two phases, the pre-equilibrium and hydrodynamic. At what moment does the evolution become hydrodynamic and is there any universality in the hydrodynamic flow? To answer these questions we briefly discuss various versions of hydrodynamics and their applicability conditions. In particular, we elaborate on the idea of "universal" (all-order resumed) hydrodynamics and propose a simple new model for it. The model is motivated by results obtained recently via the Anti-de Sitter and conformal field correspondence. Finally, charged hadron multiplicities in heavy-ion collisions at the Relativistic Heavy Ion Collider and Large Hadron Collider are discussed. At the freeze-out, the multiplicities can be related to total entropy produced in the collision. Assuming the universal hydrodynamics to hold, we calculate the entropy production in the hydro stage of the collision. We end up speculating about a connection between the multiplicity growth and the temperature dependence of the quark-gluon plasma viscosity.