  Using gauge/gravity duality, central ultrarelativistic nucleus-nucleus
collisions are modelled as collisions of shock waves in five-dimensional
asymptotic AdS space. For early times after the collision, it is possible to
analytically match the metric from the past to the future light-cone. This
allows extraction of the pre-equilibrium energy-momentum tensor of the strongly
coupled, large N gauge theory. For central collisions, this allows qualitative
statements concerning the build-up of radial flow at mid-rapidity in AA and pA
collisions. We find that the early-time radial flow buildup is identical to
that expected from ideal hydrodynamics with an entropy density proportional to
the square root of the product of the matter densities in the individual
"nuclei".
