Understanding the properties of extreme phases of nuclear matter created in relativistic heavy ion collisions is one of the major challenges in theoretical physics. Matter under such extreme conditions was present in the very early universe and is nowadays produced in relativistic heavy ion collisions. In this proposal we address two questions that are central to the understanding of the very early stages of such collisions from the perspective of both strongly and weakly coupled field theory: What is the time-scale of the thermalization and why does it happen so fast?We will study these questions in the strongly coupled setup via the gauge/gravity duality by utilizing numerical relativity techniques to describe black hole formation in the gravity side. This allows to access real-time dependent non-equilibrium dynamical quantities and will improve the understanding of pre-equilibrium flow in heavy ion collisions. In order to understand thermalization at intermediate couplings we will carry out a comparison between strong and weak coupling approaches.