  Using the Janik-Peschanski dual to a Bjorken flow, a Langevin equation is
derived for a heavy quark in an expanding N = 4 supersymmetric Yang-Mills
plasma. Such a plasma is characterized by a proper-time dependence of the
temperature and corresponds to a system out of equilibrium. The analysis first
focuses on a quark at rest in the plasma comoving frame. The case of a quark
moving across a longitudinally expanding plasma is then considered. The
two-point functions for the random noise acting on such heavy quark probes are
computed.
