Dynamic particle tracking reveals the aging temperature of a colloidal glass

Understanding glasses is considered to be one of the most fundamental problems in statistical physics. A theoretical approach to unravel their universal properties is to consider the validity of equilibrium concepts such as temperature and thermalization in these out-of-equilibrium systems. Here we investigate the autocorrelation and response function to monitor the aging of a colloidal glass. At equilibrium, all the observables are stationary while in the out-of-equilibrium glassy state they have an explicit dependence on the age of the system. We find that the transport coefficients scale with the aging-time as a power-law, a signature of the slow relaxation. Nevertheless, our analysis reveals that the glassy system has thermalized at a constant temperature independent of the age and larger than the bath, reflecting the structural rearrangements of cage-dynamics. Furthermore, a universal scaling law is found to describe the global and local fluctuations of the observables.