T he glass transition 1 is considered to be one of the most fundamental problems in statistical physics. Despite decades of effort, a general consensus on the validity of a universal theory for the large variety of glass systems is lacking 2,3 -partly because of difficulties encountered in the experimental testing of the theoretical predictions 4,5 . Here, we present experiments on a colloidal glass made of micrometresized particles in a fluid. We investigate the autocorrelation and response function to monitor the ageing of a colloidal glass. At equilibrium, all the observables are stationary, whereas 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 ageing 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 warmer than the bath, reflecting the structural rearrangements of cage dynamics. Furthermore, we find a universal scaling law to describe the global and local fluctuations of the observables.Increasing the volume fraction of a colloidal system slows down the brownian dynamics of its constitutive particles, implying a limiting density, φ g , above which the system can no longer be equilibrated with its bath 1 . Hence, the thermal system falls out of equilibrium on the timescale of the experiment and thus undergoes a glass transition 2 . Even above φ g the particles continue to relax, but the nature of the relaxation is very different to that in equilibrium. This phenomenon of a structural slow evolution beyond the glassy state is known as 'ageing' 3 . The system is no longer stationary and the relaxation time is found to increase with the age of the system, t w , as measured from the time of sample preparation.This picture applies not only to structural glasses such as colloids, silica and polymer melts, but also to spin glasses, ferromagnetic coarsening, elastic manifolds in quenched disorder and jammed matter such as grains and emulsions 2,4,5 . Theories originally developed in the field of spin glasses 6 attempt to develop a common framework for the understanding of ageing. For example, the structural glass and spin-glass transitions have been coupled by the low-temperature extension of the modecoupling theory 4,5,7 . More generally, this approach is related to analogous ideas developed in the field of granular matter such as compactivity [8][9][10][11] , and the inherent structure formalisms 12 , adapted to the energy landscape of glasses 13 .One of the important features of this scenario is a separation of timescales where the observables are equilibrated at different temperatures, even though the system is far from equilibrium. Although theoretical results have flourished, the difficulties in the experimental testing of the fundamental predictions of the theories have hampered the development of an understanding of ageing in glasses [14][15][16][17][18] . Experiments so f...