The so-called grand canonical catastrophe of the density fluctuations in the ideal Bose gas is shown to be a particular instance of the much more general phenomenon of condensation of fluctuations, taking place in a large system, in or out of equilib
rium, when a single degree of freedom makes a macroscopic contribution to the fluctuations of an extensive quantity. The pathological character of the catastrophe is demystified by emphasizing the connection between experimental conditions and statistical ensembles, as demonstrated by the recent realization of photon condensation under grand canonical conditions.
The Chapter is devoted to reviewing the main features of aging in non disordered systems relaxing via domain growth, after an istantaneous temperature quench. Using the autocorrelation and autoresponse functions to gauge the deviation from equilibriu
m, an hierarchy of processes is obtained by changing the temperature of the quench and the space dimensionality. The special role of the lower critical dimensionality is highlighted. Basic properties like the separation of the time scales and the splitting of degrees of freedom into fast and slow variables are treated in detail, both in general terms and through analytical calculations in solvable models. Though domain growth is believed to be a well understood paradigma of slow relaxation, emphasis of this review is on the non trivial aspects, not yet fully clarified, like the response function scaling behavior.
Fluctuations of energy and heat are investigated during the relaxation following the instantaneous temperature quench of an extended system. Results are obtained analytically for the Gaussian model and for the large $N$ model quenched below the criti
cal temperature $T_C$. The main finding is that fluctuations exceeding a critical threshold do condense. Though driven by a mechanism similar to that of Bose-Einstein condensation, this phenomenon is an out-of-equilibrium feature produced by the breaking of energy equipartition occurring in the transient regime. The dynamical nature of the transition is illustrated by phase diagrams extending in the time direction.
