Long-time dynamical fluctuations in glassy systems: collective relaxation, density correlations, and single-particle fluctuations

Liquids near the glass transition exhibit dynamical heterogeneity, i.e. correlated regions in the liquid relax at either a much faster rate or a much slower rate than the average. This is a collective phenomenon that has been characterized by measurements that attempt to determine the size of these regions and the intensity of the fluctuations. Here we show that the results of those measurements can be affected not only by the collective fluctuations in the relaxation rate, but also by density fluctuations in the initial state and by single-particle fluctuations, which can become dominant for very long timescales. We introduce a method to subtract the effect of those extra fluctuations, and apply it to numerical simulations of two glass forming models: a binary hard sphere system and a Kob-Andersen Lennard-Jones system. This method allows us to extend the analysis of numerical data to timescales much longer than previously possible, and opens the door for further future progress in the study of dynamic heterogeneities, including the determination of the exchange time.