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Probing non-equilibrium dynamics in critical active systems.
Gnan N., Maggi C., Paoluzzi M., Zaccarelli E., Crisanti A.
Active systems exhibit a wide range of non-equilibrium phenomena, such as motility-induced phase separation (MIPS) which leads to self-organization of active particles on large scales. Although MIPS arises from a non-equilibrium process, recent studies suggest that its critical point belongs to the equilibrium Ising universality class. It is therefore interesting to investigate what is the role played by non-equilibrium dynamics at the critical point of the MIPS. A powerful strategy to unveil non-equilibrium fluctuations is to search for violations of the Fluctuation Dissipation Theorem (FDT). By implementing massive computer simulations and a novel field theoretical description of a microscopic model of active particles, we reveal a strong violation of FDT at high frequencies and large wave-vectors even close to criticality. However, at low spatial and temporal frequencies, the response and correlator tend to coincide, validating the effective equilibrium picture at large scales. Our study unveils a strikingly different type of non-equilibrium behavior in critical active systems compared, for example, to glassy systems, highlighting the unique nature of active matter dynamics.