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Abstract

Understanding hydrodynamization in microscopic models of heavy-ion collisions
has been an important topic in current research. Many lessons obtained within
the strongly-coupled (holographic) models originate from the properties of
transient excitations of equilibrium encapsulated by short-lived quasinormal
modes of black holes. The aim of this paper is to develop similar intuition for
expanding plasma systems described by, perhaps, the simplest model from the
weakly-coupled domain, i.e. the Boltzmann equation in the relaxation time
approximation. We show that in this kinetic theory setup there are infinitely
many transient modes carrying at late times the vast majority of information
about the initial distribution function. They all have the same exponential
damping set by the relaxation time but are distinguished by different power-law
suppressions and different frequencies of very slow, logarithmic in proper
time, oscillations. Finally, we analyze the resurgent interplay between the
hydrodynamics and transients. In particular, show that there are choices of
relaxation time dependence on temperature for which the asymptotics of the
divergent hydrodynamic series is dominated not be the least damped transient,
but rather by an unphysical exponential correction having to do with
non-analyticities of the equation of motion in complexified time variable.