  Modern hydrodynamic simulations of core-collapse supernovae and neutron-star
mergers require knowledge not only of the equilibrium properties of strongly
interacting matter, but also of the system's response to perturbations, encoded
in various transport coefficients. Using perturbative and holographic tools, we
derive here an improved weak-coupling and a new strong-coupling result for the
most important transport coefficient of unpaired quark matter, its bulk
viscosity. These results are combined in a simple analytic pocket formula for
the quantity that is rooted in perturbative Quantum Chromodynamics at high
densities but takes into account nonperturbative holographic input at
neutron-star densities, where the system is strongly coupled. This expression
can be used in the modeling of unpaired quark matter at astrophysically
relevant temperatures and densities.
