  The bulk viscosity determines dissipation during hydrodynamic expansion. It
vanishes in scale invariant fluids, while a nonzero value quantifies the
deviation from scale invariance. For the dilute Fermi gas the bulk viscosity is
given exactly by the correlation function of the contact density of local
pairs. As a consequence, scale invariance is broken purely by pair
fluctuations. These fluctuations give rise also to logarithmic terms in the
bulk viscosity of the high-temperature nondegenerate gas. For the quantum
degenerate regime I report numerical Luttinger-Ward results for the contact
correlator and the dynamical bulk viscosity throughout the BEC-BCS crossover.
The ratio of bulk to shear viscosity $\zeta/\eta$ is found to exceed the
kinetic theory prediction in the quantum degenerate regime. Near the superfluid
phase transition the bulk viscosity is enhanced by critical fluctuations and
has observable effects on dissipative heating, expansion dynamics and sound
attenuation.
