Breakdown of hydrodynamic universality in causal first-order viscous neutron star oscillations
The study investigates how hydrodynamic universality breaks down in viscous neutron star oscillations using a causal first-order theory called BDNK. It shows that causality and finite-size effects restrict access to the universal regime when bulk viscosity reaches $\zeta \sim 10^{30}$--$10^{31}\mathrm{g}/\mathrm{cm}/\mathrm{s}$. This breakdown causes material changes in oscillation modes, including unphysical instabilities, suggesting first-order theories are insufficient for highly viscous neutron stars.