Determine whether second-order hydrodynamics restores universality

Determine whether a systematic second-order causal-frame treatment restores hydrodynamic universality at bulk viscosities for which first-order BDNK and Israel–Stewart descriptions exhibit universality breakdown, and whether the associated overtone and non-hydrodynamic mode features are physical or instead lie outside the second-order effective-field-theory regime.

Background

The paper finds that causal first-order hydrodynamic frames, including BDNK theory, depart from the universal hydrodynamic regime in sufficiently viscous neutron-star oscillations. In this regime, the first overtone can become longer lived than the fundamental mode, non-hydrodynamic modes can become dynamically important, and unphysical instabilities can appear.

The authors identify a systematic second-order causal-frame analysis as necessary to resolve whether these spectral features reflect genuine physics or are artifacts of leaving the applicable second-order effective-field-theory regime. They note that the second-order theory of Noronha et al. is suitable because its first-order truncation is BDNK theory and it contains Israel–Stewart theory as a special case.