Abstract

Neutron star mergers are cosmic laboratories of (not particularly well understood) physics. The modelling requires large scale simulation of extremely hot and dense matter, likely far away from equilibrium conditions. The live spacetime of general relativity is an absolute requirement, as is an understanding of fluid dynamics beyond the textbook level. Framing the discussion in the context of state-of-the-art merger simulations and the need to understand the fine print of the associated gravitational-wave signals for the Einstein Telescope and Cosmic Explorer, I will connect a robust approach to modelling relativistic fluids (based on a variational principle) with progress in understanding out-of-equilibrium aspects (like bulk viscosity). I will also outline how we hope to deal with turbulence and small-scale fluctuations, tricky problems for everyday fluids but particularly so in general relativity.