Non-equilibrium aspects of neutron star mergers

Deviations from beta equilibrium impact on the fine-print details of the gravitational-wave signal from neutron-star binary inspiral and merger. During the inspiral phase, the individual neutron stars are cold enough that the Urca reactions cannot establish equilibrium on the inspiral timescale. This leads to the presence of composition g-modes which may become resonant as the system evolves through the sensitivity band of ground-based detectors. In contrast, during the hot post-merger phase the reactions are expected to be fast enough that they need to be accounted for (at least in parts of parameter space). In this talk I will tie together the two phases of the binary problem, outlining the role of composition effects during the inspiral as well as the emergence of an effective bulk viscosity after merger. Framing the discussion in the context of state-of-the-art merger simulations, I will suggest that the non-equilibrium physics impact on the gravitational-wave signal is at the “few percent level”, likely undetectable with the LIGO instruments but plausibly within reach of third generation detectors like the Einstein Telescope and the Cosmic Explorer.