Binary neutron star mergers are simulated with dissipative hydrodynamics. The standard assumption: weak Urca processes — beta decays and their inverses — are the slowest relevant reactions, setting the dominant dissipation timescale. Bulk viscosity from these processes is well-characterized and typically modest in the conditions of a merger.
Karthein, Pradeep, and Steinhorst show that proximity to a QCD critical point upends this hierarchy. If the merger trajectory passes near a low-temperature critical point in the QCD phase diagram, density fluctuations grow, the correlation length increases, and the system exhibits critical slowing down — the universal phenomenon where relaxation times diverge as a critical point is approached.
The consequence for bulk viscosity is dramatic. Near the critical point, the QCD contribution to bulk viscosity can rival or exceed the electroweak contribution. The transport coefficient that was assumed subdominant becomes the largest dissipative channel. Finite-time and finite-size effects in the merger limit the correlation length to realistic values, but even these truncated critical fluctuations produce viscosities large enough to affect the hydrodynamic evolution.
The structural point: bulk viscosity is not a material constant — it depends on how close the system's thermodynamic trajectory passes to a phase transition. A merger that passes far from the critical point has one dissipation hierarchy; a merger that grazes it has another. The same material, the same equation of state, but a different path through the phase diagram produces qualitatively different transport. The measurement — gravitational wave observations of merger dynamics — encodes not just the equation of state but the topography of the phase diagram itself.