Causality violations in realistic simulations of heavy-ion collisions

Causality is violated in the early stages of state-of-the-art heavy-ion hydrodynamic simulations. Such violations are present in up to 75% of the fluid cells in the initial time and only after 2-3 fm/c of evolution do we find that 50% of the fluid cells are definitely causal. Superluminal propagation reaches up to 15% the speed of light in some of the fluid cells. The inclusion of pre-equilibrium evolution significantly reduces the number of acausal cells. Our findings suggests that relativistic causality may place constraints on the available parameter space of heavy-ion collision simulations when factored into more thorough statistical analyses.

Automated summary

Causality is violated in the early stages of state-of-the-art heavy-ion hydrodynamic simulations, but the number of cells that are definitely causal increases as the evolution progresses. The inclusion of pre-equilibrium evolution significantly reduces the number of acausal cells. Relativistic causality may place constraints on the available parameter space of heavy-ion collision simulations when factored into more thorough statistical analyses.