How the quark-gluon plasma becomes a fluid so quickly
In plain words
Fluid models describe heavy-ion collisions starting about 3e-24 seconds after impact, which seems too early for the matter to have settled down. How the system starts behaving as a fluid so soon is not fully understood.
Precise statement
Determine how and when the energy-momentum tensor of matter formed in $A+A$ collisions reaches viscous-hydrodynamic form (hydrodynamization) while the pressure anisotropy $P_L/P_T$ is still far from 1, at proper times $\tau$ about $0.5\ \text{to}\ 1\ \mathrm{fm}/c$. Answer: a QCD-based mechanism (attractor solutions, weak-coupling kinetic theory, or strong-coupling holography) with a predicted hydrodynamization time as a function of $\eta/s$ and energy density, tested against data.
What would settle it
QCD effective kinetic theory simulations with realistic transverse geometry, compared with photon and dilepton data sensitive to the early pressure anisotropy.
Status in the literature
Unverified note
Hydrodynamic attractor solutions found since 2015 (Heller and Spalinski) explain early hydrodynamic behavior in simplified expansions; the full three-dimensional QCD case remained open as of 2026.