Null matter and the ultrarelativistic origin of hydrodynamics at zero temperature

We uncover a universal sector of relativistic fluid dynamics by taking a novel ultrarelativistic limit in which the temperature tends to zero while the flow simultaneously approaches the speed of light. In this regime, hydrodynamics becomes an effective theory of \emph{null matter}, characterised by a preferred null vector, a preferred scale, and their gradients. We show that this theory of null matter constitutes an example of a hydrodynamic theory that can be linearly stable and causal in an arbitrary choice of frame. The framework developed here for null matter can offer insights into ultrarelativistic heavy-ion collisions, astrophysical phenomena with inherently large Lorentz factors, and the dynamics of black hole horizons.