Carrollian Origins of Bjorken Flow.

Bjorken flow is among the simplest models of fluids moving near the speed of light (c), while Carroll symmetry arises as a contraction of Poincaré group when c→0. We show that Bjorken flow and its phenomenological approximations are completely captured by Carrollian fluids. Carrollian symmetries arise on generic null surfaces, and a fluid moving at c is restricted to such a surface, thereby naturally inheriting the symmetries. Carrollian hydrodynamics is, thus, not exotic, but rather ubiquitous, and provides a concrete framework for fluids moving at or near the speed of light.