Dynamical emergence of universal horizons during the formation of black holes

Motivations for the existence of a fundamental preferred frame range from pure phenomenology to attempts to solve the nonrenormalizability of quantum gravity, the problem of time (and scale), and the cosmological constant problem(s). In many explicit constructions, such as Einstein-Aether or gravitational aether theories, k-essence, cuscuton theory, shape dynamics, or (nonprojectable) Horava-Lifshitz gravity, the low-energy theory contains a fluid (which defines a preferred frame) with superluminal or incompressible excitations. We study here the formation of black holes in the presence of such a fluid. In particular, we focus on the incompressible limit of the fluid (or constant mean curvature foliation) in the spacetime of a spherically collapsing shell within an asymptotically cosmological spacetime. In this case, ignoring the fluid backreaction, we can analytically show that an observer inside 3/4 of the Schwarzschild radius cannot send a signal outside, after a stage in collapse, even using signals that propagate infinitely fast in the preferred frame. This confirms the dynamical emergence of universal horizons that have been previously found in static solutions. We argue that this universal horizon should be considered as the future boundary of the classical spacetime.