Einstein’s theory of general relativity (GR) entwines the dynamics of matter-energy and the universe’s spacetime expansion. It is often said: matter-energy tells space-time how to curve and space-time tells matter-energy how to move. In the weak-field limits of GR, Newtonian dynamics allows us to relate the escape speed to the local potential via the Poisson equation. Objects moving faster than the escape speed will fly away on short timescales leaving behind “caustic” surfaces in the radial escape velocity profile, which can be observed in galaxy clusters using spectroscopic data. This means that the escape velocity profile of galaxy clusters can be used as a dynamical constraint for the expansion history of the universe. I will provide a description of the theory, tests using increasingly realistic simulations, applications to real data, Fisher Matrix predictions for this new cosmological technique, and future prospects, including non-standard GR models.