Einstein's theory of general relativity (GR) entwines the dynamics of matter-energy and the universe's spacetime expansion. In the weak-field limit of GR, Newtonian dynamics relates the escape speed to the local gravitational 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. Without an accelerated expansion of space-time, the gravitational escape velocity profile of galaxy clusters is defined by the observed weak-lensing shear profile. In a Dark Energy dominated universe, the escape profile surface is suppressed compared to the weak-lensing gravity-only prediction. In LambdaCDM, cluster escape velocity profiles are a dynamical probe of the acceleration of space-time. I will discuss a description of the theory, tests using realistic simulations, applications to real data, Fisher Matrix predictions for future surveys, and prospects including non-standard GR models.