As the largest bound objects in the Universe, galaxy clusters provide us an opportunity to observe galaxies escaping their gravitational potential wells at tremendous speeds. In simulated Universes, we know that bound gravitational tracers (like dark matter particles) fill their radius/velocity phase-spaces with a trumpet-like design. Caustic edges in these phase-space define the escape speeds of the particles and thus the gravitational potential profile of clusters. I will discuss the theoretical underpinnings of how we map cluster galaxy escape speeds to gravitational potential profiles in an expanding Universe. I will discuss how we observe these escape edges using projected galaxy data from here on Earth. Finally, I will show how we can use current and future data to constrain cosmology, including Chameleon-based modified gravity.