Galaxies, Cosmology and Gravitation: On Escaping Galaxy Clusters in Accelerating Universes

Abstract

The late-time cosmic acceleration of the universe is one of most profound mysteries of physical cosmology. What is at stake with this discovery is the following: either our universe is composed of some exotic ``dark energy" which drives the dynamics of the acceleration or our general relativistic theory of gravity must be radically transformed. Clusters of galaxies, some of the largest gravitationally-bound objects in our universe containing hundreds of galaxies, have been fruitful sites from which to study the consequences of our cosmological models and the gravitational theory from which these models are derived. In this work, we derive and test a novel model that takes into account the effects of our accelerating universe at the scale of galaxy clusters. More specifically, the theoretical observable we work with in this dissertation is the escape velocity profile of galaxy clusters. Our model implies that in an accelerating universe, the escape velocity profile of galaxy clusters is lower than what is expected from a universe that is not accelerating. Put differently, if the universe is accelerating, galaxies confined to their clusters have an easier time escaping them. However, testing the implications of this model is difficult given that observations can only allow us to infer the projected escape velocity profiles. Here, we study how the observed profiles can be de-projected via a function that depends on the cluster velocity anisotropy profile. To that end, we also develop a novel approach to derive cluster velocity anisotropy profiles with joint dynamical and weak lensing data. We further show that our cosmology-dependent model of the escape velocity profile can be utilized to constrain cosmological models. In particular, with the Fisher matrix formalism we show that our theoretical observable has the capacity to set competitive constraints on relativistic cosmological models of the accelerating universe in the near future. Lastly, we drop the presupposition that general relativity is the only way to describe gravitational phenomena and develop a novel probe of gravity that utilizes the sensitivity of our theoretical observable to changes in the gravitational potential.