Modified Gravity in Clusters of Galaxies

For the last century, General Relativity (GR) has been successful in explaining several phenomena which were not explained before, for instance, the perihelion precession of Mercury, the bending of light due to the gravitational field, and the prediction of Gravitational Waves (GWs). Nevertheless, GR fails to explain some of the most important phenomena in the universe, namely the Dark Energy (DE) component, and inflation. Therefore, an extension to its original framework should be adopted. There are many ways to modify GR in a way to explain the cosmological observations, among them, one of the first and trivial modification to GR is the introduction of the Cosmological Constant (CC) Λ to the original theory which can predict the accelerated expansion rate of the universe. However, this modification does not explain the origin of DE and therefore another modification is required. Moreover, the homogeneity and the flatness of the universe cannot be predicted by a mere GR theory, an early time modification to GR to contain additional scalar field called the inflation which through slow rolling can predict the homogeneity and the flatness we observe in today’s cosmology. Some of the attempts to modify GR in order to account for the DE component include f(R) gravity which are a class of models where the Ricci scalar R in the Lagrangian of the original GR is modified with another scalar function (f(R)) which represents a generalization instead of simply adding one constant Λ. Another alternative is represented by the Quintessence, which instead of a constant DE, it assumes a dynamical one through an addition of a scalar field. Our focus however, will be on a specific type of modification to GR which is represented by the Chameleon Gravity, which introduces a new scalar field, too, which locally behaves as GR but at the large scales it modifies GR and account for the constant DE, represented by Λ. There are different manners to investigate manifestations of MG in cosmology, involving CMB and LSS. In this Thesis, we bring the analysis of manifestations of specific MG scenarios in the largest cosmological objects, namely galaxy clusters, taking the advantage of new datasets which became available recently. The Thesis is divided in two parts. First, we review GR 3 and general cosmology, derived from the Einstein’s field equations including a cosmological constant and a Cold Dark Matter Component, represented by particles which were non-relativistic at their time of decoupling, interacting at most weakly with the rest of the system, the ΛCDM model. We conclude the first part by introducing Modified Gravity (MG) in a general context, which is then specialized to the specific case of Chameleon Gravity (CG) and f(R) theories. The second part consists of studying the CG phenomenology in cluster of galaxies, which can now be inspected as potential carriers of MG signature, thanks to new datasets. In particular, we study how CG affects the hydrostatic pressure within the cluster which is then used to find unprecedented constraints on the model. In addition, in the following chapters, we extend the analysis by including clusters in tension with the assumption of a Navarro Frenk and White (NFW) CDM halo, solving semi-analytical the CG field equations in relation to their mass model. A concluding chapter sums up the main parts of the Thesis, the results obtained so far, and the future prospects.