Skalerizasyonun teorik ve gözlemsel testleri

Scalarization is a phenomenon that occurs in certain modified gravity theories where the scalar field spontaneously grows in the vicinity of compact objects. This allows the theory to closely resemble general relativity in weakly gravitating environments and deviate from it in systems where gravity is strong. At the linear level, it manifests as a Tachyonic instability. This thesis will study this phenomenon using theoretical and observational tools. The first part of the thesis will be based on observations where we will introduce a technique that can test scalarization using neutron star mass and radius measurements. More specifically, we will show that the theory parameters of two scalarization models, the scalar-Gauss-Bonnet Model (sGB) and the Damour and Esposito-Farèse (DEF) model, can be constrained using neutron star mass-radius data. We will show that this method successfully constrains the sGB model, but the same is not always true for the DEF model. In the second part, we will examine theoretical tools to test ideas inspired by scalarization. Namely, we will show that a straightforward generalization of scalarization to vectors, named vectorization, suffers from not only tachyonic instability but also ghost instability. This leads to the likely conclusion that such generalizations are unphysical, which enables us to rule out most such theories by purely theoretical means.