Theories of Massive Gravity

Einstein's General Relativity has been confirmed in numerous experiments and observations through the years, the latest being the detection of gravitational waves. However there still remain cosmological phenomena, such as the accelerating expansion of the Universe and the presence of Dark Matter, whose satisfactory explanation has not been found yet in the framework of the Standard Model of Elementary Particles and Cosmology. This motivated theorists to look for modifications of gravity aiming at gaining an alternative insight into the nature of Dark Energy and Dark Matter. This Master's Thesis studies internal consistency of the theories of Modified Gravity with a massive graviton, Massive Gravity and Bigravity, and discusses their strong points as well as their weaknesses in trying to interpret the Cosmological issues of fundamental importance.