Theories of Massive Gravity in 2+1 Dimensions.

This thesis is dedicated to the study of theories of massive gravity. The formulation of higher spin gauge field theories, along with a Chern-Simons (CS) like term for fields of higher spins is presented. Through this setup, general features of theories describing massless/massive fields of higher spin in arbitrary dimensions is discussed. In 2+1 dimensions, the existence of multiple mass-generating mechanisms i.e. metric-based masses and topological masses, offers the possibility for gauge bosons to acquire mixed masses. This scenario is first introduced in a theory of photons where both Proca mass term and CS mass term is simultaneously present. The mass-mixing which occurs in this theory is further analysed through the St\"uckelberg formalism. Motivated by these results, a theory of massive gravity where gravitons obtain masses from both Fierz-Pauli mass term and CS mass term is studied. This theory allows for 3 propagating massive graviton modes, and their masses have undergone considerable mixing. This mass-mixing is made explicit through a St\"uckelberg analysis. The bimetric theory of gravity is a non-trivial generalization of the theory of General Relativity. This theory provides a consistent non-linear theory of massive gravity which is studied in detail. In this thesis, the bimetric theory of gravity is extended in 2+1 dimensiosn to the theory of Topologically Massive Bimetric Gravity (TMBG). In the theory of TMBG, the mass-mixing which arises from the interaction of the two metrics and their corresponding CS terms occurs at the nonlinear stage itself. For the version of TMBG studied in this thesis, the linearized theory shows that there are 3 massive graviton modes, which corresponds to the same 3 modes found earlier. Finally, quantum loop corrections to the graviton propagator from massive photons (which acquire mass from both CS and Proca mechanism) is calculated.