| Summary: | In my thesis, I focus on the theory of massive gravity and its cosmological implications. I show that a massive graviton leads to self-acceleration, and that a whole class of solutions possess this property, including FRW universes. Subsequently, I investigate fluctuations around these solutions, and show that they are stable, at least for spherically symmetric fluctuations. Massive gravity also contains a coordinate-independent determinant singularity that shows up in specific examples with well-defined initial conditions, and that cannot be dynamically avoided. The vierbein formulation of massive gravity is particularly suited to address this issue. It can be seen that a commonly chosen convention is not supported by the vierbein formalism. Finally, I look at the appearance and properties of similar singularities in bimetric gravity, in which the second, fiducial metric is allowed to evolve independently of the spacetime metric. I find that while determinant singularities continue to show up in bimetric massive gravity theories, physical quantities such as curvature remain finite as they cross them.
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