This work investigates the theoretical and cosmological implications of modifying Einstein's
theory of general relativity. It explores two classes of modifications to gravity: those in
which the graviton is given a small mass and those in which Lorentz invariance is
spontaneously broken. It elucidates the nature of cosmological perturbations in theories of
massive bimetric gravity including a potentially deadly instability. Theories of gravity
beyond general relativity could explain why the expansion of the Universe is accelerating
obviating the need for a dark energy and can also affect the evolution of the early Universe.
Next it investigates the nature of spacetime in massive gravity theories that contain two
different spacetime metrics. Lastly the strongest constraints to date are placed on the size
of Lorentz-violating effects in the gravity sector during inflation.
