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.

Nominated as an outstanding PhD thesis by the University of Cambridge, UK An exceptionally clear presentation of one of physics' major frontiers Investigates modified gravity from both theoretical and observational perspectives Includes supplementary material: sn.pub/extras

Autorentext
Adam Solomon is a theoretical cosmologist interested in how we can unveil the fundamental laws of physics by looking to the sky. While his research has spanned a variety of cosmological observations and fundamental theories, he has focused especially on the mystery of the accelerating expansion of the Universe, and whether this may be a sign of new gravitational physics beyond Einstein's general relativity.
He is currently a postdoctoral fellow at the Center for Particle Cosmology at the University of Pennsylvania, having obtained his bachelors at Yale University and his doctorate at the University of Cambridge.

Inhalt
Introduction.- Gravity Beyond General Relativity.- Cosmological Stability of Massive Bigravity.- Linear Structure Growth in Massive Bigravity.- The Geometry of Doubly-Coupled Bigravity.- Cosmological Implications of Doubly-Coupled Massive Bigravity.- Cosmological Implications of Doubly-Coupled Massive Gravity.- Lorentz Violation During Ination.- Discussion and Conclusions.