Cosmology as a probe of fundamental physics

Cosmology has reached maturity as a precision science. However, it leaves us with open theoretical questions. First, what is the early universe mechanism that is responsible for the observed nearly scale invariant and gaussian primordial density perturbations? Second, is general relativity the correct description of gravity on the largest observable scales? In this dissertation, we report on various theoretical developments related to these questions. We first show that inflation is the unique single field mechanism with unit sound speed capable of generating nearly scale invariant and Gaussian density perturbations over a broad range of scales. Any alternative scenario must either invoke additional fields or assume an unstable background. In the multi-field category, we present a Dirac-Born-Infeld (DBI) generalization of the pseudo-conformal scenario, an alternative to inflation which relies on the spontaneous breaking of conformal symmetry in the early universe. This scenario, like most alternatives to inflation, requires a violation of the Null Energy Condition (NEC) to smoothly bounce from a contracting phase to an expanding phase. We report that the DBI conformal galileons can violate the NEC without instabilities, while avoiding nearly all pathologies of earlier examples of NEC-violating theories. We then turn to the question of the uniqueness of general relativity as a description of the known graviton degrees of freedom. Assuming only spatial diffeomorphism invariance, we derive general relativity as the unique effective field theory of the transverse, traceless graviton polarizations. The Lorentz covariance of general relativity, having not been assumed in our analysis, is therefore plausibly interpreted as an accidental spacetime symmetry, not a fundamental spacetime symmetry.