Dartmouth Events

Abstract: In this colloquium I discuss various theoretical aspects of an accelerating universe. This includes both early universe inflation and late time cosmic acceleration. I utilize the basic method of effective field theory, wherein much of the physics can be learnt by studying the system at sufficiently large scales compared to some microscopic scale. I begin by reviewing the profound consequences of quantum mechanics and special relativity, which organizes particles into half-integer spin from 0 to 2. Under some assumptions, the spin 2 particle uniquely leads to general relativity at large distances, while the spin 0 particle allows for a new phase of matter to occur, namely inflation, under appropriate conditions with vacuum energy. I present some difficulties in embedding these accelerating phases into microscopic physics. On the other hand, I show how alternatives to general relativity with vacuum energy don't make theoretical sense, using constraints from causality and quantum mechanics. Furthermore, I show that special relativity itself follows just from locality of spin 2 particles. I discuss the observational evidence that inflation occurred and that the late time acceleration arises purely from vacuum energy, albeit with fine tuned values. To this end, I present an effective fluid description of our universe, and connections to particle physics. Since the alternative ideas do not appear to make theoretical sense and are observationally disfavored, I argue that the most plausible way to understand the fine-tuned accelerating universe may be within the framework of a multiverse.