Title: Time’s Arrow, Density Matrices, and the Quantum State of the Universe

Abstract: In a quantum universe with a strong arrow of time, we postulate a low-entropy boundary condition (the Past Hypothesis) to account for the temporal asymmetry. In this talk, I show that the Past Hypothesis also contains enough information to select a simple and unique initial quantum state of the universe. This idea leads to a new class of quantum theories formulated in terms of density matrices. They have interesting implications about the status of probabilities in statistical mechanics and the nature of the quantum state. (For more details about the project, please see here: https://arxiv.org/abs/1712.01666)