Introduction

One of the most obvious and compelling aspects of the physical world is that it has an “arrow of time.” Certain processes (such as breaking a glass or burning fuel) appear all the time in our everyday experience, but the time reverse of these processes is never seen. In the modern understanding, special nongeneric initial conditions of the universe are used to explain the time-directed nature of the dynamics we see around us.

On the other hand, modern cosmologists believe it is possible to explain the initial conditions of the universe. The theory of cosmic inflation (and a number of competitors) claims to use physical processes to set up the initial conditions of the standard Big Bang. So in one case initial conditions are being used to explain dynamics, and in the other, dynamics are being used to explain initial conditions. In this chapter I explore the relationship between two apparently different perspectives on initial conditions and dynamics.

My goal in pursuing this question is to gain a deeper insight into what we are actually able to accomplish with theories of cosmic initial conditions. Can these two perspectives coexist, perhaps even allowing one to conclude that cosmic inflation explains the arrow of time? Or do these two different ideas about relating dynamics and initial conditions point to some deep contradiction, leading us to conclude that a fundamental explanation of both the arrow of time and the initial conditions of the universe is impossible?