The Arrow of Time and the Initial Conditions of the Universe

The existence of a thermodynamic arrow of time in the present universe implies that the initial state of the observable portion of our universe at (or near) the “big bang” must have been very “special”. We argue that it is not plausible that these special initial conditions have a dynamical origin. There is no question that our present universe displays a thermodynamic arrow of time: We all have observed phenomena in our everyday lives where entropy is seen to increase significantly, but no one has ever reliably reported an observation of entropy decrease in a macroscopic system. This fact raises a number of obvious questions. Some of these questions have straightforward answers—although they largely “beg the question”: • Question: Why is there a thermodynamic arrow of time in the present universe? • Answer: Because the present entropy of the universe is very low compared with how high it could be. It is well understood that during the normal course of dynamical evolution, an isolated system will spend the overwhelming majority of its time in a state of (nearly) maximum entropy. Therefore, if we find a system to be in a state of low entropy, it is overwhelmingly likely that we will observe it to evolve to a state of higher entropy. The probability of observing it to evolve to a state of lower entropy is