Relaxing to three dimensions.

We propose a new selection principle for distinguishing among possible vacua that we call the "relaxation principle." The idea is that the Universe will naturally select among possible vacua through its cosmological evolution, and the configuration with the biggest filling fraction is the likeliest. We apply this idea to the question of the number of dimensions of space. We show that under conventional (but higher-dimensional) Friedmann-Robertson-Walker evolution, a universe filled with equal numbers of branes and antibranes will naturally come to be dominated by 3-branes and 7-branes. We show why this might explain the number of dimensions that are experienced in our visible Universe.