  Cosmologists have developed a phenomenally successful picture of structure in
the universe based on the idea that the universe expanded exponentially in its
earliest moments. There are three pieces of evidence for this exponential
expansion -- {\it inflation} -- from observations of anisotropies in the cosmic
microwave background. First, the shape of the primordial spectrum is very
similar to that predicted by generic inflation models. Second, the angular
scale at which the first acoustic peak appears is consistent with the flat
universe predicted by inflation. Here I describe the third piece of evidence,
perhaps the most convincing of all: the phase coherence needed to account for
the clear peak/trough structure observed by the WMAP satellite and its
predecessors. I also discuss alternatives to inflation that have been proposed
recently and explain how they produce coherent phases.
