In What Sense Is the Early Universe Fine-Tuned?
  It is commonplace in discussions of modern cosmology to assert that the early
universe began in a special state. Conventionally, cosmologists characterize
this fine-tuning in terms of the horizon and flatness problems. I argue that
the fine-tuning is real, but these problems aren't the best way to think about
it: causal disconnection of separated regions isn't the real problem, and
flatness isn't a problem at all. Fine-tuning is better understood in terms of a
measure on the space of trajectories: given reasonable conditions in the late
universe, the fraction of cosmological histories that were smooth at early
times is incredibly tiny. This discussion helps clarify what is required by a
complete theory of cosmological initial conditions.
