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