The universe is 2.7 degrees above absolute zero in every direction we look, a remarkably uniform glow that presents a deep cosmological puzzle. In the standard picture of the Big Bang, distant regions of space have never been close enough to exchange light or heat, so they should not be in thermal equilibrium. They should not share the exact same temperature. This is the horizon problem, and it is joined by two other mysteries: the seemingly perfectly flat geometry of space and the unexplained absence of exotic particles like magnetic monopoles.

This episode explores how a brief, explosive phase of exponential expansion known as cosmic inflation resolves all three of these issues. Proposed by Alan Guth in 1980, inflation suggests that space itself stretched by an enormous factor in a tiny fraction of a second, smoothing out the early univer ...