Alan Guth's Eureka

In 1979, physicist Alan Guth was puzzling over why the universe looks so uniform in every direction. The cosmic microwave background has nearly the same temperature everywhere, yet regions on opposite sides of the sky are too far apart to have ever traded heat. Classical Big Bang theory had no answer.

Guth proposed that a brief burst of exponential expansion stretched one tiny, connected patch into the whole visible cosmos. Former neighbors ended up on opposite ends of the sky, which neatly explains the uniformity. His first model had a flaw, the graceful exit problem: inflation never stopped cleanly.