Cosmic Inflation

Cosmic inflation is the idea that, in the very first tiny fraction of a second of its existence, the universe expanded almost unimaginably fast, ballooning by an enormous factor before settling into the steadier expansion we see today. First proposed by Alan Guth in 1980 and refined by others, it has become a central part of the modern picture of the Big Bang.

What it explains

Inflation was invented to solve several puzzles at once. One is why the universe looks so uniform: regions on opposite sides of the sky have almost exactly the same temperature in the cosmic microwave background, even though they seem too far apart to have ever been in contact. Inflation explains this by proposing that these regions were in contact, very early on, before being flung far apart by the rapid expansion. It also explains why the geometry of the universe appears so precisely flat.

The connection to Hawking

Hawking made an important early contribution to inflation theory, on the question of where cosmic structure comes from. If the early universe was so smooth, why is today's universe full of galaxies and clusters? The answer, worked out by Hawking and several others around a famous 1982 Cambridge workshop, is that tiny quantum fluctuations, the same restless quantum behaviour behind Hawking radiation, were stretched by inflation to astronomical scales. These stretched fluctuations became slightly denser regions that later grew, under gravity, into galaxies. The pattern of hot and cold spots later mapped in the microwave background fits this prediction remarkably well, tying the quantum physics Hawking studied to the largest structures in the cosmos.