cosmic inflation

Imagine the surface of a balloon, wrinkled and crumpled, with patterns drawn on it. Now blow it up a billion-fold in a fraction of a second: the wrinkles flatten out, the once-distant patterns are flung impossibly far apart, and the surface looks smooth and even everywhere. Cosmic inflation proposes that the very early universe did something like this — it underwent a brief, almost unbelievably fast burst of exponential expansion, ballooning in size by a factor of at least 10^26 in a tiny fraction of a second.

Inflation is thought to have happened around 10^-36 to 10^-32 seconds after the Big Bang, driven by a peculiar energy field (often called the inflaton) whose effect was a kind of anti-gravity — a pressure that pushed space apart and made expansion accelerate runaway-fast. During each tiny doubling-time, distances doubled, then doubled again, dozens of times over. When inflation ended, the energy stored in that field was dumped into ordinary hot particles and radiation, kindling the hot Big Bang we described earlier and setting the universe expanding at its later, gentler pace.

Inflation was invented to cure three awkward puzzles of the plain hot Big Bang: why the universe looks so uniform across regions that should never have been in contact (the horizon problem), why space is so close to geometrically flat (the flatness problem), and where the tiny seeds of all later structure came from (quantum fluctuations stretched to cosmic size). It elegantly solves all three and makes testable predictions, several of which match the cosmic microwave background beautifully. Still, inflation is a strong, well-motivated framework rather than a finished, proven fact: we do not know what the inflaton actually is, there are many competing versions, and a confident detection of its predicted gravitational-wave signature has not yet been made.