Definition
Cosmic inflation is a brief epoch of exponential expansion that the universe is thought to have undergone roughly 10⁻³⁶ to 10⁻³² seconds after the Big Bang, stretching a microscopic patch into something larger than today's observable universe.
Proposed by Alan Guth in 1980 and refined by Linde, Albrecht, and Steinhardt, inflation has become the standard prologue to the hot Big Bang model.
Why it matters
How it works
In Guth's original picture, the early universe was trapped in a high-energy false vacuum — a state with large, nearly constant energy density. General relativity then forces the universe to expand exponentially: in a Hubble time, lengths double, and many doublings happen in a tiny fraction of a second. By the end of inflation a region the size of a proton becomes larger than the observable universe today.
This exponential stretch flattens any initial curvature (a small patch of any curved surface looks flat when magnified enough) and dilutes any inhomogeneities, monopoles, or other relics from earlier eras. It also explains why opposite ends of the visible sky — which appear never to have communicated in the standard Big Bang timeline — share the same temperature: they were once in close contact, before inflation pushed them apart.
The most important quantitative success comes from quantum mechanics. Inside the inflating region, quantum fluctuations of the inflaton field are stretched along with space itself. By the time inflation ends, these fluctuations are frozen as small density variations — about one part in 10⁵. After billions of years of gravitational amplification, those same patterns are seen as the temperature ripples in the CMB and as the cosmic web of galaxies. The match between predicted and observed power spectra is one of the strongest pieces of evidence for inflation.