In physical cosmology, cosmic inflation, cosmological inflation or just inflation is the theorized exponential expansion of the universe at the end of the grand unification epoch, 10-36 seconds after the Big Bang, driven by a negative-pressure vacuum energy density. The term "inflation" is also used to refer to the hypothesis that inflation occurred, to the theory of inflation, or to the inflationary epoch. As a direct consequence of this expansion, all of the observable universe originated in a small causally connected region.

Inflation answers the classic conundrum of the big bang cosmology: why does the universe appear flat, homogeneous and isotropic in accordance with the cosmological principle when one would expect, on the basis of the physics of the big bang, a highly curved, heterogeneous universe?Inflation also explains the origin of the large-scale structure of the cosmos. Quantum fluctuations in the microscopic inflationary region, magnified to cosmic size, become the seeds for the growth of structure in the universe (see galaxy formation and evolution and structure formation).

Inflation was proposed by Alexei Starobinski in the Soviet Union, and simultaneously by Alan Guthin the United States. Guth's mechanism is different from Starobinski's, and requires a modification to allow for a graceful exit from inflation. This modification was provided independently by Andrei Linde, and by Andreas Albrecht and Paul Steinhardt.

While the detailed particle physics mechanism responsible for inflation is not known, the basic picture makes a number of predictions that have been confirmed by observation. Inflation is thus now considered part of the standard hot big bang cosmology. The hypothetical particle or field thought to be responsible for inflation is called the inflaton.

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