Cosmic inflation is the exponential expansion of the universe at the end of the grand unification epoch, 10-36 seconds after the Big Bang. This exponential expansion was driven by a negative-pressure vacuum energy density. Cosmic inflation theory states that the primeval universe went through a phase of exponential expansion, ballooning almost instantaneously from less than the size of an atom to about golf-ball size. 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.
Cosmic inflation answers the conundrum of the big bang theory: 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? Cosmic inflation predicts the existence of gravity waves, as well as fluctuations in the density and temperature of radiation left over from the big bang. 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.
Cosmic inflation was proposed almost simultaneously by by Alan Guth in the United States and Alexei Starobinski in the Soviet Union.