In cosmology, inflation is a theory expanding upon the standard Big Bang Theory. Inflationary Theory proposes that there was massive, exponential expansion very early in the creation of the universe. Inflation is involved a lot in understanding the first moments the universe existed and accounts for a number of observations that contradict the standard Big Bang model. There were three main problems that arose, solved by inflation: the Horizon Problem, the Flatness Problem, and the Monopole Problem.
The first problem that inflation address is that universe looks the same on opposite horizons, but there has not been adequate time since the Big Bang for anything to travel from one side and back. Expansion in Big Bang cosmology is the slower, power law expansion. The microwave background temperatures observed strongly indicate though that the regions were in contact at some point. This is known as the Horizon Problem. Inflation allows for the regions to have been in contact to obtain uniform temperatures, but then they separated at a speed faster than the speed of light when inflation occurred.
Next, he universe has an odd space-time geometry, creating the Flatness Problem. Relativity describes three universe shapes: open, closed, and flat. The standard Big Bang model contradicts how flat the universe actually seems to be. Power law expansion would result in a curve. The flat configuration seen in the Wilkinson Microwave Anisotropy Probe (WMAP) implies the critical density of the universe to be perpetually between a state eternal expansion and eventual collapse. To achieve a flat universe as seen today the beginning density would have needed to be exact within fifteen decimal places to avoid the open and also the closed universe shapes. The exponential growth caused by inflation results in diametrical expansion rather than curved expansion and is the only way to obtain the exact density to maintain a flat universe. In other words, exponential expansion decreases curvature, so the universe avoid the open configuration in which expansion is too rapid for galaxies to form and also the closed configuration in which expansion stops and a collapse occurs. A flat universe expands slowly enough to form galaxies without collapsing back in on itself.
The final problem created by the standard Big Bang Theory is the Monopole Problem. Current particle physics theories predict certain particles being produced during the Big Bang, but we are having trouble finding them. For example, if the Big Bang model were true there should be a large number of heavy magnetic monopoles, but none have actually been observed. Inflationary theory explains why they have not been observed. The monopoles and other particles that were created immediately prior to inflation were distributed to such an extent that their density decreased to an undetectable amount when inflation took place.
When Albert Einstein first formulated the Theory of Relativity in 1920, he proposed that the universe must either be expanding or shrinking, which was controversial at the time. Relativity is incompatible with a static universe. In 1922, Alexander Friedmann also proposed that the universe was expanding. Then in 1929, Edwin Hubble confirmed the expansion of the universe and observed that other galaxies are moving away from ours. In addition, Hubble realized that the farther away a galaxy the faster it moved. Taking that knowledge and moving backwards with it resulted in the creation of the Big Bang Theory. From the Big Bang Theory arose inflation. Exponential growth in the first moments of the universe, as proposed in inflation accounts for the shortcomings present in the original Big Bang model, resulting in the modified model many cosmologists use today.