A quantity whose rate of growth is proportional to how large it already is is said to show exponential growth. For instance, exponential growth is a model for population growth on a planet with infinite resources: The larger the population, the more children are born and hence the larger the rate of population growth.

In the context of relativity, exponential growth is interesting for cosmology. In the hypothetical inflationary phase of our universe’s evolution the cosmos underwent exponential expansion, with the rate of expansion getting ever larger as the universe itself expanded more and more.