The Inflationary Universe Theory proposes a brief period of extremely rapid accelerating expansion in the very early universe, before the radiation dominated era called the hot big bang. This acceleration is believed to be driven by a quantum field (in effect, some exotic kind of matter) with a repulsive gravitational effect. This can be achieved if the pressure of the field is extremely large and negative (unlike ordinary matter, which has positive pressure).

A specific example is a scalar field associated with a potential energy. Such a field "rolls down" the energy surface defined by the potential, and if it is slow-rolling can act like an effective cosmological constant, driving an exponential expansion with constant acceleration. During this epoch, any matter or radiation density other than that of the scalar field is negligible; one is left with an almost constant energy density of the field, often called a...