The general relativistic cosmological Friedmann equations which describe how the scale factor of the universe evolves are expanded explicitly to include energy forms not usually seen .
The evolution of the universe as predicted by the Friedmann equations when dominated by a single , isotropic , stable , static , perfect-fluid energy form is discussed for different values of its gravitational pressure to density ratio w .
These energy forms include phantom energy ( w < -1 ) , cosmological constant ( w = -1 ) , domain walls ( w = -2 / 3 ) , cosmic strings ( w = -1 / 3 ) , normal matter ( w = 0 ) , radiation and relativistic matter ( w = 1 / 3 ) , and a previously little-discussed form of energy called “ ultralight ” ( w > 1 / 3 ) .
A brief history and possible futures of Friedmann universes dominated by a single energy form are discussed .