A Friedmann model (Friedmann universe or Friedmann-Lemaitre universe) is a type of model of the universe incorporating general relativity (GR). Specific instances vary on the amount of mass, whether there is sufficient to reach or exceed the critical density. An instance exactly at the critical density is termed an Einstein-de Sitter model. Friedmann models are based the Friedmann equations, a set of equations compatible with GR worked out by Alexander Friedmann. Georges Lemaitre subsequently independently worked out the same model.