Constraints on the dark energy equation of state from recent supernova   data
  Measurements suggest that our universe has a substantial dark energy component. The most recent data on type Ia supernovae give a dark energy density which is in good agreement with other measurements if the dark energy is assumed to be a cosmological constant. Here we examine to what extent that data can put constraints on a more general equation of state for the dark energy. 
Duane A. Dicus and Wayne W.Repko