FTP name this simple form of dark energy, rejected by Einstein as his "greatest mistake."
Gliner and Zeldovich are credited with the modern interpretation, in which it contributes its value times the metric tensor divided by 8 pi times the gravitational constant to the energy-momentum tensor.  In the Friedmann equation, it appears on the right-hand side divided by three, and measurements from WMAP have suggested that its energy density is approximately 0.7 times the critical density.  Its equation of state parameter w is -1, giving a negative effective pressure that tends to accelerate the expansion of the Universe.  FTP name this simple form of dark energy, rejected by Einstein as his "greatest mistake."
Gliner and Zeldovich are credited with the modern interpretation, in which it contributes its value times the metric tensor divided by 8 pi times the gravitational constant to the energy-momentum tensor.  
cosmological constant or lambda