When he introduced the cosmological constant: Again, for the mathematically inclined, the new equation read: Gμν – 8πG ρΛ gμν = 8πG Tμν, where ρΛ is the energy density associated with the cosmological constant.

if one moves this term to the right-hand side: The equation now reads: Gμν = 8πG (Tμν + ρΛ gμν).

This is an entirely different physical: For excellent popular explanations of the cosmological constant as representing the energy of the vacuum see Krauss and Turner 2004, Randall 2011, and Greene 2011. Davies 2011 is also a short, accessible article. Theories of time, and their relations to cosmic expansion, are fascinatingly explained by Carroll 2001, and Frank 2011.

Einstein proposed in 1919: Einstein 1919.

short note on the subject: Einstein 1927.