Question of the Week, 18.10.2010
It was first introduced by Einstein to ensure that his field equations allow for a stationary universe. However, when the expansion of the universe was established experimentally, Einstein called the constant the “biggest blunder” of his life and abandoned it. Modern experimental observations suggest a small positive value, while most quantum field theories predict a huge value.
For a review of the physics and cosmology of the cosmological constant see:
http://relativity.livingreviews.org/Articles/lrr-2001-1/
Thomas Jagau