Teach Astronomy - Cosmological Constant

http://www.teachastronomy.com/ Why is the universe accelerating, and how does this relate to the more standard cosmological idea that since the big bang the expansion rate has been decelerating due to the action of gravity on all the matter of universe? For the answer to this we have to go back to Einstein in the 1920s. Einstein solved the equations of General Relativity and realized that the solutions naturally indicated expansion or contraction. When told that the universe was static, Einstein added a term to the solution of his equations called the cosmological constant to suppress the natural expansion. Thus the cosmological constant represents something that acts opposite to gravity. Gravity is an attractive force; the cosmological constant represents something that is repulsive. In the standard model of the universe with a cosmological constant the big bang is followed by a period of deceleration due to all the matter in the universe. And then at some epoch several billion years ago the deceleration changes into an acceleration, and the rate of expansion increases. We are currently witnessing a phase of acceleration in the universe and its evidence that the term in gravity is balanced by another term, the cosmological constant.