This Video Explains Albert Einstein's Biggest Blunder
"This video is about how Albert Einstein made a mistake when applying the Field Equations of General Relativity to cosmology (in particular, to a static, constant density universe), and solved the problem by introducing the cosmological constant, rather than allowing for a dynamic universe with a scale factor - that is, the Friedmann-Lemaitre-Robertson-Walker universe, first developed by Alexander Friedmann of Russia. Later, it was discovered by the Slipher and Hubble red-shift that the universe is indeed expanding, and even later, by Schwarz and company in 1998, that the expansion is accelerating - aka, dark energy. And the cosmological constant was re-introduced," states the video description.
The concepts are difficult to grasp so we won't try to explain them here. Instead, we will let you watch the video that does just that with minutephysics' usual well-illustrated and well-documented approach.