The string-theory landscape
Fernando Quevedo · Physics World · 2003
Einstein introduced the cosmological constant to his equations of general relativity because he believed the universe was static. Faced with evidence that the universe was actually expanding, however, he decided to remove it, later referring to the cosmological constant as the biggest blunder of his life. Recent observations suggest that the expansion of the universe is accelerating, which favours a small, but non-zero, positive cosmological constant with a value of 10-120 in Planck units (see article on p3).