Einstein's conversion from his static to an expanding universe
  In 1917 Einstein initiated modern cosmology by postulating, based on general
relativity, a homogeneous, static, spatially curved universe. To counteract
gravitational contraction he introduced the cosmological constant. In 1922
Alexander Friedman showed that Einstein's fundamental equation also allowed
dynamical worlds, and in 1927 Geroges Lemaitre, backed by observational
evidence, concluded that our universe was expanding. Einstein impetuously
rejected Friedman's as well as Lemaitre's findings. However, in 1931 he
retracted his former static model in favour of a dynamic solution. This
investigation follows Einstein on his hesitating path from a static to the
expanding universe. Contrary to an often repeated belief the primary motive for
his switch was not observational evidence, but the realisation that his static
model was unstable.
