"The Waters I am Entering No One yet Has Crossed": Alexander Friedman
  and the Origins of Modern Cosmology
  Ninety years ago, in 1922, Alexander Friedman (1888-1925) demonstrated for
the first time that the General Relativity equations admit non-static solutions
and thus the Universe may expand, contract, collapse, and even be born. The
fundamental equations he derived still provide the basis for the current
cosmological theories of the Big Bang and the Accelerating Universe. Later, in
1924, he was the first to realize that General Relativity allows the Universe
to be infinite. Friedman's ideas initially met strong resistance from Einstein,
yet from 1931 he became their staunchest supporter. This essay connects
Friedman's cosmological ideas with the 1998-2004 results of the astronomical
observations that led to the 2011 Nobel Prize in Physics. It also describes
Friedman's little known topological ideas of how to check General Relativity in
practice and compares his contributions to those of Georges Lemaitre. Recently
discovered corpus of Friedman's writings in the Ehrenfest Archives at Leiden
University sheds some new light on the circumstances surrounding his 1922 work
and his relations with Paul Ehrenfest.
