Ninety years ago, Russian physicist Alexander Friedmann (1888–1925) demonstrated for the first time that Albert Einstein’s general theory of relativity (GR) admits nonstatic solutions. It can, he found, describe a cosmos that expands, contracts, collapses, and might even have been born in a singularity.

Friedmann’s fundamental equations describing those possible scenarios of cosmic evolution provide the basis for our current view of the Big Bang and the accelerating universe. But his achievement initially met with strong resistance, and it has since then been widely misrepresented. In this article, I hope to clarify some persistent confusions regarding Friedmann’s cosmological theory in the context of related work by contemporaries such as Einstein, Willem de Sitter, Arthur Eddington, and Georges Lemaître.

Last year’s Nobel Prize in Physics was shared by three cosmological observers who discovered that the cosmic expansion is currently accelerating (see Physics Today, December 2011, page 14). Thus, one...

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