Ari Belenkiy’s article (Physics Today, October 2012, page 38) was thoroughly enjoyable. In it, Alexander Friedmann’s 1922 publication on the expanding universe is identified as the origin of modern cosmology. That comment motivated me to look again into the papers Paul Ehrenfest left here in Leiden, papers that were discovered a few years back. They include a draft of Friedmann’s 1922 paper, which was only published in German, and an earlier unpublished manuscript titled “On the question of the geometry of curved space.” An accompanying letter to his friend Ehrenfest clearly shows that Friedmann knew he was onto something big with his discovery that “there appears also a world, the space of which possesses a curvature radius varying with time.” I have placed the papers online at http://www.lorentz.leidenuniv.nl/history/Friedmann_archive as a tribute to this pioneer.
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1 March 2013
March 01 2013