A relic of a secular saint
On May 16, 1931, Albert Einstein delivered the second of three lectures on cosmology at Rhodes House, Oxford. The audience of over 400 academics listened in German, as no interpreter was provided, while he detailed his evolving model of an expanding universe. Using two blackboards, he sketched out equations from his April 1931 paper on the Friedmann-Einstein model, which incorporated Edwin Hubble's recent observations of galactic redshift. The chalk marks represent a specific moment in scientific thought, as Einstein abandoned his earlier static universe theory.
The last three lines on the surviving blackboard show his calculations for the density of matter in the universe (ρ), the radius of the universe (P), and its age (t). His figures estimated the universe to be about ten billion years old, with a radius of 10^8 light-years. These calculations, though later shown to contain a numerical error, show his working methods. After the lecture, a group of Oxford dons, including Robert Gunther (the museum's founder), embryologist Gavin de Beer, and chemist E. J. Bowen, recognized the blackboard's historical value. They arranged for both boards to be saved. One was later accidentally wiped clean by a cleaner in storage, but the other was transported to the museum, where it is one of its most requested objects.