Einstein’s blackboard

Einstein’s blackboard as used in a lecture in Oxford on 16 May 1931:

At that time Einstein’s theories of relativity were being combined with astronomical data to explain the shifts towards the red in the spectra of distant galaxies, which indicated that the universe was expanding. In his lecture Einstein outlined a fairly simple model to explain this apparent expansion. In the first line on the blackboard, D, the measure of expansion in the universe, is defined in terms of the expansion factor P. The expression for the density of matter in the universe, given by ρ in the third line, is derived from the field equations. The last four lines contain numerical data, giving values for density, radius and age of the universe, where ‘L. J’ stands for ‘Licht Jahr’ (light year) and ‘J’ for ‘Jahr’ (year). According to the last line, the age of the universe is about 10, or perhaps 100 billion years (the bracket indicates an alternative figure, not a product of two figures).

From Bye-bye blackboard... from Einstein and others