Edwin HUBBLE managed to calculate the rate of expansion of the Universe from observations recorded by astrophotography. Edwin Hubble began working at the Mount Wilson Observatory in 1919, shortly after the starting of the Hooker telescope was then the largest in the world. His observations made between 1922 and 1924 made it possible to establish that the Universe extended beyond the Milky Way. He identified the presence of Cepheids, giant or supergiant stars whose luminosity varies in relatively close spiral nebulae. This is for example the case of the Andromeda Galaxy (Messier 31) and the Galaxy of the Triangle (Messier 33).
Thus, between 1922 and 1936 Edwin HUBBLE solved some of the great questions of cosmology, among which the speed of expansion of the Universe. By observing the brightness variations of the Cepheids he was able to determine the escape velocity of these galaxies. By his observations HUBBLE was able to establish that the galaxies were moving away, and this proportionally to their distance. In 1929 he published what is now called Hubble’s Law. This law is the first proof of the expansion of the Universe.
It is from empirical observations that Edwin HUBBLE made his calculations. A photograph in particular, taken at the observatory of Mount Wilson and drawn in negative, shows this work of calculations and the annotations of HUBBLE. Below we can see the Barnard Galaxy photograph annotated by HUBBLE and next to the same drawn in positive.