In 1920, Harlow Shapley of the Mount Wilson Observatory and Heber D. Curtis of the Lick Observatory in California, two leading astronomers, debated each other at the Smithsonian Institution on the relationship of the Milky Way Galaxy to the Universe. Shapley's position was that the Milky Way is the only galaxy in the universe,.Curtis, however, argued that the Milky Way exists as just one of many “island universes” in the cosmos. Whereas both scientists provided a stimulating debate, it was Curtis who was vindicated for his opinion when the island universe theory was validated by Edwin Hubble, whose paper was read on 1 Jan 1925 to a meeting of the American Astronomical Society. Edwin Hubble, The Discoverer of the Big Bang Universe, by Aleksandr Sergeevich Sharov, Igor Dmitrievich Novikov. - book suggestion.