The Great Debate: Just How Big is the Universe?
On April 26, 1920, astronomer Harlow Shapley took the podium at the annual meeting of the National Academy of Sciences to argue against the existence of external galaxies, also called ‘island universes.’ His opponent, Heber D. Curtis, followed Shapley’s speech with a technical and dense speech arguing for the existence of island universes, sparking what was later called “Great Debate”.