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Dr. Edwin Hubble: Study of Extragalactic Nebulae, 1939

With a Bang

Hubble took his discovery to the next level. If galaxies twice as far away were moving away from one another at twice the speed, they must have started their expansion from the same space and time. Hubble determined, using his ratio of distance to speed, that that time was approximately two billion years ago. By today's figurings, he miscalculated by more than ten billion years, but with his estimates he laid the groundwork for the big bang theory, lending evidence to the theory that the universe has been expanding ever since it began with a tremendous burst of energy.