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Cosmology: The History and Nature of Our UniverseMeasuring the Doppler shift of galaxies in the early 20th century; observations reveal that they all were moving away from Earth; the more distant ones move faster, proportional to distance; this is the Hubble Law, implying the initial "big bang."

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Now PlayingCosmology: The History and Nature of Our UniverseMeasuring the Doppler shift of galaxies in the early 20th century; observations reveal that they all were moving away from Earth; the more distant ones move faster, proportional to distance; this is the Hubble Law, implying the initial "big bang."0Cosmology: The History and Nature of Our UniverseProfessor Whittle defines distances in cosmology; the three important distances to consider is the emission distance when the light is set out; the current distance, when the light arrives; the distance the light travels.