Edwin Powell Hubble
Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer who played a crucial role in establishing the field of extragalactic astronomy and is generally regarded as one of the most important observational cosmologists of the 20th century. Hubble is known for showing that the recessional velocity of a galaxy increases with its distance from the earth, implying the Universe is expanding. Known as "Hubble's law", this relation had been discovered previously by Georges Lemaître, a Belgian priest/astronomer who published his work in a less visible journal. There is still much controversy surrounding the issue, and some argue that it should be referred to as "Lemaître's law", although this change has not taken hold in the astronomy community. Edwin Hubble is also known for providing substantial evidence that many objects then classified as "nebulae" were actually galaxies beyond the Milky Way. American astronomer Vesto Slipher provided the first evidence for this argument almost a decade before. Edwin Hubble supported the Doppler shift interpretation of the observed redshift that had been proposed earlier by Slipher, and that led to the theory of the metric expansion of space. He tended to believe that the frequency of light could, by some so far unknown means, decrease the longer light travels through space.