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He had started measuring the Doppler shift of spectral lines from spiral nebulae .
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By 1925 he had shown that most exhibited redshifts in their spectral lines.
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In 1927 Edwin Hubble estimation of the mass of the universe cause both Einstein and De Sitter observations incorrect.
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In 1928 Edwin Hubble attended a meeting of the International Astronomical Union, held that year in Holland. There Hubble discussed cosmological theories with de Sitter.
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In 1929 the theory of the expanding universe was created.