Edwin Hubble : biography
Hubble also devised the most commonly used system for classifying galaxies, grouping them according to their appearance in photographic images. He arranged the different groups of galaxies in what became known as the Hubble sequence.
Redshift increases with distance
The Hooker telescope at [[Mount Wilson Observatory that Hubble used to measure galaxy distances and a value for the rate of expansion of the universe.]]
Combining his own measurements of galaxy distances based on Henrietta Swan Leavitt’s period-luminosity relationship for Cepheids with Vesto Slipher and Milton L. Humason’s measurements of the redshifts associated with the galaxies, he discovered a rough proportionality of the objects’ distances with their red shifts., Proceedings of the National Academy of Science vol 15 Though there was considerable scatter (now known to be due to peculiar velocities), he was able to plot a trend line from the 46 galaxies and obtained a value for the Hubble Constant of 500 km/s/Mpc, which is much higher than the currently accepted value due to errors in their distance calibrations. In 1929 Hubble formulated the empirical Redshift Distance Law of galaxies, nowadays termed simply Hubble’s law, which, if the redshift is interpreted as a measure of recession speed, is consistent with the solutions of Einstein’s equations of general relativity for a homogeneous, isotropic expanding space. Although concepts underlying an expanding universe were well understood earlier, this statement by Hubble and Humason led to wider-scale acceptance for this view. The law states that the greater the distance between any two galaxies, the greater their relative speed of separation. But two years before, in 1927, Georges Lemaître, a Belgian Catholic priest and physicist, published a paper in an obscure Belgian journal, Annales de la Société Scientifique de Bruxelles. In that paper, he showed that the data collected by Hubble and two other astronomers up to that time was enough to derive a linear velocity-distance relation between the galaxies, and that this supported a model of an expanding universe based on Einstein’s equations for General Relativity.
This discovery was the first observational support for the Big Bang theory which had been proposed by Georges Lemaître in 1927. The observed velocities of distant galaxies, taken together with the cosmological principle, appeared to show that the universe was expanding in a manner consistent with the Friedmann-Lemaître model of general relativity. In 1931 Hubble wrote a letter to the Dutch cosmologist Willem de Sitter expressing his opinion on the theoretical interpretation of the redshift-distance relation:
Today, the "apparent velocities" in question are understood as an increase in proper distance that occurs due to the expansion of space. Light traveling through stretching space will experience a Hubble-type redshift, a mechanism different from the Doppler effect (although the two mechanisms become equivalent descriptions related by a coordinate transformation for nearby galaxies).
In the 1930s, Hubble was involved in determining the distribution of galaxies and spatial curvature. These data seemed to indicate that the universe was flat and homogeneous, but there was a deviation from flatness at large redshifts. According to Allan Sandage,
There were methodological problems with Hubble’s survey technique that showed a deviation from flatness at large redshifts. In particular, the technique did not account for changes in luminosity of galaxies due to galaxy evolution. Earlier, in 1917, Albert Einstein had found that his newly developed theory of general relativity indicated that the universe must be either expanding or contracting. Unable to believe what his own equations were telling him, Einstein introduced a cosmological constant (a "fudge factor") to the equations to avoid this "problem". When Einstein heard of Hubble’s discovery, he said that changing his equations was "the biggest blunder of [his] life."