Discovery of the Expanding Universe
The astronomer Vesto M. Slipher was the first to measure the spectrum of the Andromeda galaxy in 1912. This was no simple feat in those days. He continued these measurements and by around 1925, he had measured 45 of these spectra. At that time galaxies were called “spiral nebulae”, and their nature was actually unknown. Were they galaxies like our own Milky Way and outside our galaxy or were they nebulae with a spiral shape, inside our Milky Way? At that time it was not even known what our Milky Way galaxy looked like.
Slipher discovered something very intriguing in these spectra, when he measured the red shift to find radial velocities. With the exception of the Andromeda Galaxy they were all moving away from us. This was a puzzle, also because there were not yet accurate distances to these “spiral nebulae”, but this red shift discovery looked significant for the astronomers of the day.
Edwin Hubble made observations of Cepheid variable stars in various spiral nebulae in 1922 and 1923 and proved that these objects were outside the Milky Way galaxy, using Henrietta Swan Leavitt's period-luminosity relationship for Cepheids. This technique, described earlier in this EBook, was decisive to end the discussion among astronomers about whether there was more to the Universe than just the Milky Way galaxy.
Combining these distance measurements with Slipher’s red shift measurements, Hubble and Milton L. Humason discovered a rough proportionality of the objects' distances with their redshifts. The larger the distance, the larger the receding velocity. In 1929 Hubble and Humason formulated the empirical Red shift Distance Law of galaxies, nowadays simply termed Hubble's law.
This law dictates that an object twice as far away as another object, has a velocity that is also twice as much as the velocity of the closer object. This suggestion was astounding because the general consensus in those days was that the Universe was static and not changing in size at all.
Earlier, in 1917, Albert Einstein had discovered that his newly developed General Theory of Relativity indicated that the universe must be either expanding or contracting. Einstein himself believed, like everybody else, that the Universe was static, and he therefore introduced a cosmological constant to the equations to avoid this "problem". When Einstein heard of Hubble's discovery, he was quoted as saying: "this is the biggest blunder of my life". He visited Mt. Wilson observatory to discuss the discovery with Hubble and others.