During 1922 and 1923 astronomer Edwin Hubble succeeded for the first time to identify single stars in the famous spiral nebula Andromeda, even visible with the naked eye. For this detection Hubble used world’s largest telescope at that time, the 2,5m Hooker Telescope at Mt. Wilson Observatory.
Andromeda Galaxy (M31) Source: Palomar Observatory
Several of these new-found stars were Cepheids, large and bright stars, pulsating and thus varying luminosity. Typical Cepheids pulsate with periods from a few days through to months. There is a quite relationship between cycle duration and absolute luminosity of these variable stars. As longer the period as higher the luminosity.
That is why cepheids are workable standard candles for cosmic distance measurements. It is merely necesssary to calibrate the distance of the cepheids with other techniques like geometric parallax (difference in the apparent position of an object viewed along two different lines of sight against a much more distant background) and spectroscopic parallax (comparing the luminosity of main sequence stars in Hertzsprung-Russel-Diagrams).
Edwin Hubble (1889-1953) and 2,5m (100 inch) Hooker-Teleskope at Mt.Wilson Observatory Source: Wikipedia, www.astro.caltech.edu/
Using the period-luminosity-relationship of Cepheids in Andromeda, Hubble calculated a distance of nearly one million light years (ly), too much this nebula could be a part of the Milky Way. And Hubble even undervalued Andomeda´s distance due to calibration errors. Today`s reading ist about more than two million light years (ly).
Previously most astronomers took the view that Milky Way and universe were the same. But in fact, our Milky Way was only one galaxy among many galaxies in the universe. Hubble had found the ultimate proof and from then on the world became much larger than before.
Hubble also devised a system for classifying galaxies according to their optical appearance in a diagram, resembling a tuning fork , also known as the Hubble sequence.
Hubble´s Tuning-Fork-Diagram for classification of galaxies. Source: Wikipedia
In the following years Hubble searched out a direct proportionality of the galaxies’ distances with redshifts in their spectra. This redshift is a consequence of the Doppler effect and hence a clear evidence for a flight of galaxies away from the Milky Way. The received frequency of a moving light source increases during approach (blueshift), remains constant at the moment of passing by, and decreases during recession (redshift). stretched. At first electromagnetical waves are compressed due to approach, afterwards they are stretched due to recession. Accordingly direct proportionality of galaxies’ distances with redshifts also means a direct proportionality of galaxies’ distances with the galaxies’ escape velocity.
Hubble´s Law Source: Edwin Hubble (www.pnas.org/)
The redshift of almost galaxies does not mean that Milky Way is centre of the universe. Universe acts like a proving yeast dough of a plum cake before baking. From the viewpoint of every raisin all the other raisins are going away as faster as longer the distance between raisins are. That´s because the dough itself is expanding.
In almost the same manner the space between galaxies expands, leading to an expansion of the whole universe. This discovery of Hubble was a complete surprise at that time. In place of a stable and eternal Universe there was an evolving universe with it´s own history, leading to big bang theory.
Next up Hubble wanted to know, if universe is open or closed.
In other words: Does the entire universe have enough matter and energy to stop it´s own expansion by gravitational force, warping space to a closed (four-dimensional) space-time bubble? Or will expansion last to all eternity, leading to an open universe?
Hubble tried to solve the problem by counting the galaxies as a function of their distance. He assumed in average a relationship between luminosity and distance of galaxies. If the nummber of galaxies increased overproportional with their distance, the space should have a positive curvature, leading to a closed universe and vice versa.
A two-dimensional analogy for better understanding: A spherical surface has a positive curvature, a riding saddle (hyperboloide) has a negative curvature and a flagstone is flat.
Jens Christian Heuer
Sources: Lonely Hearts of the Cosmos: The Story of the Scientific Quest for the Secret of the Universe, Dennis Overbye, Wikipedia