The Expanding Universe
During the two years 1922 and 1923, astronomer Edwin Hubble, using world’s largest telescope at that time, the 2,5m Hooker Telescope (Mt. Wilson Observatory), first identified single stars in the famous spiral nebula Andromeda, even visible with the naked eye.
Andromeda Galaxy (M31) Source: Palomar Observatory
Several of these stars were Cepheids, large and bright stars, pulsating and therefore with variable luminosity. Typical Cepheids pulsate with periods of a few days 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, http://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. But Hubble undervalued Andomeda´s distance due to calibration errors. Today`s reading ist about more than two million light years (ly).
Until this point of time the most astronomers thought, that the entire universe only consisted of the Milky Way. But in fact, the Milky Way now was one galaxy among many galaxies in the universe. The world became considerably larger than before.
Hubble also devised a system for classifying galaxies in a diapason diagram, according to their optical appearance, known as the Hubble sequence.
Hubble´s Tune-Fork-Diagram for classification of galaxies. Source: Wikipedia
During 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 recession of galaxies away from the Milky Way. The received frequency of a moving light source is higher during the approach (blueshift), it is identical at the instant of passing by, and it is lower during the recession (redshift). During approach the electromagnetical waves are compressed, during recession they are stretched. From there the direct proportionality of the galaxies‘ distances with redshifts also means a direct proportionality of the galaxies‘ distances with the galaxies‘ escape velocity.
Hubble´s Law Source: Edwin Hubble (http://www.pnas.org/)
But the redshift of almost galaxies does not mean that the Milky Way is centre of universe. It´s rather as in the raising yeast dough of a plum cake. From the viewpoint of every raisin all the other raisins are going away as faster as longer the distance between raisins. That´s because the dough itself is expanding.
In almost the same manner the space between galaxies increases, leading to an expansion of the entire universe. This discovery was a complete surprise at that time. Universe was not stable and eternal. Instead universe evolved and had to narrate it´s own history, leading to big bang theory.
All along the following years Hubble wanted to know, if universe is open or closed. In other words: Contained the entire universe matter and energy in sufficient amounts to stop its expansion by gravitational force,warping space to a closed (four dimensional) space-time bubble, or not?
Hubble tried to count 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 more than proportional with their distance, the space should have a positive curvature, leading to a closed universe. For a better understanding: a two-dimensional analogy: A spherical surface has a positive, a riding saddle (hyperboloide) has a negative curvature but 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