The Expanding Universe:The Doppler shift is the effect of sound increasing or decreasing in pitch as an object moves towards or away from you. If an object moves away, the pitch is lowered, and if moving towards you, the pitch is raised. Light works the same way, except light is shifted to redder colours when an object is moving away, and to blue colours when an object is travelling towards us.
In 1916 Vesto Slipher (to whose family I am indebted for helping fund my undergraduate education through a scholarship set up at the University of Arizona in his honour) observed about 50 nearby galaxies, spreading their light out using a prism, and recording the results onto film. The results confounded him and the other astronomers of the day. Almost every object he observed had its light stretched to redder colours, indicating essentially everything in the Universe was moving away from us.
Here we show the spectrum of a galaxy as Slipher would have seen it. The light is stretched in the bottom spectrum, so that the dark lines (the colours where elements such as Sodium absorb light), are stretched to redder colours.
Slipher's represented a cosmic conundrum for astronomers of the day: Since the time of Copernicus, astronomy has presumed that we are not a special place in the Universe. But Slipher's results seemingly contradicted this belief - we were a special place, the most unpopular place in the Universe from which all other objects were trying to move away. Slipher's results remained a mystery until Edwin Hubble came along in the 1920s with the then world's most powerful telescope, the recently completed 100inch telescope on Mt. Wilson, near Los Angeles. He used the physical law that an object becomes fainter as its distance increases to gauge the distances to Slipher's galaxies.
In 1929 Hubble
announced his results. He assumed that the brightest stars he could see in a galaxy were
all the same brightness, and found that the faster an object was moving away, the fainter
its brightest stars were, thereby showing that the more distant an object, the faster it was
moving away from us. He announced that the Universe was expanding. This may not
necessarily be obvious to everyone, but it is a natural description of Hubble's
observations as is shown next.