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.
Here is a toy model of the Universe. Imagine if we expand it by 5%, and over overlay the two images, centered on a star near the center of the two pictures. As you can see, every object appears to have moved away from the object that we have centered the images on. Furthermore, the farther an object is away from the center object, the farther it has moved in the expansion. This is exactly what Hubble saw. Another good part of this explanation is that every one in the Universe sees the same thing. Here we have centered the two pictures on a different star. From this stars' perspective everything is moving away from it - it sees exactly the same thing as the previous star. This is what all good astronomical theories should have. There is no place in the Universe which is special, and everyone sees exactly the same thing, leaving astronomy's belief that we are not a special place, intact.

We name the rate that the Universe is expanding after Hubble - The Hubble Constant. The Hubble constant tells us how fast an object is moving away from us, given its distance. If you think about it, the Hubble constant therefore tells us given an object's distance, how far apart those two objects will be at any time. If we extrapolate to the time when these two objects were on top of each other, we reach the big bang. So the Hubble constant tells us how old the Universe is.

But here we have assumed the Universe hasn't changed in its rate of expansion over time.