This picture of the universe---that it began from a small dense collection of matter and had been expanding ever since---is called the big bang theory. The theory constitutes our best idea of what the early universe looked like.
Think how different the big bang theory of the universe is from the theories of the Greeks or the medieval scholars, or even the great scientists of the nineteenth century.To them the earth went in stately orbit around the sun, and the sun moved among the starts, but the collection of starts you can see at night with your naked eye or with a telescope was all that there was. Suddenly with Hubble's work, the universe grew immeasurably. Our own collection of starts, our own galaxies in a universe in which galaxies are flying away from each other at incredible speeds. It is a vision of a universe that began at some time in the distant past an will, presumably, end at some time in the future.
To understand the origins of this idea think about why NASA named it's big eye in the sky the Hubble telescope.
Now before we begin the time line the first button is a brief history of Edwin Hubble. He is the man who is given credit with the birth of modern cosmology
