The Expanding Universe:
Hubble's observations had proved the existence of galaxies. After mapping as many galaxies as could be seen by the telescopes then'available, he turned his attention to the motion of galaxies. He was motivated to do this by a puzzling report of V.M. Slipher, an American astronomer. He had discovered in 1912 that many of the faint nebulae were moving away from the Earth at very great speeds. Their spectral lines exhibited large shifts towards the red end (what is called as red shift). This seemed peculiar because stars in the Milky Way Galaxy move at much smaller speeds, some moving away from us with others moving towards us. Slipher had made these observations a decade before galaxies were discovered. Then it was thought that the nebulae were objects in our own galaxy. He did not know what to make of his observations. But as Hubble knew that these nebulae were galaxies, he began a systematic study of the relation between their speeds and their distances alongwith his colleague M.L. Humason. What they found was very interesting. To put it simply, his observations showed that; i) all galaxies were moving away from us; ii) the farther away a galaxy was from our Galaxy, the greater was the speed at which it moved away. Hubble's discovery put forth the picture of an expanding universe. But if all the galaxies are moving away from us, are we at the centre of the Universe? No. If we were situated in another galaxy, even then the other galaxies would seem to move away from us. You can understand this picture of an expanding universe if you study Fig and also perform a simple activity.