Red shift with a bit of a balloon

 I drew a wave onto a bit of balloon.

I stretched out the balloon. The wavelength is now longer. Not rocket science, I know, but this is the RED SHIFT. Look at the wavelength pictures on this NASA website http://science.hq.nasa.gov/kids/imagers/ems/visible.html Red has the longest wavelength and violet has the shortest wavelength. The Universe is expanding. In other words, the very space between galaxies is stretching. This means that light waves are being stretched. Stretched light has a longer wavelength and is nearer to being the red wavelength. So we say the light has been red shifted. The space between the galaxies is stretching, so the galaxies end up further apart. Now put it the other way round. Edwin Hubble spotted that every galaxy had a red shift. He knew that could only be happening because the wavelength was being stretched so he concluded that space was stretching and the Universe was expanding. The final bit of logic is to run this imaginary film backwards. If it is expanding at the moment, run backwards it would get smaller and smaller until it took up no space at all. The moment at which the Universe took up no space at all is called the Big Bang.