Expansion of the Universe

Physical Environment > Expansion of the Universe

15 Billion Years ago

This site tells the story of the history of the universe. Click Earlier and Later to follow the story.
Note: Many facts have been simplified to make them easier to understand.
Our new free weekly podcast Time Crystal tells this story as sci-fi fantasy

Have you ever noticed that the sound of a moving vehicle or train changes as it goes past you? How does it change? The note drops. That is called the Doppler effect. Austrian physicist Christian Doppler (1803-1853) discovered it, and also realised that the colour of light should change in the same way. An object moving away from you should look a little redder than a staionary one (this is called the red shift), and one moving towards you should look bluer (the blue shift). But the change in colour is so slight that Doppler couldn't measure it. However we can measure it for stars and galaxies by analysing their light in a spectrometer.

By looking at many distant galaxies, Edwin Hubble found that they all have a red shift, and that the furthest objects have the biggest shift. Why should this be? Are they all afraid of humans, and so want to get away from us? No, distant galaxies know nothing about the Earth. It must be that you would see the same pattern no matter where you are in the Universe.

This implies that the whole Universe is expanding, like a cake rising in an oven. Now how could such an expansion occur? George Lemaitre suggested that the expansion must have started way back in the past, when the Universe was very small. Therefore there must have been a Big Bang. George Gamow realised that if there had been an explosion we should be able to see the light left over from it. We have since found this light, now very weak and stretched out. It is called the cosmic microwave background radiation.